To purchase this report, contact the National Technical Information Service, 5285 Port Royal

Road, Springfield, VA 22161 (call toll free 1-800-553-6847), or the Defense Technical Information

Center, 8725 Kingman Rd., Suite 0944, Fort Belvoir, VA 22060-6218.

The cover photograph was taken on the Colorado River a few miles below Potash, Utah, on June 9, 2003; it was

provided by, and is used with permission of, Doug Osmundson, U.S. Fish and Wildlife Service, Colorado River Fishery

Project, Grand Junction, Colorado. Agencies electrofishing in the Colorado River Basin have instituted measures to

minimize harmful effects on fish, including injuries such as the extreme example illustrated in the inset X-ray. That

dorsal-view X-ray of spinal misalignment and fractured vertebrae in an electrofished rainbow trout (Oncorhynchus

mykiss) was provided in 1991 by, and is used with permission of, Norman G. Sharber, Flagstaff, Arizona.
Electrofishing and Its Harmful Electrofishing and Its Harmful Electrofishing and Its Harmful

Effects on Fish Effects on Fish

Information and Technology Report

USGS/BRD/ITR--2003-0002

September 2003

By

Darrel E. Snyder

U.S. Department of the Interior

U.S. Geological Survey
Suggested citation:

Snyder, D.E., 2003, Electrofishing and its harmful effects on fish, Information and Technology Report USGS/BRD/

ITR--2003-0002: U.S. Government Printing Office, Denver, CO, 149 p.

ii
iii

Foreword ForewordForeword

This report provides a comprehensive synthesis of the literature related to electrofishing with an emphasis on

adverse effects to fish. The sections on “Electric Fields in Water,” “Responses of Fish to Electric Fields,” “Harmful

Effects of Electrofishing on Fish,” “Factors Affecting Electrofishing Injury and Mortality,” and “Conclusions” are

especially valuable to all field biologists who use electrofishing as a sampling tool. This information provides insight

on how to effectively use this valuable sampling tool while minimizing adverse effects to fish. The recommentation of

experimental testing is especially important when electrofishing is to be used to sample threatened or endangered

fishes so that necessary precautions can be taken to avoid injury or mortality. This thorough overview provides a

valuable reference to biologists, managers, and students for understanding: (1) the principles of electrofishing; (2) concepts of electrical transmission in water and fish; and (3) ways to reduce fish injury and mortality. Application of this

knowledge will ensure that studies are designed to minimize biased results and adverse impacts.

Dr. Richard S. Wydoski, Editor
iv

Preface Preface Preface

In 1990, the Lower and Upper Colorado River regions of the U.S. Bureau of Reclamation planned a three-phased

study to identify and address the potential harmful effects of electrofishing on endangered fish in the Colorado River

Basin. Phase I consisted of a comprehensive literature review and synthesis of existing information on effects of

electrofishing with recommendations for future research and interim guidelines to minimize harmful effects (Snyder,

1992a, original version of this report). Phase II consists of controlled laboratory and field experiments to address

selected questions and concerns remaining after Phase I. Phase III will field test the effectiveness of promising techniques or protocols suggested as a result of Phases I and II. Portions of the 1992 Phase I report have been abstracted

for articles to provide a brief overview of the problem (Snyder, 1992b, 1995) and specifically discuss known effects on

fish reproduction, embryos, and larvae (Snyder, 1993, 1994). Investigations concluded thus far under Phase II include

those by Cowdell and Valdez (1994), Ruppert and Muth (1995, 1997), Ruppert (1996), Muth and Ruppert (1996, 1997),

and Meismer (1999); another is nearing completion (Hawkins, personal communication).

This Final Report of Colorado State University Larval Fish Laboratory to the U.S. Bureau of Reclamation Upper

Colorado Regional Office (Salt Lake City, Utah) updates the Phase I (Snyder, 1992a) review and synthesis of electrofishing

literature based on over 60 additional technical papers, reports, and newsletter and magazine articles published on

electrofishing and its effects between 1992 and 2000. It also updates recommendations for interim guidelines on use of

electrofishing for collection of endangered fishes in the Colorado River Basin and for future research. As a recognized,

peer-reviewed publication, through and in cooperation with the U.S. Geological Survey, it is more citable in future

technical publications and available to a much wider audience.
v

Contents Contents ContentsContents

Page

Foreword ........................................................................................................................................................................ iii

Preface .......................................................................................................................................................................... iv

Abstract ..........................................................................................................................................................................1

Introduction ....................................................................................................................................................................1

Methods .........................................................................................................................................................................3

Results — Historical Overview .......................................................................................................................................4

Results — Electric Fields in Water ..................................................................................................................................9

Water Conductivity .....................................................................................................................................................9

Electrofishing Currents and Waveforms ................................................................................................................... 10

Field Intensity ........................................................................................................................................................... 12

Heterogeneous and Homogeneous Fields ................................................................................................................ 16

Bounding or Surrounded Media and Substrates ....................................................................................................... 16

Electrodes — Position, Size, Shape, and Other Matters ........................................................................................... 18

Results — Responses of Fish to Electric Fields ........................................................................................................... 21

Biarritz and Bozeman Paradigms................................................................................................................................ 21

Theory of Power Transfer from Water to Fish ........................................................................................................... 23

Major Intensity-Dependent Responses .................................................................................................................... 28

Response Thresholds .......................................................................................................................................... 31

Zone of Reactive Detection ................................................................................................................................. 37

Zones of Undirected or Inhibited Swimming and Taxis........................................................................................ 37

Zones of Narcosis and Tetany ............................................................................................................................. 38

Comparison of Currents for Electrofishing Purposes ................................................................................................ 40

Results — Harmful Effects of Electrofishing on Fish.................................................................................................... 42

Effects Other than Spinal and Related Injuries .......................................................................................................... 43

Spinal and Related Injuries ........................................................................................................................................ 46

Nature of the Injuries ........................................................................................................................................... 47

Detection and Evaluation of the Injuries ............................................................................................................. 51

Relation Between Injury and Mortality ................................................................................................................ 55

Factors Affecting Electrofishing Injury and Mortality .............................................................................................. 55

Type of Current .................................................................................................................................................... 56

 Mortality ........................................................................................................................................................ 56

 Injury ............................................................................................................................................................. 57

Field Intensity ...................................................................................................................................................... 62

 Mortality ........................................................................................................................................................ 62

 Injury ............................................................................................................................................................. 64

Duration of Exposure ........................................................................................................................................... 68

Waveform, Pulse Shape ....................................................................................................................................... 69

Pulse Frequency .................................................................................................................................................. 70

Pulse Trains ......................................................................................................................................................... 72

Pulse Duration, Duty Cycle .................................................................................................................................. 74

Voltage Spikes ...................................................................................................................................................... 75

Species................................................................................................................................................................. 75

 Mortality ........................................................................................................................................................ 75

 Injury ............................................................................................................................................................. 76

Size ....................................................................................................................................................................... 79

Condition ............................................................................................................................................................. 80

Repeated Exposures ............................................................................................................................................. 81

Summary .............................................................................................................................................................. 82

Effects on Long-Term Survival .................................................................................................................................. 84

Effects on Growth and Condition .............................................................................................................................. 86
vi

Contents (continued) Contents (continued)

Page

Effects on Reproduction and Gametes ...................................................................................................................... 89

Effects on Early Life Stages....................................................................................................................................... 90

Results — Summary of Survey Responses ................................................................................................................... 94

Experience ................................................................................................................................................................. 94

Observations of Harmful Effects on Fish .................................................................................................................. 95

Respondents’ Recommendations for Minimizing Harmful Effects ............................................................................ 96

Conclusions .................................................................................................................................................................. 97

Responses to Specific Questions .............................................................................................................................. 99

Future Research ...................................................................................................................................................... 105

Recommendations....................................................................................................................................................... 106

Interim Policy to Minimize Electrofishing Injury ..................................................................................................... 106

Further Research on Electrofishing Injuries and Responses ................................................................................... 109

Acknowledgments ...................................................................................................................................................... 114

Sources of Information ............................................................................................................................................... 115

Literature Cited ........................................................................................................................................................ 115

Unpublished Manuscripts ...................................................................................................................................... 125

Personal Communications ....................................................................................................................................... 125

Tables

Figures Figures

1. Reactions of fish in homogenous fields of direct current ............................................................................... 22

2. In-water, peak-voltage-gradient (V/cm) response thresholds by fish species, water conductivity, and

type of electrical current .................................................................................................................................. 33

3. Procedures and criteria for documenting damage to fish spinal columns and associated hemorrhages ........ 51

Figures

1. Electrofishing-induced injuries to the spinal column and associated hemorrhages in rainbow trout

(Oncorhynchus mykiss) ................................................................................................................................... 2

2. Brands (bruises or dark pigmental discolorations) in rainbow trout (Oncorhynchus mykiss) caused by

electrofishing ................................................................................................................................................... 5

3. Bent back in rainbow trout (Oncorhynchus mykiss) caused by electrofishing ............................................... 7

4. Bent backs and abnormal growth in westslope cutthroat trout (Oncorhynchus clarki lewisi) probably

caused by electrofishing (top and middle photographs) with normal trout for comparison (bottom

photograph) ..................................................................................................................................................... 8

5. Selected waveforms used in electrofishing ..................................................................................................... 11

6. Four-way logarithmic graph of relations among measures of electrical-field intensity (power density,

current density, and voltage gradient) relative to water conductivity ............................................................ 13

7. Changes in voltage gradient and current density relative to water conductivity at a constant power

density of 100 μW/cm3 . ................................................................................................................................... 14
vii

Contents (concluded) Contents (concluded)

Figures (concluded) Figures (concluded)

Appendices AppendicesAppendices

Page

8. Changes in current density and power density relative to voltage gradient in water with a conductivity

of 500 μS/cm ..................................................................................................................................................... 15

9. Hypothetical two-dimensional diagrams of heterogeneous electric fields around and between

electrodes......................................................................................................................................................... 17

10. Distortion of homogenous electric fields around fish in water that is less conductive (top) and more

conductive (bottom) than the fish ................................................................................................................... 19

11. Major intensity-dependent electrofishing response zones............................................................................. 24

12. Power-density-transfer curve .......................................................................................................................... 25

13. Power-density-transfer curves fitted to peak-power-density threshold data for twitch, taxis (attraction),

and narcosis (stun) in 6- to 9-cm-TL goldfish (Carassius auratus) subjected to homogeneous fields

of DC ................................................................................................................................................................ 25

14. Peak-voltage-gradient-threshold curves for narcosis (stun) of 6- to 9-cm-TL goldfish (Carassius

auratus) in homogeneous fields of DC; 50-Hz, square-wave PDC with duty cycles of 50%, 25%, and

10%; and 60-Hz sinusoidal AC ........................................................................................................................ 29

15. Peak-power-density-threshold curves for narcosis (stun) of 6- to 9-cm-TL goldfish (Carassius auratus)

in homogeneous fields of DC; 50-Hz, square-wave PDC with duty cycles of 50%, 25%, and 10%; and

60-Hz sinusoidal AC ........................................................................................................................................ 30

16. Dorsal- (top) and lateral-view (bottom) X-rays of a rainbow trout (Oncorhynchus mykiss) revealing

spinal misalignment and fractured vertebrae caused by electrofishing .......................................................... 48

17. Necropsy fillets of rainbow trout (Oncorhynchus mykiss) revealing hemorrhages and associated tissue

and vertebral damage caused by electrofishing, top showing multiple injuries.............................................. 49

18. Fractured vertebrae from a rainbow trout (Oncorhynchus mykiss) caused by electrofishing ........................ 50

19. X-rays of a natural spinal anomaly (A), an old spinal injury (B), and a recent electrofishing-caused spinal

injury (C) in rainbow trout (Oncorhynchus mykiss) ........................................................................................ 54

Appendices

A. Scientific names and families of fishes referenced by common name in this report ...................................... 127

B. Compendium of reported electrofishing mortalities and injuries by fish species and electrical current ....... 129
Electrofishing and Its Harmful Effects on Fish Electrofishing and Its Harmful Effects on Fish

Abstract AbstractAbstract

By

Darrel E. Snyder

Colorado State University

Larval Fish Laboratory

Department of Fishery and Wildlife Biology

Fort Collins, CO 80521-1474

Abstract. Electrofishing, a valuable sampling technique in North America for over half a century, involves a very dynamic and

complex mix of physics, physiology, and behavior that remains poorly understood. New hypotheses have been advanced regarding

“power transfer” to fish and the epileptic nature of their responses to electric fields, but these too need to be more fully explored

and validated.

Fishery researchers and managers in the Colorado River Basin, and elsewhere, are particularly concerned about the harmful

effects of electrofishing on fish, especially endangered species. Although often not externally obvious or fatal, spinal injuries and

associated hemorrhages sometimes have been documented in over 50% of fish examined internally. Such injuries can occur

anywhere in the electrofishing field at or above the intensity threshold for twitch. These injuries are believed to result from

powerful convulsions of body musculature (possibly epileptic seizures) caused mostly by sudden changes in voltage as when

electricity is pulsed or switched on or off. Significantly fewer spinal injuries are reported when direct current, low-frequency

pulsed direct current (≤30 Hz), or specially designed pulse trains are used. Salmoniae are especially susceptible. Endangered

cyprinids of the Colorado River Basin are generally much less susceptible, enough so to allow cautious use of less harmful currents

for most recovery monitoring and research. However, the endangered catostomid Xyrauchen texanus appears sufficiently susceptible to warrant a continued minimal-use policy.

Other harmful effects, such as bleeding at gills or vent and excessive physiological stress, are also of concern. Mortality,

usually by asphyxiation, is a common result of excessive exposure to tetanizing intensities near electrodes or poor handling of

captured specimens. Reported effects on reproduction are contradictory, but electrofishing over spawning grounds can harm

embryos. Electrofishing is often considered the most effective and benign technique for capturing moderate- to large-size fish, but

when adverse effects are problematic and cannot be sufficiently reduced, its use should be severely restricted.

Key Words: Behavior, electric fields, electrofishing, epilepsy, fish, injuries, mortality, power transfer, responses, stress.

Introduction

Electrofishing, the use of electric fields in water to

capture or control fish, has been a valuable sampling

technique in North America for over half a century, but

there has been increasing concern among fishery

biologists and managers regarding its potential for

harming fish. Much of this increased concern began when

Sharber and Carothers (1988) documented substantial

injury to the spinal column and associated tissues of 44

to 67% of large rainbow trout (over 300 mm TL) collected

with pulsed direct current (PDC) from the Colorado River

in Glen Canyon National Recreation Area and Grand

Canyon National Park. Most of the injuries were detected

only by X-ray analysis or necropsy in fish that appeared

externally normal (Fig. 1). This report quickly prompted

similar investigations elsewhere which also resulted in

reports of substantial numbers of PDC-caused spinal

injuries in rainbow trout (up to 98%), as well as brook

trout, brown trout, cutthroat trout, Arctic graying, river

carpsucker, northern pike, and walleye (Holmes, 1990;

1
2 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Fig. 1. Electrofishing-induced injuries to the spinal column and associated hemorrhages in rainbow trout (Oncorhynchus

mykiss). (Photograph A is both sides of a fillet exposing injuries. Photograph B is a lateral-view X-ray of the same

injuries prior to necropsy. Reproduced with permission from Fig. 2 in Sharber and Carothers, 1988.)
 SNYDER 3

Meyer and Miller, 1990; Wyoming Game and Fish

Department, 1991; Fredenberg, 1992; Hollender and

Carline, 1992, 1994; Newman, 1992; Roach, 1992; Taube,

1992; McMichael, 1993; Zeigenfuss, 1995; Dalbey et al.,

1996; Grisak, 1996; Thompson et al., 1997a).

The results of Sharber and Carothers’ (1988) study

also alarmed regional biologists and managers of the

National Park Service and U.S. Bureau of Reclamation

about continued use of electrofishing to monitor endangered humpback chub in the Grand Canyon. In a memorandum to the Glen Canyon Ecological Studies program

manager (12 July 1990), the superintendent of Grand Canyon National Park, J.H. Davis, suggested that until concerns over potential adverse effects could be resolved,

electrofishing in Glen Canyon National Recreation Area

and Grand Canyon National Park should be kept to a

minimum and be used in such a way as to minimize possible stress and injury to humpback chub. Concern also

increased about the use of electrofishing to study endangered Colorado pikeminnow, humpback chub,

bonytail, and razorback sucker in the Upper Colorado

River Basin. As a result, the U.S. Bureau of Reclamation

sponsored the three-phase study described in the Preface of this report.

The objectives of the original Phase I report (Snyder,

1992a) and this update were to: (1) review and synthesize

the literature on electrofishing including the nature of

electric fields in water, responses of fish to those fields,

its harmful effects on fish, and the factors (specific aspects of electrofishing fields and fish) potentially affecting injury and mortality in fish; (2) answer specific

questions regarding the use of electrofishing to capture

threatened, endangered, and native fishes in the Colorado River Basin; and (3) provide recommendations for

interim policy and future research to avoid or minimize

the harmful effects of electrofishing on those fishes. Although specifically intended to facilitate evaluation of

current electrofishing policies by Colorado River Basin

agencies, the review and synthesis is broad in scope and

should be useful wherever the impacts of electrofishing

are a concern. As author, I have brought little practical

electrofishing experience to this project but also no prior

biases or vested interests.

Methods Methods Methods

Publications up to year 2000 on electrofishing and

particularly its effects on fish were identified primarily

through electrofishing bibliographies (especially Burridge

et al., 1990), electronic databases of literature (e.g., Fish

and Fisheries Worldwide, Aquatic Science and Fisheries

Abstracts, Biological Abstracts, Fish and Wildlife

Reference Service, and Uncover), and the Literature Cited

sections of published papers. Copies of most Englishlanguage and translated literature and many foreignlanguage papers were obtained, scanned for content, and,

if pertinent, reviewed for inclusion in this report. Literature

identified for the earlier version of this report (Snyder,

1992a) was catalogued with keywords and content codes

in a bibliographic database (Reference Manager, Institute

for Scientific Information, Philadelphia, Pennsylvania).

An indexed bibliography was generated from this

database (Snyder and Johnson, 1991) and appended to

the original report (Snyder, 1992a). Both that bibliography

and Burridge et al. (1990) were expanded upon and

updated by Miskimmin and Paul (1997a).

Information derived from published literature and

limited-distribution reports was supplemented by data,

observations, theories, and recommendations in

unpublished manuscripts and anecdotal personal

communications.1

 Contributions regarding unpublished

and ongoing work, as well as personal observations,

experiences, and suggestions, were solicited through a

request printed in the American Fisheries Society

magazine Fisheries, vol. 16, no. 3, p. 52, May–June 1991)

and several other fishery-related bulletins and

newsletters. Approximately 30 responses were received.

Several recognized authorities and electrofishing gear

manufacturers also shared their knowledge, views, and

unpublished manuscripts. Some contacts were made and

information exchanged during special sessions on

electrofishing injuries that were held as part of annual

meetings of the Western Division of the American

Fisheries Society in 1991 (Bozeman, Montana) and 1992

(Fort Collins, Colorado). Finally, a questionnaire was

prepared to solicit local observations and

recommendations on electrofishing (Appendix II in

Snyder, 1992a). The survey forms were distributed to

researchers working in the Colorado River Basin and to

faculty and students in fishery biology at Colorado State

University.

Scientific names and families of fishes referenced by

common names herein are given in Appendix A and follow Robins et al. (1991a,b). When known, fish lengths

are specified as total length (TL), fork length (FL) or standard length (SL), conductivity as ambient or standardized to 25° C, and electrical output and field intensities as

mean (m), root-mean-square (rms), or peak (p). In many cases

these important distinctions were not reported. Except

when directly pertinent to the text, readers are referred to

Appendix B for environmental and electrical parameters

associated with electrofishing investigations discussed

herein.

1

Unpublished manuscripts and personal communications are

fully identified on page 125 after “Literature Cited.”
4 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Results — Historical Overview

Electricity has been used by humans to kill,

anesthetize, capture, drive, draw, tickle (arouse), guide,

or screen (block, repel) fish since the mid 1800’s (Vibert,

1967b; Halsband and Halsband, 1975, 1984; Hartley, 1990).

Fishery researchers and managers often rely on

electrofishing as their principal capture method for

researching, monitoring, and managing stocks of

freshwater fishes, especially salmonids (e.g., Weber, 1997).

In 1863, a British patent was granted to Isham Baggs for

electric fishing, but widespread development and use of

the technique did not occur until the 1950’s (Hartley, 1990;

Reynolds, 1995). Halsband and Halsband (1975, 1984)

provided a particularly detailed history of research on

fish in electric fields, especially with regard to German

contributions. However, man’s technological

developments are often modifications or imitations of

nature’s own. Before the evolution of modern man, certain

species of fish developed powerful electric organs which

were probably used much like their modern descendants

to detect and capture prey or ward off predators (Marshall,

1966; Hyatt, 1979). The stunning or narcotizing effects of

electric fishes were known and used for medical purposes

by the ancient Greeks, and study of electric fishes during

the 18th and 19th centuries was instrumental in our

understanding of the electrogenic nature of nerves and

muscles (Wu, 1984).

Most of our knowledge of electrofishing practice,

theory, and effects on aquatic organisms is well

represented in three English-language European symposia

publications edited by Vibert (1967a, from 1966 FAO

symposium, United Nations Food and Agriculture

Organization, Belgium), Cowx (1990, from 1988 EIFAC

symposium, European Inland Fisheries Advisory Council,

England), and Cowx and Lamarque (1990—also from 1988

EIFAC symposium); a German text by Halsband and

Halsband (1975, English translation 1984); a Russian

reference book by Sternin et al. (1972, English translation

1976); and a manual for a course on electrofishing offered

nationwide through the National Conservation Training

Center (formerly Fisheries Academy) of the U.S. Fish and

Wildlife Service (latest version, Kolz et al., 1998). A book

by Meyer-Waarden and Halsband (1975, German) and a

symposium publication edited by Maiselis (1975, Russian

with English summaries) also should be included in the

list, but English translations are not available. Fishing

with Electricity, edited by Cowx and Lamarque (1990), can

serve as a relatively up-to-date academic text and basic

reference, but not all of the information therein should be

treated as fact; there are too many uncertainties and gaps

in knowledge. Although this book is treated by

distributors as a replacement for Vibert’s (1967a) Fishing

with Electricity, Vibert’s book includes much information

not in the new book. Halsband and Halsband (1975, 1984)

is also a fine text on electrofishing, but it is based largely

on German perspectives, experience, and research, and

like Vibert (1967a), it is somewhat dated. Sternin et al.

(1972, 1976) includes marine applications and is a very

detailed treatise on the theory and practice of

electrofishing based on Soviet research and summaries

of world literature. Its Appendices 4 and 5 are tabulated

summaries of fish response thresholds (without source

references) and aftereffects on fish (reproduced in Snyder

1992a as Appendices III and IV, respectively). The manual

for the U.S. Fish and Wildlife Service classroom and

correspondence course on Principles and Techniques of

Electrofishing (Kolz et al., 1998) is a loose-leaf, periodically

updated volume in semi-outline format with a CD-ROM

disk of supplemental simulations and exercises. Except

for the article by Sharber and Carothers (1990) in Cowx

(1990), a four-page synopsis in the article by Lamarque

(1990) in Cowx and Lamarque (1990), and a few pages in

Sternin et al. (1972, 1976), Halsband and Halsband (1975,

1984), and Kolz et al. (1998), the matter of electrofishing

injury and mortality was not discussed extensively in any

of these books or manuals.

Recognized authorities on electrofishing have long

emphasized its benign qualities. For example, Halsband

(1967) stated that “the harmlessness of electric current to

fish and their food organisms has already been proved

on several occasions.” And in the foreword to their book,

Sternin et al. (1972, 1976) suggested that the theory and

practice of electrofishing in recent decades had put to

rest concerns about deleterious effects on normal activity and natural reproduction in fish. More emphatically,

Halsband and Halsband (1975, 1984) stated that “today

we are convinced that electrical collecting, repelling, and

stunning methods neither cause pain to animals nor injure them internally or externally, (apart from unavoidable

exceptions).” However, these conclusions were premature because we now have considerable evidence that

electrofishing injuries may have been more common than

they appeared or were reported.

Spinal injuries in particular were not widely recognized because most are not externally obvious and can

only be detected by X-ray analysis or necropsy. When

present, even brands (temporary dark markings on the

body; Fig. 2) were seldom associated, as they frequently

are now, with at least moderately severe spinal injuries or

hemorrhages. If captured fish had no notable external

injuries, aside from occasional brands, and appeared to

recover sufficiently to swim away, they were typically

considered “unharmed” and expected to continue to behave, grow, and reproduce normally. As a result,

electrofishing had often been considered not only the

most efficient but the least-damaging collection technique

available.
 SNYDER 5

Since Sharber and Carothers’ (1988, 1990) report of

substantial numbers of spinal injuries among electrofished

rainbow trout, some agencies have begun to verify and

further investigate the extent, conditions, and causes of

electrofishing-induced spinal injuries (e.g., Holmes et al.,

1990; Wyoming Game and Fish Department, 1990, 1991;

Fredenberg, 1992; Sharber et al., 1994; others cited in the

introduction). As a result, one agency, the Alaska Department of Fish and Game, imposed a moratorium on

electrofishing in waters containing large rainbow trout

(Holmes et al., 1990). Similarly, the Montana Department

of Fish, Wildlife, and Parks (1994) issued regulations designed to limit injuries to fish, including restricted use of

PDC over 30 Hz, and federal biologists in Idaho were

discouraged from using any electrofishing techniques

for capture of bull trout (Schill and Beland, 1995).

Many biologists across the continent and abroad

now acknowledge that potential incidence of

electrofishing injuries in otherwise normal-appearing

specimens might be a serious concern, at least for some

environmental conditions, equipment, and species. They

have been asking: what species and size groups are affected, to what degree are they affected, what equipment,

electrical parameters, and techniques are responsible, what

specific mechanisms are involved, and what can be done

to eliminate or minimize the problems?

Most of these questions are not new. Spinal injury

has been associated with AC fields for over half a century

(e.g., Hauck, 1949), but until the late 1980’s, it had been

largely overlooked as a significant problem with at least

some forms of PDC. This perception endured despite only

limited documentation of injuries caused by AC and some

early publications documenting high incidence of injury

with PDC (e.g., Horak and Klein, 1967).

Despite electrofishing’s prominent role in fishery research and management, well-designed investigations to

address many of these questions and to understand the

general reactions of fish in electric fields are relatively

few, often very limited in scope (frequently a by-product

of another investigation), and difficult to compare because of differing objectives, gear, techniques, environmental conditions, species, and terminology. With regard

to terminology, many researchers and authors fail to make

critical distinctions between PDC and continuous,

nonpulsed, direct current (DC), peak and mean output

voltages or field-intensity values, or narcosis and tetany.

Also, many reports of adverse effects are anecdotal or

lack critical data on the circumstances of the observations or experiments. Perhaps as a result of these limitations, inconsistencies, and deficiencies, reported results

sometimes seem so contradictory that they appear to follow the law of physics which states that for every action

(report) there is an equal and opposite reaction (counter

report).

Broader questions also continue to be considered.

Biologists are concerned about potential effects of

electrofishing on the survival, growth, reproduction, and

general well-being of populations and communities. Horak

and Klein (1967), Spencer (1967a), Hudy (1985), and

Schneider (1992) reported that electrofishing injuries often heal and are not necessarily lethal or debilitating to

fish. Although most fish apparently survive

electrofishing-induced spinal injuries, Lamarque (1990)

stated that growth certainly would be impaired. Sharber

Fig. 2. Brands (bruises or dark pigmental discolorations) in rainbow trout (Oncorhynchus mykiss) caused by

electrofishing. (Brands are usually temporary external manifestations of spinal injury, but injured fish often lack brands.

Photograph provided by and reproduced with permission of W.A. Fredenberg, Montana Department of Fish, Wildlife,

and Parks.)
6 INFORMATION AND TECHNOLOGY REPORT--2003-0002

and Carothers (1988, 1990) noted that we do not know

how long fish with electrofishing injuries will survive and

suggested that, at least for large rainbow trout (the subjects of their investigation), such spinal injuries might

bias age, growth, and population studies based on markrecapture techniques. Sharber and Carothers (1988, 1990)

also cautioned that the detrimental impact of such injuries might be very significant for populations of fishes

that are already low or endangered.

In an article abstracted from the 1992 version of this

report (Snyder, 1995), I concluded that in situations where

electrofishing injuries are a significant problem and cannot be adequately reduced (through gear, current, or procedural changes), use of the technique must be

abandoned or severely limited. During the next couple of

years, the message regarding potential harm to fish by

electrofishing was relayed to the public by related articles in various newspapers and fishing and outdoor

magazines (e.g., Holt, 1995; Ritchie, 1995; Cofer, 1996;

Meyer, 1997). In direct response to my article, Schill and

Beland (1995) expressed a grave concern that fishery biologists may be forced by public perception of the problem (Weber, 1997) to unduly give up or restrict use of one

of the profession’s most effective sampling tools. In particular, they observed that scientific discussion had “focused on small pieces of the puzzle” and had “largely

ignored the more important question of population significance.” They explained, by hypothetical example, that

in most cases only very small portions of populations are

sampled and even if incidences of injury and long-term

mortality were very high (e.g., 50% and 25%, respectively),

they would affect no more than one or two percent of the

population as a whole. Furthermore, they continued, annual natural mortality for some species (e.g., stream salmonids in northern states) is so high that the long-term

population effects of even greater electrofishing impacts

could be further discounted. Schill and Beland (1995) also

noted that biologists routinely sample lacustrine fish with

gill nets and accept even 100% mortality because only a

very small segment of the population is sacrificed. In some

situations, captured fish are purposely sacrificed for subsequent analysis. Similar concerns over public perception of the problem were expressed by Wiley (1996) after

Holt (1995) told “the truth about electrofishing.” However, Cofer (1996), in revealing “the shocking truth,” suggested that my article (Snyder, 1995) succeeded in stirring

debate over often-overlooked side effects and that “in

confronting the issue, scientists may have solved half

the problem by recognizing that electrofishing—in its current form—is not always so benign.”

Consistent with Schill and Beland’s (1995) suggestion of insignificant adverse effects by electrofishing on

populations, biologists such as Nehring (1991) and

Schneider (1992) have documented that years of

electrofishing, even with AC (Schneider, 1992), had not

detrimentally affected the specific populations they monitored or managed. However, adverse effects that may be

insignificant for large, widely distributed populations,

might pose a significant additional threat to the survival

or recovery of much smaller, localized populations of rare,

threatened, or endangered species.

If electrofishing injuries occur in notable numbers of

fish but do not significantly affect their population size

(long-term survival, reproduction, recruitment) or health

(growth, condition), perhaps the only real concerns in

such situations are resource quality and public perception thereof. In some fish, spinal injuries result in permanently bent backs (Fig. 3) or related deformities (Fig. 4)

which sometimes do not become obvious until well after

exposure to the electric field. In other fish, spinal injuries

might only be revealed by X-rays or dissection, possibly

on an angler’s dinner table.

The extent of concern about potential electrofishing

injuries in North America has been exemplified by the

formation of an informal working group on electrofishing

injuries within the Western Division of the American Fisheries Society, special sessions on the matter held during

annual meetings of the Western Division in July 1991

(Bozeman, Montana) and 1992 (Fort Collins, Colorado),

and the attempted establishment of an Electrofishing Injury Network through the American Fisheries Society Fisheries Management Section. In Europe, a workshop on the

harmful effects of electrofishing was organized by the

EIFAC Working Group on Electric Fishing and held on 21

and 22 May 1992 in conjunction with the 17th Session of

EIFAC in Lugano, Switzerland. Until the concerns are effectively resolved, the harmful effects of electrofishing

are likely to be the subject of still more special sessions,

workshops, and organizations.

Some state and provincial agencies have reviewed

their concerns about deleterious effects of electrofishing

and established policies, regulations, or guidelines for

use of such techniques. Emphasizing available

information on 15 species of regional interest, Miskimmin

and Paul (1997a,b) and Paul and Miskimmin (1997)

prepared a three-part report similar to this review for

consideration by the Fisheries Management Division of

Alberta Environmental Protection whose 1995 policy to

minimize adverse effects of electrofishing on fish was

being appealed. In the third part of that report, Miskimmin

and Paul (1997b) reviewed and compared existing policies

or guidelines from Canadian and U.S. jurisdictions. They

acknowledged Montana as a leader in establishing a

relatively strict state-wide policy and comprehensive

standards to minimize electrofishing injury to aquatic life

(Montana Department of Fish, Wildlife and Parks, 1994).

Alberta’s 1995 policy was similar to Montana’s. Ontario

and Washington also have official policies or guidelines
 SNYDER 7

intended to minimize injuries (established in 1986 and 1997,

respectively), and Michigan was revising its electrofishing

policy to include a section to the same end. Alaska, Idaho,

Minnesota, New York, and Wyoming have unofficial

policies or guidelines to minimize electrofishing injuries.

With regard to electrofishing in waters inhabited by

threatened or endangered species, Miskimmin and Paul

(1997b) reported that the U.S. Fish and Wildlife Service

allows use of only DC or PDC, prohibits spiked waveforms,

and requires records of pertinent water quality parameters

and electrofisher settings. Both the U.S. Fish and Wildlife

Service and the Canadian Department of Fisheries and

Oceans offer courses on electrofishing that include

consideration of adverse effects and ways to minimize

them (e.g., Kolz et al., 1998). Many provincial or state

(e.g., Colorado) and federal fishery workers are required

or encouraged to take these courses. Some states have

or are developing their own training programs and

manuals (e.g., Wyoming; Meyer and Miller, 1995).

In California, where coho salmon and steelhead (searun rainbow trout) were listed throughout the state in

1996 as threatened or endangered Evolutionarily

Significant Units under the federal Endangered Species

Act, there is serious concern about the legal and ethical

use of electrofishing for population surveys, monitoring,

and scientific investigations (Nielsen, 1998). Nielsen

(1998) reported that in response to these concerns, a

workshop was convened in Ukiah, California, on 10

February 1998 by 48 federal, state, academic, tribal,

industrial (timber), and fisheries-consultant organizations

to discuss electrofishing guidelines and protocols. A draft

of general recommendations from that meeting was still

under review late in 1998, but Nielsen (1998) expressed

concern that the recommendations would be inadequate

to effectively limit use of electrofishing under any set of

circumstances. Noting that the effective size of some

salmon and trout populations or evolutionarily significant

units can be very small (frequently less than 25 breeding

pairs), she advocated requiring use of other, non-invasive,

study methods when the cumulative effects of

electrofishing over time might significantly reduce a

population’s ability to persist or result in loss of unique

components in the genetic diversity of the species. Nielsen

(1998) concluded by suggesting that “the American

Fisheries Society should develop a set of guidelines for

least-invasive sampling methodologies and adopt a policy

on the ethical use of electrofishing. . . .” These guidelines

and criteria could then be used by federal and state

agencies to strictly (and uniformly) regulate potentially

harmful electrofishing activities under their jurisdiction.

Manufacturers of electrofishing gear are obviously

concerned about adverse impacts as well. They have a

vested interest in the technique and have begun

developing and marketing equipment intended to reduce

electrofishing injuries. As examples, see the

advertisements on both sides of the back cover of

Fisheries 16(6), November–December 1991. One is for

Coffelt Manufacturing’s CPS (Complex Pulse System, a

patented pulse train of three square pulses at 240 Hz

repeated 15 times per second), which was specifically

developed to reduce spinal injuries. The other

advertisement is for Smith-Root, Inc.’s P.O.W.

(Programmable Output Waveforms) unit, that allows users

to select from a very wide range of patterns or waveforms,

including pulse trains, some of which are likely to be less

harmful than others (Meyer and Miller, 1995). More

Fig. 3. Bent back in rainbow trout ( Fig. 3 Oncorhynchus mykiss) caused by electrofishing. (Photograph provided by and

reproduced with permission of M.S. Quinton via W.A. Fredenberg, Montana Department of Fish, Wildlife, and Parks.)
8 INFORMATION AND TECHNOLOGY REPORT--2003-0002

recently, Smith-Root, Inc. (1998) offered a special

“sweeping” PDC waveform that progressively decreases

duty cycle from 60 to 10% during the first 10 s each time

the control unit is switched on by reducing either pulse

width or frequency. The manufacturer suggested that this

new waveform will minimize injury by reducing the

percentage of time that electricity is applied as fish are

attracted from cover to the anode.

Even theories regarding the causes and mechanisms

of fish responses in electric fields are being reexamined in

an attempt to identify and explain specific factors

associated with injuries. During the workshop on

electrofishing injury held in July 1991 as part of the annual

meeting of the Western Division of the American Fisheries

Society in Bozeman, Montana, N.G. Sharber introduced

what has since often been referred to as the “Bozeman

paradigm.” His theory is that the observed responses of

fishes in electric fields, including muscular seizures

resulting in spinal and related injuries, represent

essentially the same phases of epilepsy observed in

Fig. 4. Bent backs and Fig. 4 abnormal growth in west slope cutthroat trout (Oncorhynchus clarki lewisi) probably caused

by electrofishing (top and middle photographs) with normal trout for comparison (bottom photograph). (All about 38–

40 cm TL. Top two fish were the only obviously deformed specimens among 93 trout maintained as broodstock in

Kiakho Lake, British Columbia, in June 1991. All fish were originally captured as 1 to 3-year-old juveniles a few years

earlier by stream electrofishing, and that event was considered the most likely cause for the deformities. However, such

deformities are sometimes attributed to other causes. Photographs provided by and reproduced with permission of

G. Oliver, Kootenay Region, British Columbia.)
 SNYDER 9

humans and other animals subjected to electroconvulsive

therapy (Sharber et al., 1994, 1995; Sharber and Black,

1999). As discussed later under “Responses of Fish to

Electric Fields,” he correlates these epileptic phases–

automatism, petit mal, and grand mal–with the more familiar

and well-published descriptions and explanations of

electrofishing responses, particularly those he refers to

as the “Biarritz paradigm” espoused by Blancheteau et al.

(1961); Lamarque (1963, 1967a, 1990); Vibert (1963, 1967b);

and Blancheteau (1967) following their intensive

investigations at the Biarritz Hydrobiological Station in

France.

Another new theory views electrofishing as a powerrelated phenomenon (Kolz and Reynolds, 1989a; Kolz

et al., 1998). Designated as A Power Transfer Theory for

Electrofishing by Kolz (1989a), it explores the relationship between electrical power in water and in fish as a

function of the ratio of conductivity of water to the effective conductivity of fish. This theory, like the Bozeman

paradigm, is discussed later in more detail under “Responses of Fish to Electric Fields.”

Interactions of fish, water, and electricity are a very

dynamic, complex, and poorly understood mix of physics,

physiology, and behavior. Perhaps because there are so

many variables, Reynold’s (1995) quote of W.G. Hartley

seems particularly apropos for the field of electrofishing:

“There are no experts, only those who have not been

found out.” This suggestion is not intended to discredit

or belittle the extremely valuable contributions and

knowledge of many researchers who have spent much of

their lives studying the effects of electric fields on fish or

using and developing electrofishing techniques but rather

to indicate that, despite their efforts, we still have much

to learn and many discrepancies to resolve. Noting that

most recent research focuses on descriptive comparisons

of electrofishing techniques and their injurious effects,

Paul and Miskimmin (1997) recommended that future

research include more carefully designed experiments to

test clearly defined hypotheses. Reynolds (1995)

suggested that researchers network worldwide “to unite

the techniques of electric fishing and its theoretical

foundation.” Although that theoretical foundation is still

far from complete, there is need for a coordinated program

of future electrofishing research. Such a program should

optimize resources at all levels, ensure comparability of

data, and test validity of results through independent

replication of experiments.

Results — Electric Fields in Results — Electric Fields in Water

Electrofishing (sometimes referred to as electric or

electrical fishing, electroshocking, or simply shocking),

as well as the use of electrical barriers, screens, and some

forms of anesthesia, depends on the generation of a sufficiently strong electric field around or between electrodes

in water to elicit the desired responses by targeted fishes.

The size, shape, and nature of that field, as defined by the

distribution of and changes in its electrical intensity, are

determined largely by container or basin configuration

and dimensions; conductivity of the water and bounding

or surrounded media and substrates; position, size, and

shape of the electrodes; and the peak electrical potential

(voltage differential), type of current, and waveform generated between those electrodes. These factors were discussed extensively by Cuinat (1967); Novotny and Priegel

(1971, 1974); Sternin et al. (1972, 1976); Halsband and

Halsband (1975, 1984); Smith (1989); Novotny (1990);

Meyer and Miller (1995); Reynolds (1996); and Kolz et al.

(1998).

Water Conductivity

Water conductivity, water’s capacity to conduct an

electric current, is the most critical environmental factor

in establishing an electrofishing field. The conduction of

electricity (electrical energy) in water is an ionic phenomenon. Conveyance of negative charges via electrons from

negative to positive electrodes (cathode to anode) to

complete an electrical circuit depends on electrolytic reactions at the electrodes and an almost instantaneous

chain of ionic movements and interactions (exchange of

electrons) in the water between and around the electrodes.

Accordingly, conductivity varies directly with the nature

and concentration of ions (charged atoms and molecules,

mostly from dissolved solids and dissociated water). In

nearly pure water, which has a very low conductivity,

ionization of water itself furnishes a substantial portion

of the conducting ions. When electrofishing in very lowconductivity streams with inadequate power supplies,

salt is usually added to water upstream of the sampling

area to artificially increase its conductivity (Lennon and

Parker, 1958; Zalewski and Cowx, 1990).

Conductivity is the reciprocal of resistivity (ohmscm), a term preferred by some authors, especially for very

low-conductivity (high-resistivity) waters. Conductivity

is usually measured with a conductivity meter as mhos or

siemens (S) per cm (usually μmhos/cm or μS/cm; μ = micro

or 10-6). (Mho is ohm spelled backward to indicate the

inverse relation between these units.) Following the

International System of Units, the unit name siemens is

used in the remainder of this report.

Conductivity in natural waters ranges from as low as

5 μS/cm in pure mountain streams (Gatz et al., 1986;

Zalewski and Cowx, 1990) to 53,000 μS/cm in sea water

(Omega Engineering Inc., 1990). The upper limit for potable
10 INFORMATION AND TECHNOLOGY REPORT--2003-0002

water is about 1,500 μS/cm (Wydoski, 1980). Conductivity

in a particular body of water, although generally quite

uniform, can vary considerably from one location to

another depending on substrate composition and

especially the inflow of tributaries or effluents of highly

different conductivities.

Ambient water conductivity also varies with water

temperature. As temperature rises, water viscosity decreases and ionic mobility and solubility of most salts

increase. Rates of change in conductivity depend on ionic

content and vary from about 5.2% per degree C for ultrapure waters to 1.5% per degree C for acids, alkalis, and

concentrated salt solutions (Omega Engineering Inc.,

1990). For natural waters between 10 and 25° C, the coefficient is approximately 2 to 2.3% per degree C. To approximate water conductivities at various temperatures

within this range, Reynolds et al. (1988), Reynolds (1996),

and Kolz et al. (1998) used the equation c2 = c1 /

(1.02(t1-t2 2 )

), and Sternin et al. (1972, 1976) used c = c1 /

(1 + 0.023(t1 - t2)), where c is conductivity and t is temperature. It is important to record whether measured or

reported water conductivity is ambient (actual value for

the temperature at which it was measured) or specific

(value normalized to 25° C); if the latter, it needs to be

recalculated for ambient (actual) temperature.

Electrofishing Currents and Waveforms

There are two principal types of electrical currents,

but interrupted or pulsed variations of one are sufficiently

different and important to be treated effectively as a third

type. Bipolar or alternating current (AC) is characterized

by continually reversing polarity and movement of electrons or ions of like charge (Fig. 5A). Unipolar or direct

current (DC) is characterized by movement of electrons

or ions of like charge in one direction (Figs. 5B–J). However, as used hereafter, DC specifically refers to a continuous unipolar current of constant voltage (smooth or

straight DC, Fig. 5B) or nearly constant voltage (rippled

DC, Fig. 5C). When a unipolar current is periodically interrupted or pulsed, it is specifically referred to as the

third type of current, pulsed DC (PDC; Figs. 5D–I). AC

also can be pulsed, but pulsed AC (e.g., Jesien and Hocutt,

1990) is rarely used for electrofishing.

For AC and PDC, changes in voltage amplitude or

differential (current intensity) over time define the shape

(graphical form as displayed by an oscilloscope) and frequency (Hz—hertz = cycles, pulses, or pulse patterns per

second) of their waveforms. Although other AC waveform shapes and frequencies are possible, AC used for

electrofishing usually consists of a sinusoidal waveform

at a fixed frequency of 50 or 60 Hz (single-phase generator), 180 Hz (three-phase generator), or higher (e.g., 300

or 400 Hz) as a function of generator speed (Novotny and

Priegel, 1974; Novotny, 1990).

Depending on how they are produced, PDC

waveforms used for electrofishing occur in a variety of

shapes, most commonly square (rectangular), half-sine,

quarter-sine, or exponential, and can be delivered over a

wide range of frequencies, usually between 15 and 120

Hz, but at least experimentally from 1 to about 500 Hz.

Pulse-frequency pattern can be either simple (uniform) or

complex, the latter usually consisting of a high primary

frequency interrupted secondarily at a much slower

frequency to produce bursts, packets, or trains of the

higher-frequency pulses (Fig. 5I).

PDC waveforms also are characterized by pulse width

(time current flows during each pulse, usually expressed

in ms, milliseconds) and duty cycle (percentage of time

current actually flows from the beginning of one simple

pulse or complex pulse-pattern to the next). For simple

PDC, duty cycle is a function of pulse frequency and

width. As frequency in a PDC is increased, a constant

pulse width results in a greater duty cycle, whereas a

constant duty cycle results in a proportionately shorter

pulse width.

In modern electrofishing, DC is usually produced by

conditioning power from an AC generator, or a battery

and inverter, with transformers, rectifiers, and filters

(Novotny, 1990; Novotny and Priegel, 1971, 1974). DC

produced by true DC generators is smooth (Fig. 5B),

whereas that produced by filtering rectified current from

an AC generator tends to be at least slightly rippled

(Fig. 5C). However, DC generators are heavier, more expensive, less flexible in voltage control, and less reliable

than AC generators with comparable power ratings. DC

produced by a three-phase AC generator is already relatively smooth and requires much less conditioning than

that produced by a single-phase AC generator.

In most cases, PDC waveforms also are produced

from rectified AC. Rectified sinusoidal AC directly produces half-sine PDCs at either the same or twice the AC

frequency, depending on whether the current is half or

full-wave rectified (Figs. 5D and E). Mechanical or electronic choppers (pulsators) are used to generate quartersine and exponential or capacitor-discharge waveforms

(Figs. 5G and H) from unfiltered rectified AC or square

waveforms (Fig. 5F) from rectified AC that has been first

filtered to produce DC. Square waveforms are perhaps

the easiest to adjust in pulse width and frequency. Some

very flexible electrofishing control units provide AC, DC,

and PDC–the latter with variable pulse frequencies,

widths, and sometimes shape. Some systems allow or

incorporate secondary switching or interruption of PDC

to produce complex pulse frequencies (e.g., University of

Wisconsin Engineering and Technology Center’s

Quadrapulse, Smith-Root’s P.O.W., and Coffelt’s CPS).
 SNYDER 11

Fig. 5. Selected waveforms Fig. 5 used in electrofishing.
12 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Such pulse-train waveforms were suggested by Haskell

et al. (1954) over 45 years ago. PDCs are often favored for

electrofishing because they require much less-powerful

generators or batteries than DC, and often AC, to create

electric fields of comparable size and effectiveness.

Through various manipulations of the current, DC

and PDC have even been hybridized to produce a PDC on

top of DC (Vincent, 1971; Fredenberg, 1992; Fig. 5J). In

such currents, the pulses drop only to a preset minimum

voltage level when switched off rather than to zero volts.

Strongly rippled DC (weakly filtered, rectified AC) could

be considered a hybrid current.

The various PDC waveforms generated by

electrofishing control boxes are sometimes characterized

by anomalies in the expected shape. For example,

Fredenberg (1992) reported spikes at the leading or trailing ends of square-waveform pulses; Van Zee et al. (1996)

documented under test conditions the presence of a trailing voltage spike 50 to 60% higher than the rest of a

square-waveform pulse followed immediately by a small

exponential pulse of reverse polarity (magnitude 20% of

unspiked pulse voltage); and Sharber and Carothers (1988)

described small, rounded, secondary pulses immediately

following pulses in a 60-Hz, exponential waveform. In the

latter example, Sharber and Carothers (1988) suggested

that the small secondary pulse was of sufficient voltage

near the anode to produce essentially a 120-Hz, mixed

waveform that enhanced the immobilization of fish.

Jesien and Hocutt (1990) noted that nominally square

PDC waveforms (Fig. 5F) generated by their equipment

changed shape as water conductivity increased. At conductivities of about 100 μS/cm, the trailing edge was not

perpendicular, and the voltage level was not constant

across the top of the pulse. An exponential-like voltage

spike became evident at 1,000 μS/cm and was especially

prominent at 10,000 μS/cm. In contrast, they found that

characteristics of their pulsed AC waveforms remained

constant with changes in water conductivity. Kolz (personal communication) suggested that they may have used

a faulty power source for their square-wave PDC.

Because output waveforms are not always as expected based on control box settings, it is important to

periodically calibrate, verify, and document waveform in

the output circuit with an oscilloscope. For example, an

oscilloscope tracing illustrated by Van Zee et al. (1996)

for square-wave PDC generated with control-box settings

for 80 Hz and 50% duty cycle revealed an actual frequency

of 73 Hz and duty cycle of 64%, as well as the trailing

spike and negative secondary pulse described above.

Review of the published literature and personal

communications revealed that authors and biologists

frequently fail to note the type of current and waveform

used in electrofishing. Even when noted, some

descriptions of the current are incomplete, misleading, or

erroneous. PDC is often simply referred to as DC, reflecting

its unipolar but not its pulsed nature. Also, referring to its

typical origin via an AC generator, PDCs are sometimes

incompletely called “rectified AC,” which more specifically

refers to either of the two half-sine PDC waveforms (Figs.

5D and E) or, when filtered or originating from 3-phase

AC, rippled DC (Fig. 5C). Even the term “pulsed AC” has

been improperly used for PDC. For example, Hill and Willis

(1994) used a current which they and an early manual for

the Coffelt VVP-15 electrofishing control unit referred to

as pulsed AC. Hill and Willis (1994) described it as the

positive half of a sinusoidal AC waveform, and the manual

illustrated it as quarter-sine PDC (Fig. 5G) but mislabeled

it as pulsed AC (Van Zee et al., 1996; the error has been

corrected in more-recent versions of the Coffelt manual).

Furthermore, an oscilloscope tracing of this waveform by

Van Zee et al. (1996) closely approximated a square-wave

PDC, possibly a slightly compressed quarter-sine

waveform with the trailing margin squared off near the

top.

Field Intensity

The responses of fish to electric fields in water are

dependent, at least in part, on the field’s strength or intensity. Field intensity can be described by any of three

interrelated quantities: voltage gradient, current density,

or power density. The relations between these descriptors of field intensity and water conductivity are illustrated in Figs. 6, 7, and 8.

Voltage gradient (E) is the average voltage differential per unit distance along lines of current or flux between two isopotential surfaces and is usually expressed

as volts per centimeter, V/cm. Voltage is the amount of

potential energy stored per unit of electrical charge, expressed as volts (V, joules/coulomb). Lines of flux (or

current) represent the net directions or paths of current

in an electric field around and between electrodes of opposite polarity. An isopotential surface lies perpendicular to the lines of flux and is defined by a set of points

having the same voltage differential from the surface of

the electrode. If the water is of uniform conductivity and

unbounded for a sufficient distance in all directions (an

unlikely condition), the electrode is spherical, and other

electrodes are sufficiently distant, at least the isopotential

surfaces near the electrode can be visualized as shells, all

points of which are the same distance from the surface of

the electrode.

Voltage gradient can be physically measured in the

water or approximated by calculation based on output

voltage, the surface area, size, and shape of the electrodes, the distance between them, and proximity of

bounding or surrounded surfaces or media of different

conductivity. For practical purposes, the distribution of
 SNYDER 13

voltage gradient near and between electrodes is independent of (unaffected by) water conductivity if that water

conductivity remains uniform (unstratified) in proximity

to and between the electrodes and other parameters (e.g.,

basin, electrodes, voltage differential between electrodes)

are identical. Under such conditions, a map of voltage

gradient would be the same whether water conductivity

was 10 or 1,500 μS/cm. However, this would not be true if

water conductivity was stratified as in an estuary or at

and just downstream of a tributary, spring, or industrial

outflow of substantially different conductivity.

Current density (J) is usually expressed as microamperes or milliamperes per square centimeter, μA/cm2

 or

mA/cm2, respectively (μ = 10-6, m = 10-3), and described

as the amount of current passing through a unit area of

isopotential surface (perpendicular to the lines of flux).

Current is the quantity of electrical charge flowing per

unit time, usually expressed as amperes (A, coulombs/

sec). Since instruments have not yet been developed for

direct measurement of current density, it must be calculated (J = cE).

Power density (D) is the amount of power dissipated

per unit volume between two isopotential surfaces. Power,

the mathematical product of voltage and current, is the

amount of energy expended per unit time, usually expressed as watts (W, joules/sec). Similarly, power density

is the mathematical product of voltage gradient and current density, and it is usually expressed as microwatts per

cubic centimeter, μW/cm3. Because it is a function of current density, power density is also dependent on water

conductivity. Like current density, instruments have not

yet been developed for direct measurement of power density, and it too must be calculated (D = JE = cE2 = J2 / c).

Although reintroduced to electrofishing literature a decade ago by Kolz (1989a), the term “power density” was

perhaps first introduced and used in North American literature by Monan and Engstrom (1963). Power density,

or the volumetric expression of power it represents, was

also used or discussed by Adams et al. (1972) and Sternin

et al. (1972, 1976).

Kolz (1989a) and Kolz and Reynolds (1989b, 1990a)

used a unique 4-way logarithmic graph of water conductivity, voltage gradient, current density, and power density (Fig. 6) to help explain their theory of power-density

transfer (discussed below) and for overlaying graphs of

in-water field-intensity thresholds for observed responses of fish to electric fields. Any point on the graph

simultaneously represents the corresponding values for

each quantity, and knowing any two quantities (e.g., conductivity and voltage gradient) provides a quick alternative to calculation for approximating the remaining two

quantities. Many interesting relations between these factors are revealed by studying the graph. For example,

when voltage gradient is held constant, both current density and power density increase in direct proportion to

water conductivity. At any point on the graph for which

voltage gradient is 1 V/cm, the numeric values for both

current density and power density are equal to water conductivity.

The relation between voltage gradient and current

density relative to water conductivity at a constant power

density of 100 μW/cm3

 can be visually explored in Fig. 7.

The upper and middle graphs are essentially the same

except that the upper graph uses logarithmic scales for

both axes, and the middle graph uses an arithmetic scale

for the Y-axis. The range of conductivities of particular

concern in fresh waters, about 10 to 1,500 μS/cm, is

bounded by dotted vertical lines in both of these graphs

and represented exclusively in the bottom graph for which

all axes are arithmetic with separate Y-axis scales for

current density and voltage gradient. Because of the

inverse relation between current density and voltage

gradient relative to conductivity (c = J / E), the curve for

Fig. 6. Four-way logarithmic graph of relations among

measures of electrical-field intensity (power density,

current density, and voltage gradient) relative to water

conductivity. Reproduced with permission from Fig. 7 in

Kolz, 1989a; axis labels modified.)
14 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Fig. 7. Changes in Fig. 7 voltage gradient and current density relative to water conductivity at a constant power density of

100 FW/cm3. (Top and middle graphs are the same except the Y-axis is logarithmic in the top graph and arithmetic in the

middle graph. Bottom graph is limited to the range of conductivities typical of fresh waters and bounded by dotted

vertical lines in the upper graphs; both axes are arithmetic with different Y-axis scales for voltage gradient and current

density.)
 SNYDER 15

voltage gradient in the middle graph of Fig. 7 becomes

asymptotic with the Y-axis as conductivity approaches

zero and asymptotic with the X-axis as conductivity

approaches infinity, whereas the situation is reversed for

current density. As a result, the curve for voltage gradient

at a fixed power density is relatively flat over all but the

lower end of the range for conductivity in fresh waters

(bottom graph of Fig. 7) and practically horizontal for

more saline waters. The relative stability of voltage

gradient in medium- to high-conductivity fresh waters

has important implications with respect to field-intensity

response thresholds and standardization of electrofishing

fields (discussed later in this review).

Changes in current density and power density relative to voltage gradient at a constant water conductivity

of 500 μS/cm are illustrated in Fig. 8 (the lower graph is an

expansion of the lower left corner of the upper graph).

Note that the values for current density, power density,

and water conductivity are equal when voltage gradient

is 1 V/cm (as in Fig. 6), and, as predicted by their definitions, current density increases linearly and power density geometrically with voltage gradient. For values of

voltage gradient less than 1 V/cm, power density is less

than current density.

For PDC and AC, in-water measures or calculations

of peak field intensity (maximum voltage gradient, current density, or power density through one or more waveform cycles) are substantially greater and probably more

biologically significant (Kolz and Reynolds, 1989b) than

corresponding values of mean PDC or rms AC field intensity. For sinusoidal AC (Fig. 5A), peak voltage gradient

and output voltage are approximately 41% greater than

corresponding rms values (Vp= 1.41 x Vrms). Root-meansquare values are necessary for AC because mean voltage would be zero. Concern that positive peak to negative

peak voltage differential in AC might be even more biologically significant than peak voltage differential from

zero or base level is unwarranted; the negative portion of

the waveform represents a reversal in current direction

rather than negative voltage per se (however, fish are

polarity sensitive and accordingly some responses differ

when subjected to alternating or unidirectional currents

of comparable peak voltage). For square PDC waveforms

(Figs. 5F and I), mean voltage varies directly with duty

cycle (percentage of on time); for example, with a 25%

duty cycle, peak voltage is four times greater than mean

voltage. For other PDC waveforms (Figs. 5D, E, G, and H),

mean voltage varies according to their shape as well as

duty cycle. For smooth DC, peak and mean values for

field intensity or electrical output are identical.

To facilitate comparisons, researchers and authors

must specify whether measures of field intensity or output

(voltage, amperage, power) for PDC or AC are peak or

mean (rms in AC) values. Meters on most electrofishing

control boxes register mean output values for PDC or rms

output values for AC (e.g., volt and ammeter on Coffelt’s

VVP-15 and ammeter on Smith-Root’s GPP 5.0), whereas

meters on very few units register peak output (e.g.,

ammeter on Coffelt’s Mark XXII which generates CPS).

Also, biologists should not rely on the accuracy of control

box settings and meters without periodic calibration. Van

Zee et al. (1996) revealed that voltmeters and ammeters

included on some electrofishing control units (e.g.,

Coffelt’s VVP-15) are meant to serve as references for

relative or consistent settings rather than provide accurate

measures of output. For example, using a boatelectrofishing system with a control box adjusted for an

output of 230 V and 2 A for each of two currents, they

reported oscilloscope measures of peak output to be 280

V and 1.7 A for a quarter-sine-wave PDC and 250 V and

1.5 A for square-wave PDC. The latter current also was

set for 80 Hz with a 50% duty cycle, but the oscilloscope

Fig. 8. Changes in current density and power density

relative to voltage gradient in water with a conductivity

of 500 FS/cm. (For other conductivities, adjust the values

along the vertical axis in direct proportion to the change

in conductivity—e.g., for half the conductivity, 250 FS/

cm, halve the values along the Y-axis.)
16 INFORMATION AND TECHNOLOGY REPORT--2003-0002

documented a pulse frequency of 73 Hz and a 64% duty

cycle. In addition to field intensity and output, biologists

should document water conductivity and temperature and

adequately describe or verify the waveform (shape,

frequency, pulse width, duty cycle), electrodes (position,

size, and shape), and operating procedure.

Voltage gradients are best measured in water with an

appropriate voltmeter or oscilloscope connected to insulated wires, the tips of which are exposed and set a fixed

distance apart (Kolz, 1993; Kolz and Reynolds, 1990b;

Kolz et al., 1998). The maximum voltage differential per

unit distance measured with this probe at any particular

location is the voltage gradient and will be obtained when

the exposed tips are oriented along the field’s lines of flux

(the principal direction of current flow in three dimensions around and between electrodes). When the probe

tips are rotated horizontally and vertically precisely perpendicular to the lines of flux (along an isopotential surface), there will be no voltage differential, and the

voltmeter or oscilloscope will register zero volts. Voltage

gradients can also be approximated as the difference between voltages measured from the electrode to two sufficiently close points in the water (Kolz, 1993; Kolz and

Reynolds, 1990b; Kolz et al., 1998). Like voltage-gradient

probes, fish are subject to the greatest voltage differential when they are oriented along the lines of flux. This is

often referred to as “head-to-tail voltage.” Fish are subject to the least voltage differential when oriented perpendicular to flux lines.

Voltmeters specifically designed to measure peak

voltage (e.g., peak-voltage detectors; Jesien and Hocutt,

1990) or oscilloscopes should be used for accurate

measurements of peak voltage gradient at specific

reference points in PDC (or pulsed AC) fields. However,

the presence of voltage spikes in a PDC waveform

(discussed earlier) can affect readings in some peakvoltage detectors. Oscilloscopes, although more

expensive, allow the user to observe voltage spikes and

differentiate such from normal peak voltages or voltage

gradients by ignoring any spikes, as well as to monitor

other waveform characteristics (e.g., shape, pulse

frequency, pulse width, and duty cycle). A typical

voltmeter (or multimeter) can be used to measure the

constant voltages or voltage gradients at specific

reference points in smooth DC fields (peak = mean) and

rms voltages or voltage gradients in AC fields. But

according to Jesien and Hocutt (1990) and Fredenberg

(personal communication), such meters cannot accurately

measure either peak or mean voltages in PDC or pulsed

AC. For the latter, either an oscilloscope or special

instrumentation (e.g., peak-voltage detector) is required

(Kolz, 1993). However, if a smooth DC or AC field can be

temporarily substituted using the same system and peak

output, the DC voltage or voltage gradients measured or

peak values calculated from AC rms measurements made

with a standard voltmeter in that field should be identical

to peak voltages or voltage gradients in the corresponding

PDC or pulsed AC field.

Heterogeneous and Homogeneous Fields

Because the basins occupied by rivers, streams, lakes,

reservoirs, and most other waters are irregular in shape,

and their cross-sectional areas are much larger than the

electrodes, electrofishing fields generated therein are

heterogeneous. In such fields, lines of flux (current) can

be visualized as radiating from and spreading widely

around and between the electrodes (Fig. 9). Field intensity is greatest next to the electrodes and decreases to

barely perceptible levels as distance from the electrodes

increases, even in the area directly between anode and

cathode when they are sufficiently separated. The actual

field intensity encountered by a fish in a heterogeneous

field depends on the fish’s location in the field.

Homogeneous fields are typically restricted to laboratory settings in raceways, troughs, or tanks with a constant cross-sectional profile and electrodes approximating

that profile at each end of the desired field. In homogeneous fields, the current flows parallel to the sides of the

container directly from one electrode to the other. Except

adjacent to bounding surfaces or substrates, this arrangement provides a constant voltage gradient, current density, and power density regardless of location between

the electrodes.

Controlled experiments in homogeneous fields allow

relatively precise control of field intensity and eliminate

many of the electric-field variables that are encountered

in natural waters. This greatly simplifies experimental conditions and facilitates determination of cause and effect,

but results may be difficult to extrapolate to normal

electrofishing operations.

Bounding or Surrounded Media

and Substrates

Depending on their porosity and conductivity, the

bounding media or substrates of a body of water can

affect the distribution of electricity in that body of water

(Sharber et al., 1995). The conductivity of bottom

substrates can vary considerably with location, even in

the same water body. Haskell (1954) and Zalewski and

Cowx (1990) reported that substrates of fine particles and

organic debris are more conductive than those of coarse

gravel and rubble. Because of substrate and interstitial

water conductivity, electric fields can extend well into the

bottom substrate and even onshore. Riddle (1984)

suggested that a person standing barefoot on a bank
 SNYDER 17

Fig. 9. Hypothetical Fig. 9 two-dimensional diagrams of heterogeneous electric fields around and between electrodes. (When

electrodes are sufficiently far apart, the field around each is essentially isolated as indicated in the lower diagram.

Voltage is relative to that of the water where voltage gradient is minimal. Constant-voltage (isopotential) lines are

perpendicular to the radiating lines of current. Contrary to the diagrams, current flows from negative to positive

electrodes. Reproduced with permission from Fig. 3.10 in Novotny, 1990.)
18 INFORMATION AND TECHNOLOGY REPORT--2003-0002

could be shocked. Some of the first electrofishing systems

in the United States were shore-based and used AC with

one electrode or electrode array implanted in the ground

along shore (Haskell, 1940, 1950, 1954); this practice is

still used today, including DC and PDC systems with buried

cathodes. Smith (1991) described an experimental electric

shark barrier that also incorporated electrodes implanted

onshore rather than directly in the water.

Interactions between water and a bounding or surrounded medium or substrate of different conductivity

apparently cause water conductivity near the interface to

progressively increase or decrease toward that of the

adjacent surface with corresponding changes in current

density and voltage gradient (c = J / E). If the adjacent

medium or substrate is more conductive than the water

some distance away, the current in the water near its surface progressively concentrates (current density increases) as voltage gradient correspondingly declines

(perhaps hypothetically, such that power density (D = JE)

remains the same at each point as it would have been in

the absence of the adjacent medium or substrate). Conversely, current density is reduced and voltage gradient

intensified immediately along or around less-conductive

media, including air at the water surface (Zalewski and

Cowx, 1990). As documented by Haskell (1954) and noted

by many others since, fish themselves distort the field in

their immediate vicinity if they are more or less conductive than the water (Fig. 10).

Except when used as the cathode, Riddle (1984) recommended that metal boats not be used for electrofishing.

He suggested that if a conductive vessel is positioned

between the electrodes, it would interfere with the field

(concentrate the current and thereby alter field size and

shape) and might adversely affect electrofishing efficiency. According to Sharber (personal communication),

when a metal boat is situated in an electric field and not

used as the cathode, it has an intermediate electric charge,

negative with respect to the anode and positive with respect to cathode. This concern seems to have been overlooked in much of the literature on boat electrofishing,

although some, especially earlier, workers strongly discouraged use of metal boats for reasons of safety

(Goodchild, 1990, 1991).

Electrodes — Position, Size, Shape,

and Other Matters

According to Novotny (1990), the electrodes are the

most crucial part of an electrofishing system. Their spacing, size, surface area, and shape, along with water conductivity, determine the electrical resistance of the system

and, for a specified voltage output, the distribution of

field intensity that determines the unconfined size and

shape of the effective field. Electrode systems that are

inappropriate for the power supply and waters to be

sampled can result in poor electrofishing efficiency or

unnecessary harm to fish. Novotny and Priegel (1974)

listed the following desirable characteristics for an effective electrode system:

• establishment of the largest region of effective

electric current distribution in the water to be

sampled,

• avoidance of local regions of unnecessarily large

current densities, which waste power and are potentially harmful to fish,

• adjustable to meet changes in water conductivity,

• ability to negotiate weeds and obstructions,

• ease of assembly and disassembly, and

• avoidance of unnecessary disturbance to water

to permit clear visual observation of fish.

When electrodes are positioned sufficiently far apart

(more than several radii in the case of spherical electrodes–

Novotny, 1990; 10 to 20 radii for rings–Smith, 1989), the

field around each electrode is effectively independent

and has no significant interactive effect on electrode or

system resistance. The water outside well-separated anodic and cathodic fields is considered to be at “ambient

potential” because its electrical potential does not vary

significantly (its voltage gradient is nil–Cuinat, 1967). Fish

that remain in water of ambient potential, even between

the electrodes, are theoretically unaffected by the

electrofishing operation. The level of ambient potential

relative to the electrodes depends on voltage output, total resistance (sum of anodic and cathodic resistances),

and the ratio of anodic to cathodic resistances (Kolz, 1993).

Novotny (1990) emphasized that “the most common

electrode problem is that the electrodes are simply too

small . . . .”. At the same output voltage, larger electrodes

have less electrical resistance in water and radiate larger

electric fields but with lower maximum field intensity immediately around them. Larger electrodes thereby reduce

the zone of tetany and extend the effective field for taxis

(DC and PDC fields) and narcosis (see definitions and

discussion later under “Major Intensity-Dependent Responses”). Increasing the number of anodes or cathodes

in a system has a cumulative effect similar to increasing

the size of an individual electrode. Maximum size or number of anodes or cathodes is dictated largely by practical

considerations (e.g., maneuverability, transportability,

interference with netting) and, especially in high conductivity waters, by generator capacity. When water conductivity is high, the size of the electrodes must sometimes

be reduced to prevent generator overload.

To minimize cathodic effects on fish when using DC

or PDC, cathodes should be as large as practical relative

to anodes. This will also desirably maximize anodic field

intensity and reduce the overall electrical resistance of
 SNYDER 19

Fig. 10 Fig. 10. Distortion of homogenous electric fields around fish in water that is less conductive (top) and more conductive

(bottom) than the fish. (Horizontal lines are current (flux) lines and vertical lines are constant-voltage (isopotential)

lines. Symbol = conductivity (c), E = voltage gradient, and δ = current density (J). Reproduced from Figs. 77 and 78

in Sternin et al., 1976.)

ã
20 INFORMATION AND TECHNOLOGY REPORT--2003-0002

the system. When cathodes are larger than anodes, most

of the total potential between electrodes is associated

with the anode, and the voltage differential between anode and ambient potential is proportionately greater than

between ambient potential and cathode. If cathodes are

very much larger than anodes, the very low voltage differential between the cathode and soil and water in the

vicinity may reduce the risk of severe shock or electrocution to people or animals that inadvertently approach or

touch the cathode (Smith, 1989). Because cathodic resistance for well-separated electrodes is halved each time

the surface area of the cathode is doubled, Smith (1989)

suggested that 10 m2

 may be a practical limit to the size of

the cathode. However, according to Temple (personal

communication), A. Kolz maintains that in shore-based

systems with buried cathodes, the earth itself becomes a

very large cathode. With appropriate equipment and wiring on metal boats, cathode size is often maximized for

DC or PDC electrofishing systems by using the boats

themselves as cathodes (Kolz, 1993; Reynolds, 1996); on

fiberglass vessels, cathode size is sometimes maximized

by mounting large metal plates on their bottoms (Vibert,

1967b).

Kolz (1993) discussed the importance of and procedures for determining electrode and system resistance as

well as making in-water measurements for mapping field

intensity around and between various types of electrodes. Such data are necessary for comparing electrodes

of various shapes, sizes, and designs, optimizing

electrofishing efficiency, minimizing hazardous field intensities, and standardizing electrofishing fields. Kolz

(1993) emphasized that electrofishing fields generated

through different electrode systems cannot be standardized only by output voltage, current, or power. The distribution of field intensity around and between electrodes

depends in large part on the specific size, shape, and

configuration of those electrodes and must also be known

or measured.

To this end, Kolz (1993) determined and compared

the electrical resistance and voltage-gradient and voltage-differential profiles for 18 commonly used electrodes,

including spheres, cylinders, horizontal loops, Wisconsin-ring dropper arrays, and vertical plates of various

sizes. Measurements were taken in a concrete canal (water 1.4 m deep, 3 m wide) with matching electrodes 4 m

apart (except for cylindrical electrodes which were 2.7 m

apart), water conductivities of 111 to 190 μS/cm, and an

electrical output of 100 Vrms. Electrical resistance data were

normalized for a water conductivity of 100 μS/cm but can

be adjusted by calculation for different water conductivities (electrode resistance is inversely proportional). Similarly, voltage-gradient and voltage-differential profile data

for each electrode can be calculated for different applied

voltages (directly proportional; for unmatched electrodes

data must also be adjusted by the inverse ratio of their

electrical resistances).

Spherical electrodes are considered electrically superior to other shapes (e.g., cables or narrow cylinders)

and allow more accurate calculation of electrode resistance and voltage gradient maps. Electric fields generated immediately around well-submerged spheres are

uniform and without the hot spots (localized regions of

higher intensity) produced near the corners and edges of

many other electrode shapes. For example, according to

Sharber et al. (1995), charge is not distributed uniformly

over long thin electrodes but concentrated at their distal

ends. Except near their surfaces where tetanizing voltage

gradients may exist, Novotny and Priegel (1974) and

Novotny (1990) suggested that circular and ringlike electrodes, including dropper arrays, produce electric fields

similar to those of spheres. However, Kolz (1993) documented that Wisconsin-ring dropper arrays project their

fields somewhat further in a horizontal direction than similar-size spheres. Spheres, on the other hand, project their

fields more evenly in all directions, including vertically

towards the bottom and, perhaps less advantageously,

upward to the water surface.

In addition to transfer of electrons, the process of

electrolysis at the electrodes results in generation of gases

and, more importantly, loss of metal ions from the anode

to the water and deposition of metal ions from the water

onto the cathode, usually as metallic oxides (Sharber,

personal communication). Periodically, anodes may need

to be replaced and cathodes cleaned (scraped or sanded)

to recover lost surface area and performance (oxide coatings reduce electrode resistance). When electrodes are

of the same size and type, some biologists periodically

alternate their use as anodes or cathodes to reverse the

buildup of metallic oxides (Sharber and Carothers, 1988),

but the effectiveness of this procedure has not been reported.

Riddle (1984) suggested that it was not wise to buy

aluminum punts (boats) second-hand from electrofishermen because the gauge of the metal might be

substantially reduced. According to Sharber (personal

communication), this is not a problem when a metal boat

is used as the cathode. But when a metal boat is situated

in an electric field and not used as an electrode, it has an

intermediate electric charge, negative with respect to the

anode and positive with respect to cathode. In this case,

electrolytic reactions result in both formation of

nonconductive metallic compounds on the boat’s surface

and loss of structural metal. Over time, the latter reaction

can reduce the structural integrity of the boat. When a

boat is used as a cathode, no metal is lost, but the

nonconductive metallic compounds that form on the

boat’s surface can increase its electrical resistance. This

coating can be scraped or sanded away periodically, but
 SNYDER 21

in doing so, some structural metal may be inadvertently

lost.

Results — Responses of Fish

to Electric Fields to Electric Fields

Movement toward an electrode in response to an

electric field is not unique to aquatic vertebrates, or even

organisms with nervous systems. Even individual cells

respond. Halsband (1967) noted that common carp and

trout erythrocytes placed in a powerful electric field (1,000

times more current density than used in electrofishing)

first moved towards the anode (cataphoresis), then

changed shape from oval to round, and finally

disintegrated.

As in water, ionic conductivity is responsible for electric currents in blood and interstitial fluids of living tissues (Sternin et al., 1972, 1976), but transmission of

electricity to and deep within the body of a fish is complex. Tissues and membranes have different and sometimes variable electrical qualities (e.g., conductivity,

capacitance, and impedance–Sternin et al., 1972, 1976;

Sharber et al., 1995). Skin, for example, is essentially resistive and dissipates much of the electrical energy as

heat (perhaps some observed responses in fish are actually responses to heat). Some of the electrical energy that

is transmitted across skin and other tissue membranes is

reportedly transferred by capacitance. Presumably, with

electrolytes on both sides of a membrane (e.g., water on

one side of skin and interstitial fluids and blood in capillaries on the other side), the membrane functions somewhat as a dielectric in an electrical condenser and allows

a momentary current across the membrane only as applied voltage is switched on, off, or suddenly increased

or decreased. No current is transmitted by capacitance in

PDC when the applied voltage is constant; therefore, the

amount of charge transmitted by capacitance in PDC fields

varies directly with frequency. But fish also exhibit very

distinct, field-intensity dependent, responses under continuous DC. Direct electrical stimulation of afferent nerves

probably also occurs through various external sensory

structures in the skin, including the lateral-line canal system. Although not mentioned in literature reviewed for

this report, the gills, which are the primary sites for ionic

exchange, might also have a significant role in the transmission of electrical current to the blood and from there

via the circulatory system to nerves and other tissues

throughout the body.

Neurological responses to stimuli, nerve impulse

transmission, and muscular actions in animals are

electrochemical phenomena. In accord with the “all or

none” principle of individual nerve response, each level

of reaction requires a stimulus of a specific minimum

intensity that must arrive quickly and be maintained for a

minimum time. However, if a series of stimuli below the

threshold level for nerve response are received over a

sufficiently short period, their effect may be cumulative

and still cause the nerve to respond according to the

principle of temporal summation (Best and Taylor, 1943,

as quoted by Haskell et al., 1954; Wydoski, 1980; Emery,

1984).

Biarritz and Bozeman Paradigms

In what has become known as the Biarritz paradigm,

Blancheteau et al. (1961), Lamarque (1963, 1967a, 1990),

Vibert (1963, 1967b), and Blancheteau (1967) developed a

set of principles for nerve and muscle excitation in DC

fields to explain the various responses of fishes observed

in their experiments at the Biarritz Hydrobiological Station in France (Table 1). Lamarque (1967a) summarized

these principles as follows:

“1. At a certain threshold, direct current initiates and

maintains nerve or muscle excitation by the

“autorhythm of excitation” (see Fessard 1936 and

Monnier et al. 1940).

2. Short nerves in an electric field are excited at a

higher value of current than long nerves (Laugier,

1921).

3. The greater the angle between a neurone in an

electric field and the direction of current flow, the

greater the current necessary to excite it (Fick,

cited by Charbonnel-Salle 1881).

4. A neurone can only transmit its excitation to another neurone in the soma-axon direction.

5. The stimulus being produced by catelectrotonus

at the cathode, an excitation can be conveyed to

the next structure only if the cathode is on the

soma side with regard to the axonic endings

(normodromic stimuli).

6. Inversely, if the anode is on the soma side with

regard to the axonic endings, the soma anelectrotonus can block a normodromic stimulus from

another structure, and thus create an inhibition.

7. Nerve or muscle structures of a fish in an electric

field can be excited or inhibited in situ since the

fish body has itself become an electric field. According to the potential values, certain structures

will be excited on account of their length (2), or

their position (3); others will be inhibited (6), and

yet others preserved from the action of current.”

Lamarque (1967a) also noted that nerve interaction

with PDC is further complicated by “. . . very complex

physiological processes, such as chronaxies, spatial and

temporal summations, synaptic delays, excitatory post-
22 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Table 1. Reactions of fish in homogeneous fields of direct current. Star (’) indicates that the reaction was observed, em

dash ()) that it was not observed, and blank that it was not studied. Modified from Table 1 in Lamarque (1967a).
 SNYDER 23

synaptic potential . . ., polarity inversions due to openings

of the circuit, etc.” However, some of the concepts

established by the Biarritz researchers are difficult to

understand and have been questioned by other

researchers (Hume, 1984; Sharber, personal

communication).

In 1991 during an electrofishing-injury workshop in

Bozeman, Montana, N.G. Sharber introduced another

explanation for the responses of fish to an electric field.

According to this theory, often referred to as the Bozeman

paradigm, the observed responses of fish, including

muscular seizures resulting in spinal and related injuries,

are comparable to responses of humans and other animals

subjected to electroconvulsive therapy and can be

similarly explained as phases of epilepsy, specifically

automatism, petit mal, and grand mal (Sharber and Black,

1999; Sharber et al., 1994, 1995; Sharber, personal

communication).

How the underlying concepts of the Biarritz paradigm fit in the context of the Bozeman paradigm, and vice

versa, has yet to be well explored. Because the phases of

epilepsy are understood to be disorders of cerebral function, Sharber et al. (1994), and Sharber and Black (1999)

suggested that the electric-field responses observed in

fish are due to various levels of overstimulation of the

central nervous system, either directly to the brain or

short-circuited through the spinal cord. However, other

researchers, including Haskell et al. (1954), Vibert (1963,

1967b), Lamarque (1967a, 1990), Edwards and Higgins

(1973), and Wydoski (1980), concluded that the various

responses elicited in fish by an electric field are the result

of direct stimulation of not only the central nervous system, which controls voluntary reactions, but also the autonomic nervous system, which controls involuntary

reactions, and muscles themselves. Haskell et al. (1954)

and Lamarque (1967a, 1990) demonstrated that tetany in

DC and muscular bends of the body toward the anode

upon circuit closure in DC, or repeatedly in PDC, can be

induced by direct overstimulation of efferent nerves or

nerve endings associated with muscles. In those experiments, either efferent nerves were severed from the spinal cord, or the spinal cord was destroyed or removed

prior to electric-field exposure. Muscular bends of the

body often resulted in what was or would have been

movement (taxis) towards the anode.

There is surely some truth to both paradigms, and

perhaps a better understanding of the responses of fish

to electric fields will require an integration of the two

(possibly along with aspects of the power-transfer theory

discussed below). The major intensity-dependent responses of fish described by both paradigms (reactive

detection, undirected or inhibited swimming and taxis,

and narcosis and tetany) are illustrated in Fig. 11 and

discussed later in more detail. The electro-physiological

mechanisms involved in epilepsy and electroconvulsive

therapy might or might not be much better understood

than those for the responses of fish to electric fields. In

either case, a collaboration of biologists, including experts in neuro-physiology, should be fruitful for both disciplines. Certainly, the observed results of the Biarritz

experiments and others mentioned above are valid under

the conditions in which they were performed, but a much

more complete and definitive understanding of the electrophysiological mechanisms involved is needed to better

determine what electrical-field parameters and conditions

will optimize desired electrofishing responses and minimize injury and other adverse effects.

Theory of Power Transfer

from Water to Fish

Kolz and Reynolds (1989a) suggested that

electrofishing should be viewed as a power-related phenomenon. More specifically, they hypothesized that the

responses of fish to electric fields are directly related to

the magnitude of power density (product of voltage gradient and current density) in the fish and that the in-fish

power-density threshold for each response is constant

(fixed) and independent of water conductivity. According to their theory of power transfer (Kolz, 1989a), when

water conductivity (cw) equals effective fish conductivity (cf

w

), 100% of the power density in the water is transferred to the fish (applied power density in the water, D ,

equals power density in the fish, Df ). But, as water conductivity either increases or decreases relative to the effective conductivity of the fish (conductivity mismatch),

power transfer to the fish is progressively less efficient.

To establish or maintain a desired level of power density

in the fish under conditions of conductivity mismatch

(perhaps just above the threshold for a specific response),

power density in the water must be progressively increased beyond that of the match condition in accord

with the relation Dw / Df = (1 + q)

2 / (4q), where q = cw / cf

(the conductivity mismatch ratio; Kolz, 1989a). Subscript

f represents effective in-fish values which match corresponding in-water values, subscript w, at the minimum of

the curve represented by this equation.

When plotted on the log-log graph of power-density

ratio versus water-conductivity ratio in Fig. 12, or the

unique four-way log graph (Figs. 6 and 13) used by Kolz

and Reynolds (1989b), the above relation yields what Kolz

and Reynolds referred to as a normalized curve for predicting the increase in applied power-density needed to

maintain a constant level of power-density in a fish (the

curve minimum) as water conductivity changes. Note that

the curve is symmetrical with a rounded bottom, like an
24 INFORMATION AND TECHNOLOGY REPORT--2003-0002

inverted normal curve, but with limbs that in the four-way

enhancement of the graph (Figs. 6 and 13) become asymptotic to 45° lines to the left and right which respectively represent corresponding quantities of current

density and voltage gradient. According to the theory, if

such a curve fits in-water power-density threshold data

for a specific response (as in Fig. 13) the coordinates at

curve minimum represent the minimum in-water power

density threshold, the fixed in-fish power-density threshold, the point at which 100% of the power density in the

water is transferred to the fish, and the point at which

water conductivity equals effective fish conductivity, all

for that specific response. Hereafter in this report, the

above relation and its corresponding curve are respectively referred to as the power-density-transfer equation

and curve.

Although introduced a decade ago by Kolz and

Reynolds (1989a) as a theory to be considered, further

tested, and perhaps further developed and refined, the

theory of power transfer has never been critically scrutinized in the literature for compliance with the laws of

physics, electrical theory, or long-standing principles and

theories regarding passage of electrical currents or charge

to and through biological organisms or tissues. According to Kolz (personal communication), the theory has been

substantiated using hard-wired techniques with dead fish

and gelatins suspended in water, but his data have not

yet been published. Nor have data beyond that presented

by Kolz and Reynolds (1989b, 1990a) been generated to

significantly support the theoretical relation between fieldintensity response thresholds and the power-densitytransfer equation and curve. Yet, the theory has been

promoted as a critical concept for understanding and practicing electrofishing by the authors and instructors of

the widely taught U.S. Fish and Wildlife Service course

Principles and Techniques of Electrofishing (latest course

Fig. 11. Major intensity-dependent electrofishing response zones. (The outer boundaries of response zones for a 1

spherical anode at the surface and sufficiently distant from the cathode are more-or-less hemispherical shells around

the anode that represent field-intensity thresholds for the associated responses. Actual and relative sizes of the zones

are specimen dependent (species, size, condition, and orientation) and vary with electrical output, electrode size and

shape, and environmental conditions. Labels in italics represent corresponding phases of epilepsy as suggested by

Sharber and Black, 1999, except that here the phase of tonic-clonic contractions (quivering or pseudo-forced swimming)

between petit mal and grand mal (narcosis and tetany) is treated as the initial part of grand mal (partial tetany). Zones

of taxis, narcosis, and tetany represent the effective range for fish capture using direct and pulsed direct currents.)
 SNYDER 25

manual by Kolz et al., 1998) and by Reynolds (1996) in his

chapter on electrofishing in the second edition of Fisheries Techniques (Murphy and Willis, 1996). Upon further

development and testing, the theory or at least portions

of it, might prove valid and useful, but questions regarding basic tenets remain.

Perhaps the first question to be asked is whether

power itself is a quantity that can be transferred (or in

part reflected as per some explanations of what happens

under conditions of conductivity mismatch–Reynolds,

1995, 1996). Kolz (1989a) suggests in his text that electrical

power is potential energy. However, in Kolz’s (1989a) own

glossary, and generally in physics textbooks, electrical

power is technically defined as the rate of doing work or

the amount of energy expended per unit time and power

density as energy dissipated per unit time in a given

volume of matter. As measures of expended or transformed

energy, it follows that electrical power and power density

cannot be transferred from water to fish (or anything else).

According to Sharber (personal communication), only

current (electrical charge conveyed per unit time by

electrons or negative ions) is transferred in an electric

field from water to fish, not power, and the amount of

current transferred to and through the fish, or induced in

it, is determined solely by the voltage gradient or

differential across the tissues of the fish and the

impedance of those tissues. Perhaps the differences in

opinion are in part semantic, and the term “power transfer”

should be replaced with “power induction” or a more

appropriate term.

Possibly the earliest consideration of relationships

among water resistivity (inverse of conductivity), fish

resistivity, in-water power density, and in-fish power density in North American literature was that presented by

Monan and Engstrom (1963). Although not considered

further herein, a comparison between their theory and

Fig. 12 Fig. 12. Power-density-transfer curve. (Represents the

relation Dw/Df = (1 + q)2 /(4q), where q = c w/c f (the

conductivity mismatch ratio, water to fish) for predicting

the increase in applied power density in water (Dw,)

necessary to maintain a constant transfer of required

power density (e.g., that for a desired response) to the

fish (Df) as the conductivity ratio (ä) increases or decreases

from unity with changes in water conductivity. This

relation was derived from concepts of normalized power

and load mismatch by Kolz, 1989a and is fundamental to

his theory of power transfer for electrofishing.

Reproduced with permission from Fig. 1 in Kolz and

Reynolds, 1989b; axis labels modified.)

Fig. 13 Fig. 13. Power-density-transfer curves fitted to peakpower-density threshold data for twitch, taxis (attraction),

and narcosis (stun) in 6- to 9-cm-TL goldfish (Carassius

auratus) subjected to homogeneous fields of DC. (Range

and means of experimental data are represented by vertical

bars and associated point symbols, respectively; ma =

milliamperes or 1,000 microamperes. Reproduced with

permission from Fig. 6 in Kolz and Reynolds, 1989b; axis

labels modified.)
26 INFORMATION AND TECHNOLOGY REPORT--2003-0002

mathematical derivations and Kolz and Reynolds’ (1989a)

theory of power transfer might be interesting and enlightening.

The response-threshold experiments by Kolz and

Reynolds (1989b, 1990a) with 6- to 9-cm-TL goldfish appear to support at least the mathematical relationships of

power-density theory. Using various currents (60-Hz AC;

50-Hz, square-wave PDC with 10, 25 and 50% duty cycles;

and DC) across a range of water conductivities (~10, 100,

1,000, and 10,000 μS/cm), they determined the mean lowest peak-power densities at each conductivity required

in water to initiate specific responses (thresholds for

twitch, taxis, and stun). When plotted on log-log graphs

relative to water conductivity (e.g., Fig. 13 for DC data),

Kolz and Reynolds (1989b, 1990a) found that the response-threshold data for each tested response and current approximately fit the shape of their

power-density-transfer curve. The data were fitted to the

equation by nonlinear least squares regression, but some

data, especially for DC as represented in Fig. 13, do not fit

the power-density-transfer curve as well as others. Kolz

and Reynolds (1989b) acknowledged the higher variability of their DC data and attributed it to inexperience in

conducting their first set of experiments.

However, the position of these fitted power-densitytransfer threshold curves, as defined by their minima

(effective fish conductivities and fixed in-fish powerdensity thresholds), varied with each response and current

tested. For example, they calculated that the effective

conductivity of the goldfish for twitch was 69 μS/cm under

DC, 83 to 99 μS/cm under PDCs, and 119 μS/cm under AC;

for narcosis, the effective conductivity was 83 μS/cm

under DC and 137 to 160 μS/cm under PDCs and AC. The

existence of a different effective conductivity for each

combination of response and current suggests that each

response probably involved different tissues, electrical

pathways, or physiological mechanisms. Otherwise,

effective conductivities would be the same and threshold

curves for each response would be vertically aligned.

Although comparing across types of current, Kolz and

Reynolds (1989b) particularly noted an increase in

effective fish conductivity (averaged for all responses)

relative to increasing pulse or current frequency (i.e., from

78 μS/cm for DC to 120 μS/cm for 50-Hz PDC to 138 μS/cm

for 60-Hz AC) and suggested that it might be due to

capacitive reactance. In-fish power density and effective

fish conductivity probably also vary with species, size,

condition, orientation in the field, and water temperature.

As Kolz and Reynolds (1989b) pointed out, effective

fish conductivities based on the minima of power-densitytransfer curves for specific responses in living fish are

not the same as, and generally have much lower values

than, fish conductivities determined by other methods.

In their experiments, as discussed above, they reported

mean effective conductivities of 69 to 160 μS/cm for

goldfish depending on the specific response observed

and current tested. In contrast, Monan and Engstrom

(1963) reported fish conductivities of 505 to 1,266 μS/cm

for sockeye salmon, Sternin et al. (1972, 1976) reported a

range of conductivities from 319 to 3,571 μS/cm for a

variety of freshwater fishes, and Haskell (1954) reported

an approximate conductivity of 667 μS/cm (resistivity of

1,500 ohm-cm) for the flesh of brown trout. Haskell (1954)

considered effective resistivity of the fish (inverse of

conductivity) to be equal to water resistivity when a fish

or its flesh failed to distort the distribution of voltage or

voltage gradient around it (Fig. 10) when placed in a

homogeneous field in a long, narrow trough.

Congruence of a single set of experimental responsethreshold data with Kolz’s (1989a) power-density-transfer

curve does not necessarily validate that mathematical

relation, much less the underlying concepts of powertransfer theory as presented by Kolz and Reynolds

(1989a). Comparable response-threshold evidence based

on other species and independently replicated tests of

goldfish are needed. Despite an attempt by Jesien and

Hocutt (1990) and implications by Fisher and Brown (1993),

no comparable data have been reported in the literature

to further support or disprove the theory of power transfer.

Jesien and Hocutt (1990) determined in-water fieldintensity thresholds for 50% tetany in 18- to 22-cm channel

catfish exposed for 1 s to 30-Hz and 120-Hz PDCs and two

pulsed ACs in water conductivities of 100, 1,000, and

10,000 μS/cm at 20° C. However, they needed at least one

more set of trials at a lower water conductivity to

demonstrate congruence with or divergence from the

normalized curve predicted by power-transfer theory.

Depending on the type of current and fish orientation

upon exposure under the PDCs (towards anode or

cathode), peak voltage-gradient thresholds ranged from

0.22 to 0.37 V/cm at 100 μS/cm, 0.12 to 0.29 V/cm at 1,000 μS/

cm, and 0.09 to 0.23 V/cm at 10,000 μS/cm; corresponding

peak power densities were 4.8 to 13.4, 14.4 to 84.1, and 81

to 515 μW/cm3

. Voltage gradients decreased and power

densities increased progressively with increasing water

conductivity for all but one current, waveform, and fishorientation combination (threshold voltage gradient for

the response was lowest at 1,000 μS/cm for 30-Hz PDC

with fish facing anode). No in-water, power-density,

threshold minima were apparent in or calculated for the

data.

In a personal communication reported by Jesien and

Hocutt (1990), A. Kolz suggested that the power-density

minima and corresponding effective fish conductivities

for the fish response tested in each of their treatments

probably occurred at slightly lower water conductivities

than tested. However, if Jesien and Hocutt’s data conform

to Kolz’s (1989a) power-density-transfer curves, the curve
 SNYDER 27

Reynold’s (1989b, 1990a) goldfish threshold curves for

twitch and stun under AC, Fisher and Brown (1993) suggested that the results were congruent with power-transfer theory. However, if those data are plotted on a log-log

graph like those used by Kolz and Reynolds (1989b, 1990a;

e.g., Fig. 13), it is obvious that the resulting curve is much

too tight (left and right limbs too steep) to approximate

Kolz’s (1989a) normalized power-density-transfer curve.

Also, unlike Jesien and Hocutt’s (1990) data above, no

combination of pairs of data points including the powerdensity value for the 60-μS/cm stream can be made to

approximate the normalized threshold curve (calculations

result in negative values for effective fish conductivities

and threshold minima). Using data points for the 35 and

120 or 125 μS/cm streams, calculated minimum coordinates for fitted power-density curves are 1,499 μS/cm at

21 μW/cm3

, or 884 μS/cm at 35 μW/cm3, respectively.

Fisher and Brown’s (1993) data certainly do not approximate a power-transfer curve, nor were their field experiments designed or intended to test the mathematical

relations of power-transfer theory; experiments to that

end need to be much more precise and controlled.

Fisher and Brown (1993) conducted a series of cagedfish experiments with “prepositioned areal” electrofishing

gear to determine effective distances from the electrodes

for 100% immobilization of fishes in streams of varying

conductivity. Each cage contained a mixed assemblage

of locally caught species. With measurements made from

the center of cages set at various distances from the electrodes, Fisher and Brown (1993) reported that the average peak-field intensities for cages farthest from the

electrodes with 100% of the fish immobilized (very coarse

threshold approximations) were 237 μW

Ten years after Kolz and Reynolds (1989a) proposed

their theory of power transfer in fish, I found only two

published accounts of practical field applications.

Burkhardt and Gutreuter (1995) described a procedure by

which aspects of the power-transfer theory were used to

standardize electric fields in a major long-term monitoring

program using 60-Hz PDC (25% duty cycle) in waters

with conductivities ranging from 250 to 700 μS/cm and

water temperatures from 15 to 35° C. Chick et al. (1999)

adopted the procedure to evaluate use of airboat

electrofishing with 60 or 120-Hz PDC for sampling large

fishes in shallow, vegetated habitats with water

conductivities of about 200 to 950 μS/cm and temperatures

of 15 to 25° C. The procedure consists of preparing a

table of peak-power-output goals for the anticipated range

of water conductivities and temperatures, then using that

table to adjust electrical output for measured water

conductivity and temperature with the expectation that

the resulting distribution of power densities in the water

will cause comparable responses (e.g., narcosis) by fish

of the same species and size at the same relative position

(distance from an anode) during each sampling effort.

The tables for each study were based on electrical and

physical parameters recorded for the most successful

(presumably highest catch-per-unit-effort) unstandardized

collections taken earlier in each program and an effectivefish-conductivity value of 150 μS/cm. The latter value

was selected from the upper end of the range of effective

conductivities reported by Kolz and Reynolds (1989b,

1990a) for goldfish subjected to a waveform and duty

cycle similar to that used by Burkhardt and Gutreuter

(1995).

minima for at least some treatments might have occurred

at a water conductivity (and effective fish conductivity)

somewhat above 100 μS/cm with the value at 100 μS/cm

being on the left upside portion of the curve. This

possibility can be explored by assuming that the

experimentally derived threshold values at the three

conductivities tested for each treatment do fit a powerdensity-transfer curve and calculating the fixed in-fish

power density (Df) and effective fish conductivity (cf) for

each curve (the coordinates of the curve minimum) based

on any two data points presumably on the respective

curve. The coordinates for the minimum of each curve

can be calculated by: (1) rearranging the power-densitytransfer equation given in the first paragraph of this

section to solve for Df and setting the resulting equations

for the coordinates of each of any two data points on the

curve equal to each other [Df = 4Dw1 w(c 1 f / c ) / (1 + (cw1

/ cf

2 )) = 4Dw2 (cw2 f w / c ) / (1 + (c 2 f / c ))2

f

]; (2) solving for

c ; and (3) substituting the value of cf

f

back into the powerdensity-transfer equation for either data point to

determine the value of D . Doing so using threshold

approximations for the 120-Hz-PDC, fish-facing-cathode

treatment at 100 and 10,000 μS/cm (approximately 5 and

100 μWp

3 /cm , respectively), the calculated minimum

coordinates would be an effective fish conductivity of

126 μS/cm at an in-fish power-density threshold of

4.9 μW/cm3 . For threshold approximations at 100 and

1,000 μS/cm (14.4 μW/cm3) and at 1,000 and 10,000 μS/cm,

the calculated minimum coordinates would be 106 μS/cm

at 5.0 μW/cm3 and 227 μS/cm at 8.7 μW/cm3, respectively.

For whatever reason (perhaps inadequate or imprecise

data), these results do not bode well for the fit of all three

data points on the power-density-threshold curve. A

similar extrapolation of coordinates for threshold-curve

minima for Jesien and Hocutt’s (1990) other treatments

might prove interesting, but would probably also be

inadequate to support or disprove application of the

mathematical relations of power-transfer theory to fish.

p/cm3 in a 35-μS/

cm stream, 10 μW/cm3in a 60-μS/cm stream, 77 μW/cm3

 in

a 120-μS/cm stream, and 80 μW/cm3 in a 125-μS/cm stream.

Because a plot of the mean threshold values relative to

water conductivity was V-shaped (lowest value at the

intermediate water conductivity) and fit between Kolz and
28 INFORMATION AND TECHNOLOGY REPORT--2003-0002

However, for standardizing electrofishing operations

in waters with similar ranges of conductivities and

temperatures, perhaps the procedures used by Burkhardt

and Gutreuter (1995) and Chick et al. (1999) are more

complex than they need to be; a direct in-water, voltagegradient-measurement approach might be simpler, just as

effective, and more certain. With this approach, electrical

output would be adjusted at the beginning of each

sampling effort (and again whenever conductivity and

temperature are likely to differ significantly) until a target

voltage gradient is measured at a standardized position

in the electric field (e.g., 1 m from an anode towards the

boat). Based on Kolz and Reynolds’ (1989b, 1990a)

experiments (Fig. 14), voltage-gradient response

thresholds for water conductivities beyond 200 μS/cm,

and especially 500 μS/cm, decrease so gradually with

increasing water conductivity (about 0.7 to 0.5 V/cm for

the stun threshold in 6- to 9-cm-TL goldfish using a current

comparable to that used by Burkardt and Gutreuter, 1995)

that one value (e.g., 0.6 V/cm) could be effectively used

as the target voltage gradient for standardization over the

full range of conductivities encountered in the above

discussed investigations.

For a little more precision when using the in-watermeasurement approach for standardizing electrofishing

fields or when working in lower conductivity waters, voltage-gradient thresholds for the desired response and

current using the goldfish model could be read directly

from a portion of the appropriate Kolz and Reynolds’

(1989b, 1990a) graph enlarged for the conductivity range

of interest. Alternatively, a comparable curve of in-water

voltage-gradient thresholds (Ew) versus water conductivity (cw) could be generated by using the equation

Ew = Ef x (1 + (cf / cw)) / 2) with the voltage gradient (Ef)

and water conductivity (cf) values corresponding to the

power-density minimum for the pertinent power-densitytransfer threshold curve by Kolz and Reynolds (e.g.,

Fig. 14; the referenced equation is derived from the powerdensity-transfer equation based on the definition of power

density, D = cE2). Of course, these suggestions assume

that the mathematical relations of the power-density theory

are valid and that Kolz and Reynolds’ (1989b, 1990a) data

for goldfish and the electrical currents tested are suitable

models for the targeted fish and electrofishing operation.

Voltage-gradient and power-density threshold curves

based on Kolz and Reynolds’ (1989b, 1990a) goldfish data

for narcosis (e.g., stun in Fig. 13 for DC) are compared

relative to water conductivity for each tested current in

Figs. 14 and 15, respectively. Upper and lower graphs in

each figure are the same except that the upper graphs use

unequal logarithmic scales and the lower graphs use

arithmetic scales for the more limited range of freshwater

conductivity. Regardless of whether field intensity is

represented by units of voltage gradient or power density,

threshold curves for narcosis in these graphs are similar

for AC and the three PDCs but notably higher for DC at

moderate to high conductivities (about 60% higher among

voltage gradient curves). Comparable reactive detection

(twitch) curves would be similar for each of the currents

(Kolz and Reynolds, 1989b, 1990a). As suggested above,

beyond water conductivities of 250 μS/cm, and especially

beyond 500 μS/cm, voltage-gradient thresholds decrease

so gradually that one approximate value (for each species,

size range, water temperature, and waveform) can

effectively approximate the threshold for a particular

response at all higher levels of conductivity in fresh water

(Fig. 14). For moderate to high water conductivities,

corresponding in-water power-density or current-density

thresholds would increase significantly with increasing

water conductivity (Fig. 15).

Despite possible problems with semantics and technical aspects of the power-transfer theory (including the

concept implied by its name) and apparent support by

just one set of threshold data for one species, the mathematical relations of the theory appear to be valid and

useful at least for defining or predicting field-intensity

threshold curves for selected responses over a wide range

of water conductivities. Aside from this and the standardizing procedure described by Burkhardt and

Gutreuter (1995), the utility of in-fish power-density response thresholds and effective fish conductivities in

electrofishing operations has not yet been realized. Until

such utility is realized, response-threshold data are probably more easily understood and used in terms of peakvoltage gradients, which, unlike power density, can be

measured directly in the water. Also, power-density can

only be determined by calculation from measurements of

voltage gradient and water conductivity, and for use in

the field, including standardization of electric fields based

on in-water measurements, it must be converted back to

voltage gradient. Accordingly, most field-intensity data

in the remainder of this review, including response thresholds, are presented as voltage gradients (when water conductivity data are also available, corresponding current

density and power density values can be calculated).

Major Intensity-Dependent Responses

The sequence of generally observed, intensitydependent responses by fish as they approach the anode

in an electrofishing field are illustrated in Fig. 11. Except

for their relation to epileptic responses and the distinction

between narcosis and tetany (an important distinction

overlooked in much of the literature), most of these

responses were documented as early as the 1920’s (e.g.,

Scheminzky, 1924, according to Lamarque, 1990). Vibert

(1963) and his associates at the Biarritz Station found

that not all fishes exhibited the same set of responses in
 SNYDER 29

Fig. 14 Fig. 14. Peak-voltage-gradient-threshold curves for narcosis (stun) of 6- to 9-cm-TL goldfish (Carassius auratus) in

homogeneous fields of DC; 50-Hz, square-wave PDC with duty cycles of 50%, 25%, and 10%; and 60-Hz sinusoidal AC.

(These in-water curves are an alternative representation of the power-density-transfer curves for narcosis in Figs. 6B10

of Kolz and Reynolds, 1989b. The vertical ranges denoted by small capital letters and associated with each curve

approximate the corresponding ranges of experimental threshold data provided in Kolz and Reynolds= graphs.)
30 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Fig. 15 Fig. 15. Peak-power-density-threshold curves for narcosis (stun) of 6- to 9-cm-TL goldfish (Carassius auratus) in

homogeneous fields of DC; 50-Hz, square-wave PDC with duty cycles of 50%, 25%, and 10%; and 60-Hz sinusoidal AC.

(These in-water curves are an alternative representation of the power-density-transfer curves for narcosis in Figs. 6B10

of Kolz and Reynolds, 1989b. The vertical ranges denoted by small capital letters and associated with each curve

approximate the corresponding ranges of experimental threshold data provided in Kolz and Reynolds= graphs.)
 SNYDER 31

DC that they observed for brown trout and European eel

(Table 1). Vibert (1963) suggested that there is “. . . a sort

of competition between the reaction to the particular

electric stimulus and the general behavioral response to

normal ecological stimuli.” Biarritz biologists also reported

that some responses differed with the type of current.

Lamarque (1990) specifically warned that, because of the

dynamic behavior and unlimited types of PDC available,

responses of fish in PDC can be quite different from those

in DC and that confusion between them would lead to a

considerable misunderstanding of electrofishing

procedures.

Based on either laboratory or field observations,

other biologists reported results that contradict the

Biarritz observations and sometimes each other. For

example, in PDC fields with a sufficient range of voltage

gradients to bound response thresholds, the Biarritz

researchers observed anodic taxis (and tetany) in trout

(brown or rainbow) and European eels but no narcosis

(100 Hz, 1-ms pulses). Conversely, Kolz and Reynolds

(1989b), in experiments with goldfish, observed narcosis

but no taxis (50 Hz; 2, 5, and 10-ms pulses). Yet, in practical

electrofishing operations, it is the strength and range of

both responses, taxis and narcosis, that generally make

PDC so useful. Contrary to the findings of Kolz and

Reynolds (1989b), Bird and Cowx (1993) documented both

taxis and narcosis in goldfish under a variety of PDC

waveforms and frequencies (30 to 600-Hz square, 50-Hz

quarter-sine, 50-Hz exponential; pulse widths 0.2– 30 ms).

Taxis towards the anode is the forte of electrofishing with

DC, but Haskell et al. (1954) reported that, even under

uniform laboratory conditions, the response was very

erratic; certain individuals were quickly drawn to the

anode, but others exhibited only partial or no taxis. Kolz

(personal communication) noted that participants in the

U.S. Fish and Wildlife Service electrofishing course have

reported that taxis may or may not occur in PDC fields

(species not specified, presumably field observations).

Meismer (1999) compared responses and response

thresholds for Colorado pikeminnow and rainbow trout

in homogeneous fields of DC, 15- or 60-Hz, square-wave

PDC, or CPS, and reported that both species exhibited

expected classical responses in all currents. However, in

both species, CPS caused more of a shudder than a sharp

twitch as observed with other currents. Also, taxis was

notably strongest in DC and not quite as strong in CPS

as in simple PDCs, and when current was switched off,

fish recovered more quickly from DC than other currents

tested. Despite these reports of variable and contradictory

responses, Sharber and Black (1999) suggested that

although the threshold levels for and intensity of the

various responses might differ, the general responses of

fish to an electric field are essentially the same regardless

of whether AC, DC, or PDC is used.

The responses indicated in Fig. 11 are those expected

of fish in DC and possibly all electric fields when facing

the anode (or either electrode in AC). According to the

Biarritz paradigm, responses and thresholds differ when

fish face the cathode or are perpendicular to the lines of

current (Table 1). Jesien and Hocutt (1990) found that

channel catfish in homogeneous PDC fields are more sensitive to tetany when facing the cathode than when facing the anode. Changes in other environmental or

experimental conditions may also affect fish responses.

The Biarritz and most other experiments mentioned above

were conducted in homogeneous fields. Whether responses or thresholds specific to fish facing in either

direction (toward the anode or cathode) would differ in a

heterogeneous field might depend on whether the fish

are closer to the anode or the cathode (the matter was not

addressed in literature reviewed for this report). Vibert

(1963) and Northrop (1967) noted that under field conditions it is impossible to distinguish each of the responses

documented in laboratory experiments, especially in flowing water or a moving field wherein fish are continually

reoriented relative to the lines of current and can be moved

quickly from one response zone to another.

Response Thresholds

Electrofishing fields are nearly always

heterogeneous, with field intensity highest at the

electrode surface and decreasing geometrically from that

surface to barely perceptible levels a few meters away.

The outer boundary for each response zone illustrated in

Fig. 11 represents the minimum in-water field intensity

(i.e., voltage-gradient, current-density, or power-density)

or threshold for that response. The specific values for

these thresholds vary with water conductivity and

temperature, electric-field waveform and frequency, and

the pertinent electrical and physiological characteristics

of the fish, which, considered as a whole, define its

effective conductivity. According to Whitney and Pierce

(1957), Halsband (1967), and Emery (1984), the electrical

conductivity of a fish (not necessarily its effective

conductivity) depends on its species, size, shape, condition,

surface area, and possibly even the size of its scales.

Response zones shrink or expand for individual fish

according to their orientation in the field. As suggested

above and in Table 1, both response threshold and nature

of response can vary with orientation. A fish in taxis when

facing the anode might at the same location be only in the

zone of reactive detection when oriented perpendicular

to lines of current. In the latter situation, the fish would

retain voluntary control of its movements and could dart

sufficiently away to escape further influence by the field.

If the fish turns from the perpendicular position instead

of darting directly away, the voltage differential across
32 INFORMATION AND TECHNOLOGY REPORT--2003-0002

the fish (from head to tail) would increase until at some

point the fish loses voluntary control and enters a state

of automatism. The fish might then remain in this state or,

through random movement and changes in orientation,

return to the zone of reactive detection or possibly begin

anodic taxis. Most fish in a state of anodic taxis continue

in this state until they reach the zone of narcosis.

Momentum from taxis or drift can sometimes carry a fish

from the zone of narcosis into the zone of tetany near the

anode. In experimental homogeneous fields, Edwards and

Higgins (1973) occasionally observed fish passing into

and out of a state of paralysis (stun) as their orientation

changed relative to the lines of flux. They also noted that,

when homogeneous field intensity was not sufficient to

stun, fish which could not escape or move to less-intense

zones tended to align themselves perpendicular to the

lines of flux (parallel to isopotential planes), where total

voltage across their bodies was least. Even when all

factors noted above as affecting response thresholds are

the same, including orientation, observed threshold

values apparently vary somewhat with individual

specimens and probably even in repeated tests of the

same individual with or without adequate stress-recovery

periods between tests.

For specific species, size ranges, and other

conditions, in-water field-intensity thresholds can be

approximated for various responses by fish and used to

define effective electrofishing fields. In-water, peakvoltage-gradient threshold data by species, water

conductivity, and current type for twitch (reactive

detection), anodic taxis, and narcosis (or stun) are

summarized from selected references in Table 2. However,

data in Table 2 from Sternin et al.’s (1976) Appendix 4 are

summarized across species and may include mean (PDC)

or rms (AC) voltage-gradient thresholds. The term stun

covers both narcosis and tetany, which may be difficult

to distinguish. Some stun threshold data from Sternin

et al. (1976) might actually be the threshold for loss of

equilibrium prior to narcosis. All known threshold data

for endangered cypriniform fishes from the Colorado River

Basin (Ruppert and Muth, 1997; Meismer, 1999) are

included in Table 2. Some relative response-threshold data

is also provided in Table 1.

Most experiments to determine field-intensity thresholds for specific responses by fish are conducted in homogeneous fields. However, one set of threshold

experiments by Taube (1992) was uniquely conducted in

premapped heterogeneous fields to better simulate

electrofishing conditions in the field. The results were

extremely wide ranges for twitch and narcosis thresholds

with unexpectedly high means but minima comparable to

thresholds determined by Taube (1992) and most other

investigators using homogeneous fields in the same range

of water conductivity (Table 2). Whether the unusually

wide and high ranges of threshold values from Taube’s

(1992) heterogeneous-field experiments are artifacts of

experimental methodology or reflect a real difference between response thresholds in the two types of electric

fields is a matter that deserves further investigation.

As noted earlier, Kolz and Reynolds (1989b)

suggested that peak-intensity threshold values for AC

and PDC are probably more biologically significant than

mean-intensity values. This suggestion was based on

their observation that across all tested waveforms, the

ranges of power-density-curve minima for the thresholds

of each response in goldfish were much narrower (values

more similar) using peak rather than mean field-intensity

data. For example, using corrected data for twitch

threshold curves from Table 1 in Kolz and Reynolds

(1989b), the range of peak field-intensity minima, 0.13–

0.19 Vp/cm (2.1–2.7 μWp/cm3

), is much narrower than that

for mean field-intensity minima, 0.015–0.19 Vm/cm (0.023–

2.4 μWm/cm3). Matching water and effective fish

conductivities at those curve minima were 69–119 μS/

cm). Mean (effective) power-density minima for 50%, 25%,

and 10% (duty cycle) PDC in Kolz and Reynolds’ (1989b)

Table 1 were miscalculated; respective corrected minima

are 0.68, 0.17, and 0.023 μW/cm3 (rather than 1.4, 0.7, and

0.2 μW/cm3) for twitch and 32, 6.3, and 1.0 μW/cm3 (rather

than 64, 25, and 10 μW/cm3) for stun. Voltage-gradient

threshold data for AC and PDC discussed earlier from

Edwards and Higgins (1973) and that summarized in

Table 2 (except possibly data from Sternin et al., 1976)

represent peak field intensities.

Typical voltage-gradient response thresholds reported for fish in fresh waters of moderate to high conductivity range from about 0.01 to 0.1 V/cm for twitch to

about 0.5 to 1.5 V/cm for tetany (Vibert, 1963; Lamarque,

1967a, 1990; Sternin et al., 1972, 1976; Kolz and Reynolds,

1989b, 1990a; Bird and Cowx, 1993; Ruppert and Muth,

1997). However, the range for a particularly sensitive species can be much lower. Jesien and Hocutt (1990) found

this to be the case for channel catfish tested at water

conductivities of 100, 1,000, and 10,000 μS/cm; voltage

gradient thresholds for 50% tetany ranged from 0.1 to 0.4

V/cm. For water conductivities greater than 100 μS/cm,

Reynolds (1996) concluded that voltage gradients of 0.1

to 1.0 V/cm are generally effective for inducing narcosis

(and possibly tetany) in most species.

However, in low conductivity waters, voltagegradient thresholds can be much higher than the typical

figures noted above. In water of just 4 μS/cm (16–18° C),

Bird and Cowx (1993) reported twitch thresholds of 1.4 to

3.0 Vp/cm for juvenile rainbow trout ( = 14 cm) subjected

to a variety of PDCs, but at 1,000 μS/cm, even narcosis

thresholds for respective currents were much lower,

ranging from 0.5 to 1.8 Vp/cm. As summarized in Table 2,

thresholds in waters with conductivities between 9 and

0
 SNYDER 33

Table 2. Table 2. Table 2. Table 2. 

Sternin et al. (1976), all species and lengths combined from various studies

In-water, peak-voltage-gradient (V/cm) response thresholds by fish species, water conductivity, and type of

electrical current. Summarized from selected literature.a

 Source, species, length Response threshold, peak V/cmb

conductivity, temperature Twitch Anodic taxis Narcosis or stunc

 Currentd Mean Range Mean Range Mean Range

Sternin et al. (1976), all species and lengths combined from various studies

20 μS/cm, (no temperature data)

DC (no data) 0.28–0.32e (no data)

50 to 100 μS/cm, 2 to 13° C (+?)

DC 0.29f f 0.1–0.62 0.94 0.25–2.0 2.1 0.99–2.8

AC (50, 60 Hz) 0.04f f 0.01–0.06 (not applicable) 0.13 0.10–0.19

143 to 160 μS/cm, 0.2 to 18°C

AC (50, 60 Hz) 0.03f f 0.02–0.04 (not applicable) 0.12 0.08–0.41

174 to 286 μS/cm, 4 to 21°C

DC (no data) 0.25e (no data)

PDC (5, 10, 15 Hz) 0.13f f 0.06–0.30 0.35 0.10–0.71 1.4f 0.30–4.4

AC (50, 60 Hz) 0.04f f 0.01–0.06 (not applicable) 0.21 0.05–1.1

300 to 740 μS/cm, 2 to 16° C (+?)

DC 0.07f f 0.01–0.14 0.23 0.08–0.66 0.51f 0.22–0.82

PDC (5, 10, 15 Hz) 0.06f f 0.06–0.10 0.15 0.10–0.21 0.37f 0.30–0.53

AC (50, 60 Hz) 0.03f f 0.01–0.06 (not applicable) 0.15 0.04–0.47

880 to 2,000 μS/cm, (no temperature data)

DC (no data) 0.24d,e 0.22–0.27 (no data)

AC (50, 60 Hz) 0.02f f 0.01–0.03 (not applicable) 0.09 0.07–0.14

>10,000 μS/cm (saltwater, brackish and marine), 7 to 30°C

DC 0.02f f 0.01–0.04 0.11 0.04–0.20 0.28f 0.11–0.50

PDC (4–500 Hz) 0.07f f 0.03–0.14 0.20 0.06–1.0 0.45f 0.13–0.82

AC (50 Hz) 0.02e (not applicable) 0.12e

Kolz and Reynolds (1989b), goldfish ( Kolz and Reynolds (1989b), goldfish (Carassius auratus g ), 6 to 9 cm TLg

9 to 19 μS/cm, 20°C

DC 0.52 0.26–0.76 2.4 1.1–3.6 4.9 2.2–8.1

PDC (50 Hz, 10%) 0.60 0.42–0.69 (not observed) 4.8 4.5–5.1

PDC (50 Hz, 25%) 0.60 0.45–0.65 (not observed) 4.8 4.4–5.2

PDC (50 Hz, 50%) 0.60 0.55–0.68 (not observed) 5.1 4.6–5.5

AC (60 Hz) 0.50 0.40–0.55 (not applicable) 4.5 3.7–4.8

110 to 160 μS/cm, 20°C

DC 0.12 0.10–0.15 0.56 0.35–0.67 1.5 1.1–1.7

PDC (50 Hz, 10%) 0.16 0.13–0.18 (not observed) 1.1 1.0–1.2

PDC (50 Hz, 25%) 0.17 0.13–0.20 (not observed) 0.95 0.85–1.00

PDC (50 Hz, 50%) 0.17 0.10–0.22 (not observed) 1.05 0.95–1.10

AC (60 Hz) 0.10 0.06–0.14 (not applicable) 1.00 0.94–1.10

1,000 to 1,600 μS/cm, 20°C

DC 0.11 0.02–0.15 0.40 0.29–0.46 0.85 0.70–0.91

PDC (50 Hz, 10%) 0.10 0.08–0.11 (not observed) 0.49 0.46–0.54

PDC (50 Hz, 25%) 0.09 0.07–0.10 (not observed) 0.46 0.43–0.50

PDC (50 Hz, 50%) 0.10 0.09–0.11 (not observed) 0.58 0.50–0.62

AC (60 Hz) 0.08 0.06–0.09 (not applicable) 0.55 0.50–0.60
34 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Table 2. Table 2. Table 2. Table 2. Continued.

 Source, species, length Response threshold, peak V/cmb

conductivity, temperature Twitch Anodic taxis Narcosis or stunc

 Currentd Mean Range Mean Range Mean Range

9,700 to 10,000 μS/cm, 20°C

DC 0.09 0.08–0.11 0.37 0.25–0.42 0.80 0.60–0.88

PDC (50 Hz, 10%) 0.08 0.07–0.09 (not observed) 0.47 0.44–0.50

PDC (50 Hz, 25%) 0.10 0.07–0.11 (not observed) 0.40 0.38–0.45

PDC (50 Hz, 50%) 0.08 0.08–0.09 (not observed) 0.48 0.45–0.53

AC (60 Hz) 0.07 0.06–0.08 (not applicable) 0.45 0.42–0.50

Taube (1992), rainbow trout ( Taube (1992), rainbow trout (Oncorhynchus mykiss h

Taube (1992), rainbow trout ( Taube (1992), rainbow trout (Oncorhynchus mykiss g

Meismer (1999), rainbow trout ( Meismer (1999), rainbow trout (Oncorhynchus mykiss g

Meismer (1999), Colorado pikeminnow ( Meismer (1999), Colorado pikeminnow (Ptychocheilus lucius g

Ruppert and Muth (1997), humpback chub ( Ruppert and Muth (1997), humpback chub (Gila cypha Gila cyphaGila cypha g

Oncorhynchus mykiss), 31 to 48 cm FLh

103 μS/cm, 11°C

DC 0.37 0.18–0.56 (no data) 4.4 0.44–6.4

PDC (20 Hz, 25%) 0.30 0.19–0.43 (no data) 2.3 0.47–5.0

PDC (20 Hz, 75%) 0.28 0.15–0.50 (no data) 3.1 0.53–10.4

PDC (30 Hz, 50%) 0.36 0.15–0.71 (no data) 3.6 0.92–6.5

PDC (60 Hz, 50%) 0.35 0.11–0.97 (no data) 2.8 0.61–6.4

CPS (240:15 Hz, 12%) 0.18 0.09–0.28 (no data) 1.7 0.30–3.4

Taube (1992), rainbow trout (Oncorhynchus mykiss), 33 to 60 cm FLg

100 to 121 μS/cm, 9 to 13°C

DC (no data) (no data) 0.51 0.26–0.71

PDC (30 Hz, 50%) (no data) (no data) 1.4 0.90–4.8

PDC (30 Hz, 75%) (no data) (no data) 1.3 0.53–2.6

PDC (60 Hz, 50%) (no data) (no data) 0.73 0.54–1.3

CPS (240:15 Hz, 12%) (no data) (no data) 1.0 0.54–1.3

AC (60 Hz) (no data) (no data) 0.30 0.27–0.41

Meismer (1999), rainbow trout (Oncorhynchus mykiss), 21 to 50 cm TLg

530 μS/cm, 18°C

DC 0.05 0.03–0.07 0.17 0.12–0.21 0.53 0.34–0.63

PDC (15 Hz, 6%) 0.08 0.06–0.10 0.16 0.13–0.19 0.63 0.54–0.70

PDC (60 Hz, 24%) 0.04 0.03–0.05 0.07 0.05–0.09 0.16 0.14–0.20

CPS (240:15Hz, 12%) 0.10 0.06–0.12 0.21 0.18–0.26 0.56 0.43–0.69

Meismer (1999), Colorado pikeminnow (Ptychocheilus lucius), 30 to 39 cm TLg

530 μS/cm, 18°C

DC 0.09 0.05–0.13 0.16 0.14–0.19 0.38 0.28–0.57

PDC (15 Hz, 6%) 0.11 0.08–0.13 0.18 0.16–0.21 0.32 0.25–0.36

PDC (60 Hz, 24%) 0.05 0.02–0.10 0.16 0.09–0.20 0.22 0.18–0.27

CPS (240:15 Hz, 12%) 0.11 0.09–0.14 0.18 0.13–0.20 0.30 0.26–0.35

Ruppert and Muth (1997), humpback chub (Gila cypha), 5 to 10 cm TLg

940 μS/cm, 15°C

PDC (30 Hz, 12%) (no data) 0.42 0.38–0.45 0.63 0.62–0.65

 (tet. 0.76 0.74–0.78)

PDC (60 Hz, 24%) (no data) 0.36 0.35–0.38 0.56 0.53–0.62

 (tet. 0.75 0.71–0.82)

PDC (80 Hz, 40%) (no data) 0.34 0.31–0.38 0.51 0.45–0.58

 (tet. 0.72 0.69–0.75)

CPS (240:15 Hz, 12%) (no data) 0.47 0.43–0.53 0.67 0.65–0.69

 (tet. 0.80 0.76–0.83)
 SNYDER 35

160 μS/cm ranged from about 0.01 to 0.97 V/cm for twitch,

0.25 to 3.6 V/cm for taxis (DC and PDC only), and 0.08 to

10 V/cm for narcosis. In field and laboratory trials with a

variety of species in water conductivities of 35 to 125 μS/

cm, Fisher and Brown (1993) found that most fish were

stunned with 60-Hz AC at minimum peak voltage gradients

from 0.2 to 1.1 V/cm, the upper end of which matches Kolz

and Reynolds’ (1989b) range of stun threshold values for

goldfish at 110 to 160 μS/cm (0.94–1.1 Vp/cm; Table 2). As

water conductivities increase from very low levels,

voltage-gradient-response thresholds decrease rapidly

through 100 μS/cm, decrease more slowly through about

800 μS/cm or less, then stabilize at relatively low levels

for all higher conductivities (Table 2).

Voltage-gradient thresholds reported for different (or

even the same) types of currents are often difficult to

compare among investigations due in part to differences

in experimental conditions and methodology and the size,

condition, and species of fish tested. With that in mind,

data in Table 2 suggest that response thresholds are generally lowest for AC and often highest for DC (exceptions

include DC thresholds occasionally as low as AC for twitch

and lower than PDC for taxis). Thresholds for PDCs of

various configurations vary widely from lowest (especially when compared only to DC) to highest, but in most

comparisons, range between values for AC and DC. In

comparisons among CPS, simple PDCs, and DC, taxis

thresholds are almost always highest for CPS and usually lowest for simple (constant-frequency) PDCs, but

narcosis thresholds vary from lowest to highest for CPS

and simple PDCs and from intermediate levels to highest

for DC. For goldfish, Kolz and Reynolds’ (1989b) reported

similar stun thresholds for AC and PDC within each range

of conductivity from 110 to 10,000 μS/cm (Table 2). Jesien

and Hocutt (1990) similarly found that 50%-tetany thresholds for catfish subjected to pulsed ACs (0.l1 to 0.37 Vp/

Table 2. Table 2. Table 2. Table 2. Concluded.

 Source, species, length Response threshold, peak V/cmb

conductivity, temperature Twitch Anodic taxis Narcosis or stunc

 Currentd Mean Range Mean Range Mean Range

Ruppert and Muth (1997), bonytail ( Ruppert and Muth (1997), bonytail (Gila elegans g ), 5 to 8 cm TLg

940 μS/cm, 15°C

PDC (30 Hz, 12%) (no data) 0.48 0.46–0.49 0.73 0.69–0.75

 (tet. 1.10 1.04–1.16)

PDC (60 Hz, 24%) (not recorded) 0.45 0.43–0.46 0.61 0.60–0.64

 (tet. 1.00 0.95–1.04)

PDC (80 Hz, 40%) (not recorded) 0.40 0.38–0.42 0.60 0.58–0.62

 (tet. 0.98 0.97–0.99)

CPS (240:15 Hz, 12%) (not recorded) 0.78 0.75–0.80 1.00 0.98–1.04

 (tet. 1.40 1.38–1.42)

a

Sternin et al. (1976, Appendix 4: data for many species, further summarized here for all species combined), Kolz and

Reynolds (1989b, Figs. 6–10, data approximated from graphs), Taube (1992, Appendix Table 8), Ruppert and Muth (1997,

Table 1), and Meismer (1999, Table 6; ranges by personal communication).

b

Data from Sternin et al. (1976) may include mean PDC and AC (rms) voltage-gradient thresholds.

c

Thresholds for narcosis and stun (narcosis and tetany combined) are assumed to be the same, but some data

summarized for stun by Sternin et al. (1976) may represent the threshold for loss of equilibrium prior to narcosis. Distinct

thresholds for tetany (tet.) from Ruppert and Muth (1997) are given in parentheses under narcosis thresholds.

d

DC = direct current, PDC = pulsed direct current, CPS = Coffelt’s complex pulse system (a PDC pulse train), and AC =

alternating current. PDC parameters are pulse frequency and duty cycle.

e

Data for only one species from only one investigation.

f

Average of summarized data across species, not mean for individual specimens or treatments.

g

Data from homogeneous-field experiments.

h

Data from heterogeneous-field experiments.
36 INFORMATION AND TECHNOLOGY REPORT--2003-0002

cm) were nearly the same as those they determined for

PDCs (0.09 to 0.28 V/cm). Interestingly, at comparable levels of conductivity, and regardless of current and waveform, Jesien and Hocutt’s (1990) 50%-tetany thresholds

for channel catfish are much lower than Kolz and

Reynolds’ (1989b) stun (narcosis) thresholds for goldfish (Table 2). However, the thresholds and effects of

pulsed AC may be significantly different from those of

continuous AC.

Vincent (1971) concluded that PDC induces DC-like

responses but at lower field-intensity thresholds. As summarized in Table 2, this suggestion of lower thresholds

for PDC than DC is supported by Kolz and Reynolds’

(1989b) data for stun but not for twitch, for which PDC

and DC thresholds are broadly comparable. Nor is

Vincent’s (1971) statement supported by the data of

Sternin et al. (1976), which suggest lower thresholds for

DC. For goldfish that were tested in approximately 300-

μS/cm water with DC and a multitude of PDCs, Bird and

Cowx (1993) reported threshold data (mistakenly attributed to crucian carp—Cowx, personal communication)

that are very similar to those of Kolz and Reynolds (1989b),

except Bird and Cowx observed a taxis response in PDC

treatments and Kolz and Reynolds did not. Bird and Cowx

(1993) found that DC and the various PDC threshold ranges

were similar for twitch (0.04–0.08 V/cm) and taxis (0.10–

0.20 V/cm) but not stun (DC threshold of 0.9V/cm was

notably greater than the 0.2–0.6 V/cm thresholds for

PDCs).

Within most species-size groups tested with DC, a

wide variety of simple PDCs, and a PDC pulse train,

Edwards and Higgins (1973) also found that stun thresholds (only response tested) were generally highest for

DC and least for the PDC pulse train with mutually exclusive ranges (e.g., for 8–22-cm bluegill, stun thresholds

were 1.3–1.9 V/cm for DC vs. 0.3–0.6 V/cm for the PDC

pulse train). In a set of experiments with rainbow trout

(31–48 cm FL) in premapped heterogeneous fields with

DC, various PDCs, and CPS, Taube (1992) also found that

the thresholds for stun and twitch (taxis not recorded)

were generally highest for DC and lowest for a PDC pulse

train, in this case CPS (Table 2). However, in a set of homogeneous-field experiments, also with rainbow trout (33–

60 cm FL), Taube (1992) found the opposite, with stun

thresholds notably lower for DC than CPS.

Relative to DC and a PDC pulse train, Edwards and

Higgins (1973) found the ranges of stun thresholds among

the various constant-frequency PDCs they tested for several species-size groups to be generally more variable.

The threshold ranges were sometimes intermediate and

sometimes matching or occasionally exceeding the ranges

for either DC and the pulse train. However, among these

PDCs, there was a tendency for the thresholds of lowerfrequency currents to more closely approximate the higher

threshold values for DC and the higher-frequency currents to approximate the lower threshold values for the

PDC pulse train. Similarly, data in Table 2 suggest that

response thresholds generally decrease with increasing

pulse frequency. For example, Meismer (1999) consistently

found twitch, taxis and narcosis thresholds for 60-Hz PDC

notably lower than those for 15-Hz PDC, and the latter

generally comparable to thresholds for DC and CPS

(Table 2). Bird and Cowx (1993) observed the opposite

tendency for twitch and taxis (thresholds increasing with

increasing pulse frequency) among the PDCs they tested

with a 10% duty cycle, but not PDCs with 50% or 90%

duty cycle or for the stun thresholds.

In general, biologists have found that response

thresholds vary with species and size of fish. Comparing

twitch, taxis, narcosis, and tetany thresholds for two

closely related endangered species of similar size (5–10 cm

TL) but different ages, Ruppert (1996) and Ruppert and

Muth (1997) reported that year-old humpback chub were

8 to 43% more sensitive to a variety of PDCs and CPS

than 7-month-old bonytail. Differences were greatest for

CPS and tetany treatments. With a few exceptions,

Meismer (1999), who tested DC, 15 and 60-Hz PDC, and

CPS, found that thresholds for Colorado pikeminnow (30–

39 cm TL) were generally similar to thresholds for rainbow trout (21–50 cm TL) for twitch and taxis but not

narcosis (narcosis thresholds were notably lower for Colorado pikeminnow). Edwards and Higgins (1973) also compared thresholds (stun only) among species but found

that apparent differences were confounded by differences

in the size of the fish that were tested. Combining data for

currents and ignoring differences in species, they found

that stun thresholds decreased with increasing fish length

(e.g., 0.6 to 1.8 V/cm for 4 to 8-cm bluegill, 0.2 to 1.4 V/cm

for 17 to 27-cm channel catfish, and 0.2 to 0.9 V/cm for 28

to 61-cm bowfin; water about 100 μS/cm, 24° C).

Consensus of biologists experienced in

electrofishing and in general texts on electrofishing is

that large fish are easier to capture than smaller fish. The

relation is supported by at least some studies comparing

the size distribution of fish collected by electrofishing

with the known size distribution of populations or

comparable data collected by other techniques (e.g.,

Sullivan, 1956; McFadden, 1961). Taylor et al. (1957)

investigated the relation between DC response

thresholds and fish length by subjecting 4 to 34-cm

(probably SL) rainbow trout to homogeneous fields of

0.1 to 0.5 V/cm for up to 6 s. They recorded four levels of

responses, from inhibited motion or minor signs of distress

to narcosis or tetany, and reported decreasing response

thresholds as size increased to 25 cm; beyond 25 cm the

relationship was not clear. Similarly, Maxfield et al. (1971)
 SNYDER 37

subjected young-of-the-year (YOY) rainbow trout

averaging 5 cm TL to 30 s of homogeneous, 8-Hz PDC at

1 Vp/cm and yearlings averaging 19 cm TL to 5-Hz PDC at

0.75 Vp/cm, but despite the lower field intensity and

slightly slower pulse frequency, they observed narcosis

only among the yearlings. Lamarque (1990) noted that

the threshold for nerve response deceases with increasing

nerve length only for nerves shorter than about 4 cm and

that the threshold remains constant for nerves of greater

length. Accordingly, he concluded that any size-response

relation (except for small fish) is probably due to factors

other than the direct effect of the electric field on nerves.

Emery (1984) suggested that the effect of size is a function

of total surface area rather than the length or weight of

the fish.

Zone of Reactive Detection

The outermost region of a heterogeneous electric

field to which fish respond in some fashion is usually

referred to as the zone of reactive detection, fright, or

perception (Fig. 11). Field intensity in this zone is sufficient to elicit momentary involuntary twitches, shudders,

or convulsions but low enough that fish can still remain

mostly indifferent to the stimuli, move away voluntarily if

irritated, or respond with instinctive reactions such as

flight, taking cover, and possibly aggressive displays if

startled. Fish might actually perceive the field but may or

may not react to it at substantially lower field intensities

and notably greater distances from the electrodes than is

required to evoke twitches or fright responses. The commonly referenced threshold for the twitch response occurs when field intensity is sufficient to elicit a sudden

movement, shudder, or muscular convulsion, the latter

most likely occurs only when the current is switched on

or off, pulsed, or possibly alternated with sufficient voltage differential. Although not indicated in Fig. 11, Sharber

and Black (1999) consider at least some of the responses

attributed to this zone, particularly twitch in the form of

muscular jerks or convulsions, to be epileptic automatisms.

A fright response usually reflects the fish’s normal

behavior when startled. It is most likely an unconditioned

defensive reaction (Sternin et al., 1972, 1976) that results

in many fish escaping the more intense and effective portions of the field (Novotny and Priegel, 1974). Fright or

other responses to detection of an electric field vary with

species. Meismer (1999) observed that rainbow trout (21–

50 cm TL) exposed to homogeneous fields of gradually

increasing intensity in shallow test chambers (~2.0 x 0.5 x

0.5 m) reacted very violently with much thrashing, flailing, and rapid, forceful swimming, sometimes leaping 10

to 15 cm out of the water in an apparent attempt to escape

the field. In a few cases, swimming was so forceful that

the fish broke through nylon-mesh screens intended to

prevent fish from contacting the electrodes. In contrast,

similarly treated Colorado pikeminnow (30–39 cm TL) reacted much less violently without thrashing, jumping,

trying to break through screens, or otherwise desperately trying to escape. Vibert (1963) noted that flatfishes

may take cover by burrowing. In some cases, the fright

response attributed to an electric field might actually be a

reaction to noise, motion, or related, nonelectrical stimuli

produced during an electrofishing operation.

Some biologists (e.g., Reynolds, personal communication) suspect that fish in this zone cannot perceive a

directional component to the electric field. If so, fish may

be just as likely to dart farther into the field as away from

it. However, if fright response or flight results in escape

by most fish in this zone, then the majority of fish captured by electrofishing were probably present in the effective zones of the field (taxis, narcosis, and tetany) when

the current was switched on. Captured fish initially in the

zone of reactive detection may have been trapped against

a shoreline, bar, shallow riffle, or purposely set net as the

electric field approached. Such possibilities should be

considered when planning the approach to a sampling

area and deciding where, when, how often, and how long

the electric field should be applied.

Zones of Undirected or Inhibited Swimming and

Taxis

The combined zones of undirected motion or inhibited swimming and taxis (forced swimming towards the

anode, anodic taxis, electrotaxis, or oscillotaxis) represent the epileptic phase of automatism according to the

Bozeman paradigm (Sharber, personal communication;

Sharber and Black, 1999). Without introducing a nonelectrical stimulus, it might be difficult to behaviorally distinguish fish that respond indifferently to an electric field in

the zone of reactive detection from those that exhibit undirected or inhibited motion in the portion of the zone of

automatism represented by undirected or inhibited swimming. Fish in the latter state may blunder (Northrop, 1967)

into the zone of taxis or be engulfed by that portion of a

moving field and subsequently be forced to swim towards

the anode until they are netted or reach the zone of narcosis. Some fish exhibiting taxis have enough momentum

to carry them through the zone of narcosis into the zone

of tetany. The threshold for taxis by targeted fish defines

the outer limits of an effective electrofishing field.

Vibert (1963) noted that flatfishes “may burrow or

remain on the bottom resisting the swimming response

until narcosis or tetany take over.” Whether flatfish actually resist taxis, respond in a different, perhaps species-
38 INFORMATION AND TECHNOLOGY REPORT--2003-0002

specific manner, or experience different electrical-field parameters at the substrate interface is unknown.

Haskell et al. (1954) suggested that due to continually changing orientation of a fish’s body, especially in a

moving field, taxis towards the anode in DC and PDC is a

composite of natural swimming movements caused by

the central nervous system, involuntary bends of the

body toward the anode (especially upon initial circuit

closure in DC and with each pulse in PDC), and anesthesia (narcosis). They reported that involuntary bends toward the anode were strongest when fish were

perpendicular to the lines of current, whereas the anesthetic response was greatest when fish were parallel to

the lines of current.

Lamarque (1990) suggested that anodic taxis under

PDC is distinctly different from that under DC. Haskell

et al. (1954) also observed differences and concluded that

DC “modifies the normal swimming motion and guides

the fish toward positive pole,” whereas PDC causes an

“involuntary . . . turn toward the positive pole and forward motion at each circuit closure.” Haskell et al. (1954)

and Lamarque (1990) also noted that motion resulting

from PDC required a lower voltage threshold and was

more pronounced than that from DC. Some biologists

(e.g., Fredenberg, personal communication) have observed that taxis can be so powerful in some PDC currents that fish sometimes appeared to swim rapidly by

and beyond the anode without succumbing to narcosis

or tetany (sometimes ultimately circling back towards the

anode). Other biologists reported no taxis under PDCs

for certain species and experimental conditions (e.g., Kolz

and Reynolds, 1989b, 1990a for goldfish). In AC, taxis

cannot be sustained towards either electrode because

the current continually reverses direction and the fish

ultimately aligns itself perpendicular to the lines of current in a “swimming” response referred to as transverse

oscillotaxis.

Zones of Narcosis and Tetany

Narcosis and tetany represent two distinct forms of

stunned immobility (Vibert, 1963). The zone of narcosis

or petit mal (Sharber, personal communication; Sharber

and Black, 1999) is characterized by a loss of equilibrium,

limp or relaxed muscles, and reduced or discontinued

breathing motions (apnea). The zone of tetany or grand

mal (Sharber, personal communication) is represented by

a partial to full state of sustained muscle contraction. In

full tetany, fish are rigid and apnea persists. Fish in the

outermost, (lowest intensity) portions of the zone of tetany

sometimes quiver or exhibit a very confined and rapid

swimming motion, usually while lying on their sides or

backs. Although treated here as the initial phase of tetany

or grand mal, Biarritz researchers considered it a separate

transitory response between narcosis and tetany and referred to it as pseudo-forced or second swimming towards the anode. Similarly, Sharber and Black (1999), in

accord with conventional epilepsy terminology, considered it as a transitory phase of tonic-clonic contractions

between petit mal and grand mal. During this transitory

phase or as fish progress from it to a state of full tetany or

grand mal, Sharber and Black (1999) also noted that

chromatophore stimulation can result in a patchy or barlike discoloration of the skin (brands).

When fish in narcosis or the beginning of tetany are

removed from the electrofishing field (by netting, switching off the current, or moving the field away from the

fish), they usually recover immediately and behave in a

relatively normal manner. For goldfish ( = 16 cm) exposed for 5 s at 1.1 Vp/cm (294–320 μS/cm; 16–18° C), Bird

and Cowx (1993) reported that recovery of breathing motions was immediate after exposure to DC but variously

delayed from 4 to 45 s after exposure to various PDCs.

Mitton and McDonald (1994a) similarly reported ventilation recovery times averaging 19 s (but sometimes requiring up to 3 min) for rainbow trout exposed to 20 s of

60-Hz PDC at a field intensity sufficient to induce tetany.

Barham et al. (1989b) reported that over a wide range of

field intensities and exposure times in 50-Hz AC and 50-

Hz, half-sine PDC, common carp (25–60 cm) shuddered

convulsively a few seconds after current ceased and recovered respiratory motions within 30 to 100 s but otherwise remained narcotized for an additional 2 to 40 s;

recovery of equilibrium and swimming motions took another 4 to 44 s. Meismer (1999) noted that adult rainbow

trout recovered equilibrium immediately after being narcotized at threshold level for 5 s in DC or CPS, but that

recovery was somewhat delayed in 15 or 60-Hz PDC.

Fully tetanized fish or those in the zone of tetany for

excessive periods may require several minutes to recover

normal muscle response, respiratory movements, and

equilibrium. Full physiological recovery takes much longer,

more than 24 h according to Barton and Dwyer (1997).

Some fish kept in a state of tetany too long never recover.

Meismer (1999) reported that among Colorado

pikeminnow (30–39 cm TL) and rainbow trout (29–50 cm

TL) subjected to gradually increasing field intensity

through the threshold for tetany, then held at 1.0 Vp/cm

for 5 s in DC, 15- or 60-Hz, square-wave PDC, or CPS, all

Colorado pikeminnow exposed to 15-Hz PDC or CPS

required more than 5 min to recover equilibrium, some

individuals of both species exposed to 60-Hz PDC required

more than 15 min to recover equilibrium, and 10% of

rainbow trout exposed to the 60-Hz, square-wave PDC

died. Among specimens of each species tetanized with

abrupt 10-s exposures of 60-Hz, square-wave PDC (via

Coffelt’s VVP-15) or 60-Hz, quarter-sine-wave PDC (via

Smith-Root’s GPP 5.0) at 1.5 Vp/cm, Meismer (1999)

 0
 SNYDER 39

reported that all fish recovered equilibrium except 30% of

the rainbow trout exposed to 60-Hz, square-wave PDC.

Like electrofishing, fishery biologists have found

immobilization by controlled electrical narcosis to be a

useful tool when tagging fish, gathering specimen-specific data, or collecting eggs and milt from fish, especially

large fish (Hartley, 1967; Gunstrom and Bethers, 1985;

Barham et al., 1987, 1988, 1989a; Orsi and Short, 1987;

Walker et al., 1994). The technique is usually referred to

as electrical anesthesia, but Hartley (1967) emphasized

that although the fish are temporarily paralyzed and appear unconscious, we do not know whether they are insensitive to touch or pain. For anesthesia, fish are usually

subjected to a relatively homogeneous electric field in a

small chamber where voltage gradients are easily controlled. Generally, smooth DC is preferred to minimize the

risk of tetany and because the operator can handle the

fish in the water without feeling the current, unless he

has cuts on his hands (Hartley, 1967). According to

Hartley (1967) and Kynard and Lonsdale (1975), fish can

be instantly immobilized by initially applying twice the

minimum voltage subsequently needed to maintain narcosis. These voltage levels are arrived at experimentally

or through experience and depend primarily on water conductivity, species, and size of the fish. Unless fish are

physically restrained, the higher initial field intensity is

probably necessary because many fish will not be aligned

parallel to the lines of current when the field is switched

on. Ellis (1974) narcotized 2-year-old channel catfish with

60 s of 60-Hz AC, 15 to 25-Hz PDC, or DC at 1.5V/cm then

monitored the fish in cages in a pond for 133 days. He

reported that the fish regained consciousness within 2 h

and exhibited no significant effects on survival, growth,

and feed conversion. Barham et al. (1987, 1988, 1989a,b)

found anesthesia in both AC and half-sine PDC preferable to benzocaine for Mozambique tilapia, but unsuitable for common carp. Tipping and Gilhuly (1996) noted

that in preliminary experiments with adult steelhead (rainbow trout), electrical anesthesia using CPS at a calculated 1.7 Vp/cm produced better narcosis than carbon

dioxide anesthetization but also induced compression fractures in the spines of 8% of the fish.

Lamarque (1967a, 1990) observed that in a DC field

just sufficient for narcosis, a fish facing the anode can

remain safely narcotized for several hours. However,

Kynard and Lonsdale (1975) reported that yearling

rainbow trout (~12 cm) held under DC narcosis for 6 h

(0.25 V/cm, 13–21° C, 450 μS/cm) suffered 7% mortality

and that survivors required up to half a day to resume

normal swimming and feeding behavior, but that growth

and phototropic response over the next 25 days were

unaffected. In contrast, recovery was generally

instantaneous for fish narcotized for only 1 or 2 h and no

mortalities were reported for trout held under electrical

narcosis for up to 4 h. Kynard and Lonsdale (1975) also

documented a decrease in ventilation rate for narcotized

fish, up to 52% reduction for yearlings held under narcosis

for 4 h versus an 18% decrease among controls.

Walker et al. (1994) investigated the use of homogeneous 50-Hz ACs (sine and triangular waveforms) and

50-Hz PDC over a range of field-intensities and exposure

times for successful narcosis (anesthetization) of northern pike juveniles and adult broodstock. Their criterion

for successful narcosis was sustained immobilization for

at least a minute after exposure without externally obvious physical injury (enough time to strip adults of eggs

or milt). Fish were oriented parallel to lines of current and

faced the cathode in PDC. For juveniles (13–19 cm SL),

the field intensities and exposure times that induced narcosis or injury using either AC were variable and unpredictable (e.g., injury after 30-s exposure at 0.4 Vrms/cm,

narcosis without injury after 60 s at 0.7 Vrms/cm, and neither narcosis nor injury after 30 s at 2.1 Vrms/cm). In contrast, 10- to 60-s exposures of juveniles to the PDC over a

range of 0.4 to 2.1 Vp/cm produced no externally detected

injuries or obvious behavioral impairments and at 1.4 Vp/

cm or greater, consistently induced successful narcosis.

Post-exposure narcosis time increased with increasing

field intensity from just under 2 min after a 10-s exposure

at 1.4 V/cm to 12 min after a 60-s exposure at 2.1 V/cm.

Although 10-s exposures always resulted in the shortest

times, narcosis time was not statistically correlated with

time of exposure. Walker et al. (1994) also exposed over

300 broodstock northern pike (45–97 cm SL) to 10-s exposures of the PDC and successfully narcotized them for

approximately a minute with breathing movements reestablished within 2 min and upright swimming within 3

min. There were no deaths or external signs of injury within

24 h of exposure, but the fish were not X-rayed to assess

internal injuries.

Another method to anesthetize fish is to place them

in direct contact with the electrodes, usually on a table

with the anode contacting the head and the cathode contacting the body (Kolz, 1989b). As long as the body of

the fish conducts an adequate current, the fish is immobilized; when the circuit is broken, the fish recovers instantly unless it was maintained under narcosis more than

a couple of hours.

The terms narcosis and tetany are often confused

and used interchangeably in practice and in the literature.

In some cases, failure to distinguish these terms is due to

difficulty in identifying the initial states of partial tetany.

The terms stun or stunned are used herein to refer to

immobilization (paralysis) in either state when the

distinction is unnecessary or the specific state is

undefined. The term shock is sometimes used as a

synonym for stun (Sternin et al., 1976), but it is more

generally defined as any response to an electrical stimulus
40 INFORMATION AND TECHNOLOGY REPORT--2003-0002

(especially a sudden reaction), the electrical stimulus

producing such a response, and, among fish biologists,

as the act of electrofishing.

Comparison of Currents for

Electrofishing Purposes

AC is often considered to have a larger effective

field than either DC or PDC (Lamarque, 1990), but at the

same mean output (voltage, amps, or watts) this might

not always be the case relative to DC and is unlikely

relative to most PDCs. Kolz and Reynolds (1989b) found

peak-voltage gradient thresholds for narcosis in goldfish

were lower in AC than in DC but except at the lowest

conductivities, comparable to those in PDC (Table 2;

Figs. 14 and 15). Also, as discussed earlier, the effective

anodic fields for DC and PDC include the zone of taxis

whereas the effective fields around electrodes for AC are

limited to narcosis and tetany. Kolz and Reynolds (1989b,

1990a) found that the DC threshold for taxis in goldfish

(0.7 Vp/cm) is lower than the threshold for AC narcosis

(0.9 Vp/cm), hence a slightly larger effective field for DC if

peak outputs are the same. Ignoring taxis and assuming

equal peak-field-intensity thresholds for narcosis, the

same peak output, and all other conditions the same, distribution of peak-field intensity (voltage gradient, current density, or power density) and the size of the effective

field will be identical regardless of the type of current and

waveform. However, if mean (rms for AC) rather than peak

output are matched (generator capacity is limited mostly

by mean output), distribution of peak-field intensity and

size of the effective field will always be greater for AC and

PDC than for DC (peak and mean output or field intensity

are identical for DC, and, in this case, distribution of meanfield intensities would be the same for all currents). For

AC and PDC, the difference between peak and mean output and field intensities varies according to waveform

characteristics and is frequently greater, sometimes much

greater, for PDC. For single-phase sinusoidal AC, peakvoltage gradient and peak-power density are about 1.4

and 2 times greater, respectively, than corresponding mean

values (for sinusoidal AC, Vrms = 0.71 Vp) or square-wave

PDC with a 71% duty cycle. Likewise for peak-voltage

and peak-power output. If a PDC duty cycle is less than

71% (regardless of wave shape), its peak-field intensity

will always be greater than in sinusoidal AC fields at the

same mean output. For example, with a 25% duty cycle,

square-wave PDC fields will have a peak-voltage gradient and peak-power density about 2.8 and 8 times greater,

respectively, than sinusoidal AC and 4 and 16 times

greater, respectively, than for DC or corresponding mean

values for this PDC.

In some cases, larger fields might not be

advantageous. Vincent (1971) suggested that because

the zones of narcosis and tetany, as well as taxis, are

larger in PDC than DC fields, fish might be more difficult

to net and more susceptible to tetany and tissue damage.

Stunned fish are usually easier to net than rapidly moving

fish in taxis, but fish that are stunned beyond the reach of

netters may not be seen and escape capture. Chmielewski

et al. (1973) noted that fish stunned while taking cover

are less likely to be captured, and those initially stunned

in flowing water may be washed away before they can be

netted.

Although Haskell (1950) suggested that DC is more

dangerous to man than AC, most electrofishing authorities consider AC, with its reversing polarity and presumably large zone of tetany, to be more dangerous to fish

and perhaps to the electrofishing team and observers

than either DC or PDC (e.g., Hauck, 1949; Taylor et al.,

1957; Lamarque, 1967a, 1990; Northrop, 1967; Vibert,

1967b; Vincent, 1971; Novotny and Priegel, 1974;

Reynolds, 1983, 1996; Kolz et al., 1998). Lamarque (1967a)

specifically observed that AC and PDC can provoke violent tetanus. As discussed later, excessive exposure to

tetanizing currents can result in severe stress, unrecoverable fatigue, or respiratory failure. Still, some state agencies (e.g., Illinois, Michigan) continue to make extensive

use of AC electrofishing (Schneider, 1992). Hudy (1985)

and Schneider (1992) maintain that AC can be used effectively without significant harm to the populations being

studied. If so, the substantial zones of narcosis and tetany

in AC might be desirable to improve capture efficiency

under certain conditions–usually in shallow, clear, slowmoving water where fish can be easily netted and rapidly

removed from the electric field. In low-conductivity

streams along the Appalachian Mountains, where AC is

considered the most effective electrofishing current, experienced field biologists report few, if any, mortalities,

brands, or other external signs of injury because they are

able to net the fish quickly and minimize time of exposure

(Wydoski, personal communication). However, as discussed later, spinal injuries, which may not be externally

obvious, are not necessarily dependent on time of exposure and if AC’s reputation for causing greater harm than

DC or PDC is warranted, its use is probably best reserved

for situations in which fish will be permanently removed

and injuries or mortalities are not a serious concern

(McCrimmom and Berst, 1963).

In most electrofishing operations, taxis and narcosis

are the responses to be sought and optimized, whereas

tetany is considered dangerous and to be minimized or

avoided. DC is generally considered the least damaging

current, in part because it is believed to have a higher

threshold for tetany than AC or PDC. Grisak (1996), who
 SNYDER 41

noted that all fish he captured with PDC or DC succumbed

to tetany, observed that in PDC (40-Hz), most fish reacted

mildly and appeared to simply rise from the depths to the

surface (apparently tetanized on the spot some distance

from the anode), except common carp which at the surface swam violently away from the electric field (no carp

were captured). In contrast, he observed that most fish in

DC swam directly toward the anode and some, particularly goldeye, swam so strongly that once stunned near

the anode, momentum frequently carried them into contact with the anode. As discussed earlier, the zone of

tetany for any current can be controlled to some degree

by careful selection of output voltage and the size, shape,

and configuration of the electrodes (Novotny and Priegel,

1971, 1974; Chmielewski et al., 1973; Novotny, 1990).

Lamarque (1990) suggested that DC generated by fullwave rectification of three-phase AC (600 Hz) has less

ripple (4%) and a correspondingly less tetanizing effect

on fish than DC that is only half-wave rectified (300 Hz,

17% ripple).

PDC is a diverse family of waveforms with different

wave shapes, simple and complex frequencies, pulse

widths, and duty cycles, each of which might affect the

responses of fish in the electric field. In sea water

(>50,000 μS/cm), Groody et al. (1950) compared the responses of Pacific sardine (20–30 cm) and topsmelt (11–

12 cm) in DC and several PDC and hybrid currents

(3–12-Hz, square-wave PDC with various pulse widths,

some hybridized with lower-intensity DC; 45–68-Hz, halfsine-wave PDC—half-rectified AC; 120-Hz, half-sinewave PDC—fully rectified AC; the latter as a pulse train

switched on and off at 3–30 Hz; and 4–8-Hz, exponential

PDC). They reported that square-wave PDCs were by far

best at inducing taxis (observed in 36% of the fish vs.

none to 3% for the other currents, including 3% for DC).

Most effective and least injurious of all currents tested

were 3- to 4-Hz, square-wave PDCs with 67 to 75% duty

cycles (168–250-ms pulses). Groody et al. (1950) also observed that the strength of current most effective in producing taxis was inversely related to the size of the fish.

In fresh water, Haskell et al. (1954) tested brown trout (8–

18 cm) in fields of square-wave PDC at frequencies of 60

Hz or less but observed no significant reactions until the

frequency was reduced to about 15 Hz, after which response strength increased as the frequency was further

reduced to 1 or 2 Hz with an 80% duty cycle (800 and 400

ms for latter frequencies). Perhaps as pulse duration increases in high-duty-cycle currents, fish respond more

as normally expected with DC in fresh water. Kolz and

Reynolds (1989b) also failed to observe taxis among goldfish (6–9 cm TL) subjected to 50-Hz, square-wave PDC

(duty cycles of 10, 25, and 50% and pulse widths of 2, 5,

and 10 ms, respectively), but they did document twitch

responses and narcosis; all three responses were observed in DC.

Contrary to the preceding findings, other researchers have reported not only twitch or random movement,

narcosis, and tetany but substantial taxis for PDC frequencies over 20 Hz. Vincent (1971) concluded that with

frequencies at or below 50 Hz, PDC is as effective or more

effective than DC in producing anodic taxis. Based on

experiments with trout (brown or rainbow, 20 cm) at 18° C,

Lamarque (1976) concluded that the optimum PDC frequency for taxis was around 100 Hz (30% duty cycle, 3-

ms pulses), but he noted that lower frequencies might be

better for electrofishing because tetany would be less

likely near the electrode (higher threshold for tetany at

lower frequencies). In contrast, Northrop (1962, 1967)

found that square-wave PDC was most effective at inducing taxis in brown trout (20–25 cm) when operated at

33 Hz with a 67% duty cycle (20-ms pulse width) and that

fish were immediately stunned and showed no significant electrotaxic behavior when subjected to 100-Hz PDC

with a 50% duty cycle (5-ms pulse width). Perhaps when

using 100-Hz PDC, Northrop’s effective zones for both

taxis and narcosis were so large, so distant from the anode, that he only observed and netted narcotized fish;

that is, taxis may have occurred beyond his range for

observing and netting fish. Based on field observations,

Sharber (personal communication) found that taxis in

square-wave PDC is not only evident at 60 and 30 Hz, but

also much better than at 15 Hz (duty cycles of 24, 12, and

6%, respectively; pulse width 4 ms each). Based on mean

times for rainbow trout to swim toward the anode and

succumb to narcosis in raceway experiments, Sharber et al.

(1994) reported that 30-Hz and 60-Hz PDCs (as well as

CPS) were equally effective for taxis in rainbow trout (25–

35 cm). Bird and Cowx (1993), unlike Kolz and Reynolds

(1989b), documented taxis, as well as narcosis, in goldfish ( = 16 cm) under a variety of PDC waveforms and

frequencies (30–600-Hz square, 50-Hz quarter-sine, 50-

Hz exponential; pulse widths 0.2–30 ms). Ruppert (1996)

and Ruppert and Muth (1997) also observed taxis in juvenile 5- to 10-cm humpback chub and bonytail subjected

to 30-, 60-, and 80-Hz PDC, as well as CPS (4-, 4-, 5-, and

2.6-ms pulses, respectively). Similarly, Meismer (1999)

documented taxis in adult rainbow trout and large subadult Colorado pikeminnow subjected to 60-Hz, squarewave PDC, as well as 15-Hz PDC and CPS.

Taylor et al. (1957), using a triangular PDC waveform

and a fixed duty cycle of 33%, not only observed taxis in

rainbow trout (20 cm) at frequencies as high as 120 Hz,

but also reported lower thresholds for strong taxis at 48

to 120 Hz (0.33–0.25 Vp/cm) than at 36, 24, and 12 Hz (0.48,

0.78, and 0.87 Vp/cm, respectively). A similar inverse

relation between frequency and voltage-gradient

0
42 INFORMATION AND TECHNOLOGY REPORT--2003-0002

thresholds was observed for narcosis. Although

differences were sometimes not very great (and probably

not significant), tendencies for similar inverse relations

were reported by Taube (1992) for narcosis (no data for

taxis) in adult rainbow trout subjected to homogeneous

fields of 30- and 60-Hz PDC, Ruppert and Muth (1997) for

taxis and narcosis in juvenile bonytail and humpback chub

subjected to 30-, 60-, and 80-Hz PDC, and Meismer (1999)

for twitch, taxis, and narcosis in adult rainbow trout and

in large subadult Colorado pikeminnow subjected to 15-

and 60-Hz PDC. Taylor et al. (1957) also reported that for

currents of the same pulse frequency, those with greater

duty cycles (47 and 88%, resulting from greater pulse

widths) also had higher thresholds for taxis and were

therefore less efficient at inducing taxis (smaller effective

ranges from anode). This relation between duty cycle (or

pulse width) and taxis thresholds for currents of the same

pulse frequencies has not been reported by other

investigators. However, data by Kolz and Reynolds

(1989b), who assessed response thresholds for goldfish

subjected to 50-Hz PDC with duty cycles of 10%, 25%,

and 50% and failed to observe taxis, suggest no similarly

consistent relation for either twitch or stun thresholds.

As might be expected based on the above discussed

relation between threshold levels and PDC frequency,

some researchers have found PDCs less than 20 Hz to be

less effective for taxis and capture of fish than higherfrequency PDCs. Northrop (1967) reported poor taxis for

frequencies of 10 Hz or less and Sharber (personal communication) suggested that taxis using 15-Hz PDC is unsatisfactory for effective electrofishing.

Like low-frequency PDCs, CPS, with its train of three

240-Hz pulses delivered 15 times per second, has also

established a reputation for poorer performance than

higher-frequency PDCs. In one-on-one boatelectrofishing comparisons in Alaskan streams, Taube

(1992) reported catch rates 56 to 68% lower for CPS than

DC or 25-Hz PDC, but he failed to report whether peak

outputs or field strengths were the same. Ruppert and

Muth (1997) reported higher thresholds for taxis in juvenile humpback chub and bonytail subjected to CPS than

30-, 60-, or 80-Hz PDCs. Similarly, Meismer (1999) found

twitch, taxis, and narcosis thresholds for adult rainbow

trout and subadult Colorado pikeminnow generally much

higher under CPS than 60-Hz PDC but comparable to

those under 15-Hz PDC and DC. Perhaps the pulse trains

of CPS are physiologically similar to single pulses in lowfrequency PDC; if so, fish might be expected to respond

to CPS as if the current were a simple 15-Hz PDC. However, despite reporting taxis thresholds similar to those

for 15-Hz PDC, Meismer (1999) noted that once established, taxis under CPS did not appear to be quite as strong

as under either 15 and 60-Hz PDC.

As noted above and contrary to Meismer’s (1999)

observation, Sharber et al. (1994) conducted raceway time

trials with rainbow trout and concluded that taxis is comparable under 30-Hz PDC, 60-Hz PDC, and CPS, but they

failed to note that field intensity under CPS had to be

about 20% greater to initiate that taxis (Sharber, personal

communication). Consistent with this observation, Meyer

and Miller (unpublished manuscript, 1991; also Wyoming

Game and Fish Department, 1991) used output voltages

about 20 to 25% higher for CPS (460–470 V) than 40-Hz

PDC (370–390 V) to maintain comparable sampling efficiency. In a set of heterogeneous-field experiments, Taube

(1992) doubled output voltage for CPS to elicit

electrofishing responses comparable to those for DC and

20- to 60-Hz PDC. Whether increasing field intensity

would similarly improve taxis and capture efficiency under simple low-frequency PDCs (e.g., 15 Hz) has not been

documented.

Electrofishing efficiency apparently varies with species, habitat, and timing as well as the electric field. Although Pugh and Schramm (1998), like other investigators

discussed above, found 15-Hz PDC often less effective

for capture of some species (especially shad, Clupeidae)

and generally took fewer specimens overall than 60-Hz

PDC in the lower Mississippi River, they reported that 15-

Hz PDC actually captured a slightly greater diversity of

species (35 species vs. 33 species for 60-Hz), was nearly

as effective for many species, and was usually more effective for flathead catfish and blue catfish than 60-Hz

PDC. Vincent (1971) concluded that DC is the best current for capture efficiency in rivers with brushy bank cover

or high turbidity, whereas PDC is best in large open rivers

with less bank cover and clearer water or in waters that

are too conductive for effective use of DC. He also observed that a hybrid DC-PDC current (e.g., Fig. 5J) has

qualities intermediate to DC and PDC, implying that it

might be a good compromise. Roach (1992) and Reynolds

et al. (1992) reported capture of three times more northern

pike with 60-Hz PDC (50% duty cycle; 3 fish/h) than with

either DC (0.9 fish/h) or 30-Hz PDC (25% and 75% duty

cycle; 1.1 and 0.9 fish/h, respectively). Roach (1992) also

noted that there is a general belief that 60-Hz PDC has

better holding power than 30-Hz PDC and that when

electrofishing conditions for capture of northern pike are

ideal (timing), capture rates can be as high as 30 per hour

using 60-Hz PDC (Roach, personal communication).

Results — Harmful Effects of Results — Harmful Effects of

Electrofishing on Fish Electrofishing on Fish

Possible detrimental effects of electrofishing on

individual fish include cardiac or respiratory failure, injury,
 SNYDER 43

stress, and fatigue. Mortality can be immediate or delayed.

Small fish whose normal behavioral responses are slowed

or inhibited may be more susceptible to predation. Fish

that survive despite electrofishing injury or other adverse

impacts, may suffer short-term, long-term, or lifetime

handicaps that affect their behavior, health, growth, or

reproduction. Significant numbers of surviving but

adversely affected fish may ultimately impact community

structure, population size, quality of the fishery resource,

and management strategies. Harmful effects on fish

(except embryos) reported in published literature, agency

reports, and personal communications are summarized

by species in Appendix B. For most pertinent

investigations discussed in the remainder of this review,

Appendix B also includes (if provided by the source)

selected specimen, environmental, and electrical data that

might not be referenced in the text.

In most cases, the harmful effects of electrofishing

can be traced to one of two causes–excessive exposure

to high intensity portions of electric fields resulting in

tetany or aspects of electric fields that result in sudden

and powerful, but unsustained, contractions of the body

musculature, sometimes referred to as myoclonic jerks or

seizures. The field characteristics and specific mechanisms responsible for the muscular convulsions have not

been conclusively identified, but field intensities for these

responses apparently extend well below those for tetany,

perhaps even beyond the threshold for taxis in DC and

PDC. Injuries due to such seizures are generally classified as spinal injuries but may include damage to tissues

or organs not associated with the vertebral column (or

notochord in cartilaginous fishes).

Effects Other Than Spinal

and Related Injuries

Among nonspinal injuries, the most extreme would

probably be electrocution when fish are sufficiently exposed to very high voltage gradients. In humans and

other mammals, fibrillation of the heart and death by cardiac arrest are common results of exposure to strong electric currents, but electrofishing mortalities are generally

rare, and such effects in fish are inadequately documented

in the published literature. Northrop (1962, 1967) suggested that “temporary” cardiac arrest might occur in electrically narcotized (perhaps tetanized) fish, whereas Kolz

and Reynolds (1990b) stated that cardiac arrest is seldom

a factor in fish mortality. However, neither evidence nor

references were provided to support either statement.

Based on an experiment with tetanizing DC on a rainbow trout, Taylor et al. (1957) reported that although they

observed an arrhythmia (an extra beat followed by skipped

beats) when the current was initially applied, normal heart

beats quickly resumed as the current continued to be

applied. They concluded, based on this one experiment,

that the heart was not severely affected by electrofishing

currents. However, the kymogram in their paper indicates

that the current was interrupted momentarily after its initial application, skipped heart beats continued during that

interruption, and normal beats resumed only after the

current was reestablished. The events in Taylor et al.’s

(1957) experiment are open to alternative interpretations,

none of which can be effectively supported by a single

kymogram. Perhaps cardiac arrest had indeed occurred,

and the next impulse was required to start the heart again.

In any case, the effects of an electric field on a fish’s heart

might be different using PDC or AC.

In experiments by Schreck et al. (1976) recovery of

normal heart activity required much more time. Their fish

also exhibited irregular cardiac activity immediately after

being shocked (probably tetanized) with DC but required

4 to 5 min to return to normal. For two fish that were

shocked for 45 and 60 s and failed to resume respiration,

heart beats initially appeared to recover, then decreased

in frequency and amplitude, and finally ceased in about

15 to 25 min (probably due to lack of oxygen).

The visceral organs of fish may also be affected by

electric fields. Shparkovskij and Vataev (1985) stimulated

the brain of Atlantic cod using square-wave PDC of 0.1 to

0.5 mA and a “burst” frequency of 300 Hz (in this case,

the meaning of “burst” is uncertain). When lateral areas

of hindbrain and midbrain were stimulated, peristalsis of

the stomach and gut was inhibited. When the rostral cerebellum was stimulated, muscle contraction of the digestive tract was accelerated. Marriott (1973) described two

ripe female pink salmon that had been electrocuted with

110-V, 60-Hz AC as having severely ruptured internal organs. However, Taylor et al. (1957) compared sections of

various organs and tissues from an electrocuted rainbow

trout with those from an untreated trout and reported no

abnormalities.

Bleeding from the gills was perhaps first reported as

an electrofishing injury by Hauck (1949) in his description

of injuries to rainbow trout. Barham et al. (1989b) reported

bleeding from the gills of many common carp narcotized

with either 50-Hz AC or 50-Hz, half-sine PDC. However,

this injury seems to be particularly prevalent in mountain

whitefish electrofished in Montana regardless of the type

of current or equipment used (Fredenberg, personal

communication). According to Fredenberg (personal

communication), “it is not unusual, on some streams, to

see literally dozens of mountain whitefish come to the

electrode under taxis with blood streaming in the water.”

The injury apparently occurs at field intensities much

less than required for narcosis. However, neither the
44 INFORMATION AND TECHNOLOGY REPORT--2003-0002

specific cause of this injury nor its relation to other types

of electrofishing injuries or subsequent survival has been

investigated.

Walker et al. (1994) reported hemorrhages in both

paired and median fins of juvenile northern pike exposed

to 50-Hz, triangular-wave AC but not 50-Hz, sine-wave

AC or 50-Hz PDC. Such injuries are likely often overlooked and may be more common than reported in the

literature.

Respiratory failure is probably the ultimate cause of

mortality in most electrically stunned fish. Because respiration may be reduced in partially narcotized fish and

effectively ceases in fully narcotized or tetanized fish,

those that are stunned but not removed from the electric

field soon enough will likely die of asphyxiation. Synaptic fatigue occurs when fish are overexposed to a tetanizing current and results in a continuation of tetany for an

extended period after removal from the field, a condition

referred to as post-tetanic potentiation (Lamarque, 1990).

Schreck et al. (1976) observed that after the current was

switched off, tetanized rainbow trout either did not resume breathing movements for 60 s or they “coughed”

violently for the first 30 s. Once respiratory movements

resumed, hypoxic conditions were addressed by substantially increased buccal pressure rather than breathing frequency. However, other biologists reported increases in

respiratory rates during recovery (e.g., Kraiukhin and

Smirnova, 1966; Kynard and Lonsdale, 1975). Kolz and

Reynolds (1990b) noted that oxygen debt can take hours

to pay back. Respiratory failure in eels, and perhaps certain other fishes, can also be caused by a suffocating

excess of mucus produced on the gills while under the

influence of an electric field (Lamarque, 1990).

Stunned fish should be removed quickly from the

electric field and placed in an uncrowded tank or pen with

fresh, well-oxygenated water for recovery. Chmielewski

et al. (1973) noted that trout not breathing (through the

gills) for 5 min have little chance of survival without

artificial respiration (e.g., moving fish back and forth or

otherwise pumping or forcing fresh, oxygenated water

over the gills). Based on experiments with brown trout,

they reported that reestablishment of equilibrium and

normal respiratory movements usually required under 1

to 2 min and that recovery time increased with field

intensity and fish length but decreased with successive

exposures (indicating decreased sensitivity to the electric

field). For rainbow trout ( = 126 g) exposed for 20 s to 60-

Hz PDC at a field intensity sufficient to induce tetany

within 2 to 3 min, Mitton and McDonald (1994a) reported

ventilation recovery times averaging 19 s but sometimes

up to 3 min after removal from the current. Northrop (1967),

however, noted that recovery from AC-induced

electronarcosis (probably tetany) is relatively slow, taking

as long as 5 to 10 min for some larger species. Schreck

et al. (1976) noted a similar “apparent” recovery time for

yearling hatchery-reared rainbow trout subjected to DC.

Adams et al. (1972) narcotized 5- to 9-cm-TL common

shiners, with 5- to 30-s exposures in homogenous fields

of DC at about 1.5 to 3.6 V/cm and found that recovery

times increased with field intensity, exposure time, and

length of the fish. Shiners requiring over 2 min for recovery

frequently died. In a set of homogeneous-field

experiments, Bird and Cowx (1993) exposed goldfish ( =

16 cm) to 5 s of DC and various frequency, duty cycle,

and types of PDC at a fixed field intensity of 1.1 Vp/cm.

They reported that recovery of breathing motions

(recovery of equilibrium not noted) was immediate for

DC, and variously delayed 4 to 45 s for the various PDCs

tested. Among the latter, respiratory-movement recovery

times were greatest for 50-Hz, 25%-duty-cycle (5-ms

pulses), quarter-sine PDC and shortest for the highestfrequency (400- and 600-Hz), highest-duty-cycle (90%,

2.3- and 1.5-ms), square-wave PDCs (6 and 4 s), and

intermediate for all other PDCs tested (13 square-wave

and one exponential, Appendix B). For square-wave PDCs

with frequencies of 100, 400, and 600 Hz, breathingmovement recovery times decreased with increasing pulse

frequency when duty cycle was 10% or 90% but remained

the same as for lower frequencies when duty cycle was

50%.

Stress and fatigue are physiological responses that

disrupt physicochemical balance, osmoregulatory functions, and normal behavior but usually require only a

short time for recovery. According to Vibert (1967b),

Halsband reported that the average duration of residual

effects after fish were stunned and removed from an electric field was 20 min for exponential (capacitor or condenser-discharge) PDC, 60 min for DC, and 120 min for

AC. However, full physiological recovery can require more

than 6 h for electrofished rainbow trout (Schreck et al.,

1976) or 24 h or longer for other species (Whaley et al.,

1978; Barton and Dwyer, 1997). Some species are so sensitive to certain stresses that recovery can take weeks or

months (e.g., handling and confinement stresses in some

sharks–Smith, 1991). Stress can be so great, or fish so

sensitive, that some fish eventually die. Because of the

effects of electrofishing on blood chemistry, the U.S. Environmental Protection Agency recommended that

electrofished specimens not be used in physiological or

bioassay studies (Weber, 1973, according to Emery, 1984).

In response to tetany in a DC field, Schreck et al.

(1976) reported immediate increases in blood

concentrations of plasma corticoid (adrenal hormones,

steroids), lactate or lactic acid (by-product of anaerobic

muscular activity), and thrombocytes (white blood cells

instrumental in blood clotting) in yearling, hatchery-reared

rainbow trout. Increases in thrombocytes might be at least

partially a response to tissue trauma, minor bleeding, or

0

0
 SNYDER 45

hemorrhage. Blood glucose exhibited a delayed response,

not increasing significantly until after lactic acid levels

returned to normal, about 3 h after being tetanized. Schreck

et al. (1976) found no immediate effect on blood levels of

packed cells (hematocrit), plasma protein, calcium,

magnesium, or androgen, nor did they find any effect on

electrophoretic patterns of 13 tested isoenzyme systems

(proteins often used in systematic analyses).

Other biologists also reported rapid increases in

plasma cortisol in shocked fish. For rainbow trout ( =

113 g) exposed to 20 s of homogeneous 60-Hz PDC (240–

270 μS/cm; 16° C), Mitton and McDonald (1994a) found

that plasma cortisol increased more than two-fold and

lactate about six-fold within the first 1 h then returned to

near resting levels by 8 h and 4 h, respectively, after exposure. Maule and Mesa (1994) exposed juvenile chinook

salmon ( = 8 g, 9 cm FL) to 1.5 s of 120-Hz PDC (73 μS/

cm; 13° C) and reported that plasma cortisol remained the

same in survivors (16% and 25% mortality in test tanks)

as in controls for fish sampled within 4 s of exposure but,

as reported by Schreck et al. (1976) and Mitton and

McDonald (1994a), rose rapidly within the next 15 min,

continued to rise to a peak (four to five-fold) by 1 h after

treatment, then declined gradually to control levels by

6 h. Barton and Dwyer (1997) also reported an increase in

plasma cortisol to a peak during the first hour, but the

increase was greater than ten-fold for juvenile bull trout

subjected to 10 s of either 60-Hz PDC at a lethal field

intensity (calculated as possibly 2.8 Vp/cm; ambient conductivity 219 μS/cm; 9° C) or 60-Hz PDC or DC at a lower,

non-lethal intensity (possibly 1.3 or 1.4 Vp/cm). Fish subjected to the non-lethal fields recovered from narcosis

within about 1 min and showed no external signs of injury (Barton, personal communication) but required at

least 24 h for plasma cortisol levels to gradually return to

pre-shock levels, a much longer time than reported by

Maule and Mesa (1994) and Mitton and McDonald (1994a).

Like Schreck et al. (1976), Mitton and McDonald

(1994a) also reported that plasma glucose rose more slowly

and less extensively (60% increase) to a peak about 4 h

after treatment, then gradually returned to resting levels

in 8 h. Unlike Schreck et al. (1976) and Mitton and

McDonald (1994a), Barton and Dwyer (1997) found that

plasma glucose in shocked fish rose immediately (within

the first hour) to about twice pre-shock levels, then remained at that raised level for the remainder of a 24-h

monitoring period.

Other physiological indicators of stress also have

been investigated. Mitton and McDonald (1994a) reported

immediate increases in catecholamines (greater than threefold from non-detectable levels), metabolic acid, and

carbon dioxide (about two-fold) and a decrease in pH

(more than 0.2 units). Catecholamines and carbon dioxide

returned to resting levels within an hour or two, whereas

metabolic acid and pH overshot their return to resting

levels within that time but stabilized back to near resting

levels within 8 h after treatment. Burns and Lantz (1978)

reported results similar to those of Schreck et al. (1976)

for lactate, hematocrit, and plasma protein in adult

largemouth bass. They also tested electrofishing effects

on hemoglobin concentrations in the blood and the

percentage of water in muscle tissue but found no

differences from control fish or changes during a 19-h

period after electrofishing. Contrary to the findings of

Burns and Lantz (1978) and Schreck et al. (1976), Bouck

and Ball (1966) reported changes in plasma protein

concentrations (and composition) in rainbow trout

captured by electrofishing, as well as by seining and hookand-line fishing.

Based on significantly lower levels of plasma cortisol and glucose in juvenile bull trout subjected to handling stress (30 s aerial exposures in a dip net) and controls

that were transferred between tanks, Barton and Dwyer

(1997) concluded that the physiological stresses of DC

and 60-Hz PDC electrofishing are significantly greater

than handling stresses. Mitton and McDonald (1994a)

compared the physiological effects of combined electroshock and 1 min of aerial exposure with electroshock

only for rainbow trout and reported similar elevations of

cortisol and glucose; significant increase in catecholamines; non-significant increases in lactate, carbon dioxide, and metabolic acids; and decrease in pH. In another

experiment, they monitored survival of rainbow trout exposed to the combined stress of 20 s of 60-Hz PDC and

up to 4 min aerial exposure and reported no fatalities during the next 2 weeks.

All capture methods are stressful to some degree

(Wydoski, 1980). Schreck et al. (1976) concluded that

stress induced by electrofishing is similar to that caused

by hypoxia and extreme muscular activity. Similarly, Mitton

and McDonald (1994a) emphasized that in salmonids,

stress response to electrofishing is comparable in magnitude to other acute stressors such as handling and exhaustive exercise, including those resulting from capture

by angling. On the other hand, and as noted above, Barton

and Dwyer (1997) found the stress caused by

electrofishing to be significantly greater than handling

stress. Stresses can be cumulative; electrofishing stresses

added to existing environmental stresses (e.g., pollution)

can increase mortality significantly over either alone

(Wydoski, 1980). Increased mortality can occur directly

as a result of stress and fatigue or indirectly through

greater susceptibility to predators, disease, and parasites.

In some cases, delayed, stress-related mortality may be

more significant than immediate electrofishing mortality.

Injury-related stresses may persist and affect the fish’s

physiology, behavior, growth, and reproduction for a

long time.

0

0
46 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Mortality, stress, and some injuries can result as much

from poor, improper, or careless handling after capture as

from electrofishing itself (Hudy, 1985; Barrett and

Grossman, 1988). Because stress can also be induced by

confinement, fish not being held for longer-term observation should be released as soon as possible after recovering equilibrium and normal respiration. Earlier release

might make them especially easy prey for predators

(Whaley, 1975; Whaley et al., 1978). Waiting until equilibrium and respiration are adequately reestablished also

allows more opportunity to observe, document, and aid

injured or distressed specimens. If undesirable effects

are observed, electrofishing procedures should be adjusted to minimize those effects. Emery (1984) suggested

adding salt (1.5%) and a light anesthetic to the holding

water to help fish replace lost ions and reduce additional

stress. However, if the anesthetic slows recovery of respiration in fish that have been tetanized, it might do more

harm than good. Eloranta (1990) reported that recovery

of electrofished specimens was slower and mortality (70–

80%) significantly higher in unaerated containers treated

with MS-222 (tricaine methanesulfonate) than in containers without the anesthetic.

Electrofishing also affects subsequent fish behavior. Mesa and Schreck (1989) observed that rates of feeding

and aggression decreased in hatchery-reared and wild

cutthroat trout immediately after the they were

electrofished and marked in an artificial stream. In a natural stream, they reported that similarly electrofished and

marked wild trout immediately sought cover, remained

relatively inactive, did not feed, and were easily approached by a diver. An average of 3 to 4 h was required

for 50% of the fish to return to normal behavior. In contrast, fish that remained uncaptured in the same section

of the stream, even after successive passes, exhibited

little change in normal behavior. Either uncaptured fish

were insufficiently affected by the electric fields, or handling and marking of captured fish were responsible for

differences in behavior. Callahan (1996) reported reduced

feeding by large and small bluegill for up to 5 h after

being electrofished. In associated predator experiments

in a 2.4 m diameter pool, he found small bluegill more

likely to be eaten by largemouth bass immediately after

being shocked than unshocked bluegill but that differences in susceptibility to predation decreased with time

and after 10 min shocked bluegill recovered sufficiently

to behave like unshocked bluegill. However, Callahan

suggested that these temporary effects on feeding and

susceptibility to predation would have a negligible effect

on a population. Horak and Klein (1967) experimented

with rainbow trout and found that swimming performance

was significantly reduced in fish captured by

electrofishing. For juvenile rainbow trout (3–12 g, 6–

12 cm) exposed to 20 s of 60-Hz PDC, Mitton and

McDonald (1994b) reported that the reduction in swimming performance was comparable to that for fish forced

to exercise for 5 min. Swimming performance in both

shocked and exercised fish dropped gradually for 1 h

after treatment to 53% of control values then recovered

to near control performance within 2 to 4 h of treatment.

Swimming performance deceased beyond that of exercised fish when shocked fish were subsequently exposed

to air for 1 to 4 min; those fish exposed to air for 4 min

experienced a 62% drop in swimming performance between 0.5 and 1 h after treatment and required more than

6 h for recovery of normal endurance. Fatigue from long

exposure or high-intensity fields can also reduce a fish’s

short-term sensitivity to subsequent exposures

(Chmielewski et al., 1973). Cross and Stott (1975) suggested that electrofished specimens might be less

catchable for the next 3 to 24 h and that this response

could substantially affect population estimates based on

short-term mark-recapture or depletion techniques.

Spinal and Related Injuries

Hauck (1949) provided perhaps the most detailed

description of electrofishing injuries. In a rescue attempt,

503 rainbow trout (0.7–2.3 kg), were electrofished from a

canal (14–21° C) in Idaho using hand-held electrodes and

a portable (truck-mounted), 110-V, 60-Hz, 495-W AC generator. Voltage was set by rheostat at 80 to 90 V, just

enough to momentarily stun fish within 3 m of the electrodes. Hauck (1949) noted that reactions of fish in the

field varied. Respiratory activity increased in all fish, and

most fish experienced at least partial muscular paralysis.

Fish exhibiting partial paralysis swam in an arc around

the electrode (oscillotaxis), whereas those exhibiting total paralysis (probably tetany, including cessation of respiratory movements) would float momentarily on their

sides then sink slowly to the bottom.

Hauck (1949) described the injuries in captured fish

as follows: “A number of fish hemorrhaged from the gills

or vent, or both. Others showed dilated and hemorrhaged

blood vessels in the skin near the vent. Several were observed with the intestine protruding from the vent. Physical contact with the electrode caused the appearance of

dark vertical bars on that area of the fish which touched

the electrode.”

The fish were transported to a nearby hatchery pond

where they were observed for 2 to 5 days before release.

During this time, 131 fish (26%) died either as a result of

electrofishing or subsequent handling. Although not

stated, incidence of injury was probably much higher than

mortality (Reynolds and Kolz in Reynolds et al., 1988).

Hauck (1949) noted that “Paralysis of swimming muscles

persisted in some fish for several days. This loss, or partial

loss, of locomotion would indicate an injury to the nervous
 SNYDER 47

system. The dark, vertical bars remained in evidence. Dead

or dying tissues in the caudal peduncle and caudal fin

appeared on several fish which fact would indicate loss

or impairment of circulation to this region. Several fish

lost their sense of balance.”

Hauck (1949) dissected 10 specimens with representative injuries from among the rescued fish. “One 5-pound

rainbow trout had a fractured sixth caudal vertebra. As a

result of this fracture the haemal artery and vein had ruptured in the seventh caudal vertebra. The breakdown of

circulation of blood at this point caused the death of the

entire body posterior to the injury, including muscles and

skin. Blood clots and hemorrhaging were evident throughout the caudal peduncle, particularly in the region adjacent to the fracture. This fish suffered total paralysis of

the swimming musculature before its death.”

“A 1.5-pound specimen had three fractured vertebrae, the 11th, 29th, and 30th abdominals. Curvature of

the spine appeared through the abdominal vertebrae 18

to 22, and the ligamentous connections between ribs and

parapophyses in this region were broken. This fish also

had blood clots in the afferent branchial arteries and had

hemorrhaged through the membranes of the gill filaments.”

He described 4 more of the 10 fish as having fractured vertebrae or spinal curvature, which he described

as ligamentous fractures. One of these fish had 12 ruptured dorsal (probably segmental) arteries anterior to a

fracture in a single abdominal vertebra. Another, that had

an impaired sense of balance before it was killed, had

bloody fluid in the semicircular canals. Six of the 10 fish

suffered injury in the region of the brain, as evidenced by

dilated blood vessels or blood clots. Hauck (1949) suggested that the latter brain injuries might have been secondary to electrofishing, perhaps caused by collisions

with rocks or other structures. He concluded his 1949

publication with the suggestion that further investigations on the injurious effects of electrofishing were needed

before the technique was widely employed in fishery

management.

Nature of the Injuries

Compressed, broken, or misaligned vertebrae and

related electrofishing injuries, including separated or damaged ribs, damaged swim bladders, ruptured dorsal and

haemal arteries, and other internal hemorrhages (Figs. 16–

18), are believed to be caused by momentary but powerful convulsions of the body musculature. Bleeding at the

vent could be caused by related damage to the viscera,

but bleeding at the gills is probably a separate phenomenon. Lamarque (1990) suggested that such convulsions

are the result of direct excitation of the muscles (perhaps

via motor nerves) and “hyper-reflexivity.” Sharber et al.

(1994, 1995) and Sharber and Black (1999) surmised that

these convulsions or myoclonic jerks are random seizures

similar to those sometimes experienced by people with

epilepsy or subjected to electroconvulsive therapy before chemicals were available to block stimulation of motor neurons.

Myoclonic jerks or seizures are thought to occur simultaneously, or nearly so, on both sides of the body,

thereby subjecting the vertebral column to opposing

forces that can break, crush, or dislocate the vertebrae

(Lamarque, 1990; Sharber et al., 1994; Sharber and Black,

1999; Figs. 16 and 18). Stewart (1967; according to

Lamarque, 1990) reported that spinal injuries by DC (perhaps actually PDC) are primarily compression fractures,

whereas those produced by AC are primarily

misalignments. However, Hollender and Carline (1994)

reported that among brook trout electrofished with 250 to

300-Hz AC or 60-Hz PDC, the frequency of compressiononly injuries was greater in AC, whereas the frequencies

of fractures, complete separation of vertebrae, and combinations of vertebral misalignments and compressions

were similar in both types of current. Using PDC, Sharber

and Carothers (1988, 1990) and Fredenberg (1992) observed both compression fractures and misalignments.

Comparing DC and several PDCs, Fredenberg (1992) concluded that there were no notable differences in the types

of injuries caused by the various currents, only differences in their frequency and severity; he particularly noted

that misalignments were relatively rare in DC. Like

Fredenberg (1992), Dalbey (1994) and Dalbey et al. (1996)

reported substantially greater incidences of spinal injury

among rainbow trout captured with PDC (54%) or a hybrid of PDC over DC (40%, Fig. 5J) than with DC (12%),

but most of the differences were manifest in a substantially greater percentage of fish having less severe spinal

damage (compression between vertebrae and misalignment) when exposed to PDC or the hybrid current (44%

and 34%, respectively, vs. 6% for DC). As a result, the

percentages of fish afflicted with the most severe spinal

damage (fractures of vertebrae, Fig. 18, or complete separation of two or more vertebrae, Fig. 16) were similar for all

three currents (6–10%).

Electrofishing-induced vertebral damage is usually

accompanied by ruptured blood vessels, torn muscles or

ligaments, and perhaps other soft-tissue damage (Fig. 17;

Hauck, 1949; Taylor et al., 1957; Spencer, 1967a; Sharber

and Carothers, 1988, 1990; Holmes et al., 1990; Wyoming

Game and Fish Department, 1990; Fredenberg, 1992).

However, Holmes et al. (1990), Fredenberg (1992), and

others also observed hemorrhages along the spine or in

the musculature without apparent corresponding damage

to vertebrae. Sometimes the incidence of such

hemorrhages was much greater than the incidence of

obvious vertebral damage. Grisak (1996) found that the
48 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Fig. 16 Fig. 16. Dorsal- (top) and lateral-view (bottom) X-rays of a rainbow trout (Oncorhynchus mykiss) revealing spinal

misalignment and fractured vertebrae caused by electrofishing. (Photographs provided by and reproduced with the

permission of N.G. Sharber, Coffelt Manufacturing, Inc., Flagstaff, Arizona.)
 SNYDER 49

relative incidence of vertebral damage and muscular

hemorrhages varied among species and with different

electrical currents. For goldeye collected by DC, he

reported incidences of 21% for each type of injury, but

for goldeye taken by 40-Hz PDC, he reported only 4%

with spinal damage and a high of 39% with hemorrhages.

All but two goldeye had only muscular hemorrhages (all

class 1 or 2—Table 3) or only vertebral damage (nearly all

class 1). Among other species injured with 40-Hz PDC,

Grisak (1996) reported that all flathead chub injuries were

spinal damage (8%) and that twice as many river

carpsucker injuries were spinal damage (18%) than

hemorrhages (9%). Among controls also X-rayed and

necropsied (105 fish collected with other sampling gear),

Grisak (1996) reported only one fish (a goldeye) with a

fresh internal injury, a spinal compression (class 1).

Ruppert (1996) and Ruppert and Muth (1997), subjected

juvenile humpback chub and bonytail (5–10 cm TL; n =

390) to a variety of PDCs at thresholds for taxis, narcosis

and taxis, and reported hemorrhages associated with the

spine in 13% of the fish (up to 27% for individual treatment

means) but no apparent vertebral damage; only one of

120 control fish suffered an internal hemorrhage. To a

lesser extent, the reverse situation, vertebral damage

Fig. 17 Fig. 17. Necropsy fillets of rainbow trout (Oncorhynchus mykiss) revealing hemorrhages and associated tissue and

vertebral damage caused by electrofishing, top showing multiple injuries. (Photographs provided by and reproduced

with the permission of N.G. Sharber, Coffelt Manufacturing, Inc., Flagstaff, Arizona.)
50 INFORMATION AND TECHNOLOGY REPORT--2003-0002

without associated hemorrhages, has been observed in

other studies. Among brook trout subjected to AC or

PDC, Hollender and Carline (1994) reported two to three

times more incidences of spinal damage without

hemorrhages (16% for AC, 11% for PDC) or hemorrhages

without vertebral damage (10% for AC and PDC) than

incidences of both in the same fish (5% for AC, 4% for

PDC). Fredenberg (1992) observed that when only

hemorrhages or damaged vertebrae were detected, the

injuries were usually minor to moderate, but Hollender

and Carline (1994) found most such injuries to be

moderate to severe.

Electrofishing-induced spinal injuries can occur anywhere along the spinal column, including immediately

behind the head, but most have been observed near or

posterior to the middle of the spine. Predominant location varies with species. Spinal injuries in Salmoninae are

most frequently located near or between the dorsal or

pelvic fins and the anal fin (Sharber and Carothers, 1988,

1990; Meyer and Miller, unpublished manuscript, 1991;

Fredenberg, 1992; Hollender and Carline, 1994; Kocovsky

et al., 1997), whereas those in centrarchids and ictalurids

are predominantly located in the caudal region, posterior

to the vent (Spencer, 1967a). Among ripe razorback sucker

(an endangered species) experimentally shocked by Muth

and Ruppert (1996), most injured fish had spinal hemorrhages near the origin of the dorsal fin. The only fish with

an obviously damaged spine had two vertebral injuries

(class 2 and 3) with associated hemorrhages, one located

just posterior to the dorsal fin and the other slightly behind the anal fin. Ruppert and Muth (1997) reported that

most spinal hemorrhages observed in humpback chub

and bonytail were located between the dorsal and caudal

fins. Grisak (1996) reported most spinal damage and hemorrhages for goldeye and cypriniform fishes taken in the

Missouri River occurred in the middle and posterior portions of the body.

The number of vertebrae involved in each incident

of spinal damage varies considerably, from one to as many

as 20, depending on species and severity of the injury.

For example, Hollender and Carline (1994) reported that

most spinal injuries in brook trout subjected to AC or

PDC involved five to seven vertebrae with an extreme

range of two to 18. Spinal dislocations, fractures, or both

reported for large rainbow trout by Sharber and Carothers

(1988, 1990) involved a mean of eight vertebrae. For

warmwater fishes, Grisak (1996) reported that most spinal

injuries involved three to nine vertebrae. Fredenberg

(1992) found that misalignments in trout typically involved

two to five vertebrae among a larger series of compressed

vertebrae.

Multiple injuries, especially spinal hemorrhages, are

quite common (Fig. 17 top). Ruppert and Muth (1997)

reported a mean of three and up to eight hemorrhages for

individual juvenile humpback chub and bonytail injured

by electric fields. Over 60% of injured trout examined by

Fredenberg (1992) and his associates were characterized

by two or more hemorrhages, with up to eight in one

specimen; multiple, well-spaced vertebral injuries were

also common. Among electrofished rainbow and brown

trout X-rayed by Meyer and Miller (1990), up to 41%

(50% of injured fish) experienced two spinal injuries and

up to 11% (14% of injured fish) had three injuries.

However, in the next year they successfully X-rayed 220

electrofished trout and observed spinal injuries in 17% of

those trout but only one fish with multiple injuries

(Wyoming Fish and Game Department, 1991). For annually

electrofished salmonids examined by X-ray and found to

have either old (healed) or new spinal injuries or both,

Fig. 18 Fig. 18. Fractured vertebrae from a rainbow trout

(Oncorhynchus mykiss) caused by electrofishing.

(Photograph provided by and reproduced with the

permission of W.A. Fredenberg, Montana Department of

Fish, Wildlife, and Parks.)
 SNYDER 51

Kocovsky et al. (1997) reported that 77% had one injury,

17% had two injuries, and 6% had three injuries (also that

56% were class 1 injuries, 19% class 2, and 25% class 3).

Some of the multiple injuries they reported had

accumulated from prior years of electrofishing.

Fredenberg observed that multiple hemorrhages

frequently alternated from side to side, sometimes in

evenly spaced patterns. Explanations for multiple

hemorrhages occurring on only one side or alternating

side to side, have yet to be studied. If multiple injuries are

the result of multiple, temporally separated seizures and

all myomeres contract simultaneously, it would be logical

to expect that the weakest portion of the spine (that which

is already injured) would be most susceptible to

subsequent injury. However, multiple injuries at different

locations might be likely if the nerves associated with the

original injury were also damaged or otherwise made, at

least temporarily, nonfunctional (i.e., no longer subject to

stimulation or overstimulation). Alternating side-to-side

hemorrhages might be the result of successive

convulsions as the fish bend from side to side during

taxis towards the anode. If multiple injuries result from

single convulsive events, perhaps the muscular

contractions sometimes differ in strength or intensity on

each side of the body or in different regions of the body.

Detection and Evaluation of the Injuries

Participants in a special session on electrofishing

injuries, held at the June 1991 meeting of the Western

Division of the American Fisheries Society (Bozeman,

Montana), considered, modified, and agreed on a set of

procedures and criteria recommended by J. B. Reynolds

for standard documentation of the presence and severity

of damage to the spine and associated hemorrhages

(Table 3). These procedures and criteria have since been

detailed in many publications, including Reynolds (1996).

By these criteria, vertebral damage (usually based on Xrays) and hemorrhages (based on clean fillets of muscle

tissue along the spine) are separately ranked from zero to

three according to severity. Since its introduction, this

severity rating system for spinal injuries and hemorrhages

has been used by many biologists (e.g., Roach, 1992;

Taube, 1992; Dalbey, 1994; Hollender and Carline, 1994;

Table 3. Procedu Table 3 res and criteria for documenting damage to fish spinal columns and associated hemorrhages. From

Box 8.2 in Reynolds, 1996.

 Procedures Criteria

Spinal damage

Fish should be dead or anesthetized to insure good resolution on X-ray 0 - No spinal damage apparent

negatives. Photograph (X-ray) the left side of each fish, positioning it 1 - Compression (distortion) of

to include all vertebrae. Photographs (X-rays) from the dorsal aspect vertebrae only

also may be necessary to clarify the injury rating. X-rays of two or more 2 - Misalignment of vetebrae, includfish per plate will save money. Record the position of every affected ing compression

vertebra, counting the first separate vertebra behind the head as number 1. 3 - Fractures of one or more vetebrae

Rate the worst damage to the spine. or complete separation of two or

more vertebrae

Internal hemorrhage

Fish should be killed within 1 h after capture and either frozen or held on 0 - No hemorrhage apparent

ice to allow clotting in blood vessels. Fish should not be filleted 1 - Mild hemorrhage; one or more

immediately after death because fillet-related bleeding will mask injury- wounds in the muscle, separate

related hemorrhages. Fillets should be smoothly cut close to rays and spine from the spine

and through the ribs and back to the caudal peduncle. Rate the injury from 2 - Moderate hemorrhage; one or more

the actual specimen, then photograph the worst side of fish with the fillet small wounds on the spine (< width

inside up (color slides are best for follow-up evaluation). Rate the worst of two vertebrae)

hemorrhage in the muscle mass. 3 - Severe hemorrhage; one or more

large wounds on the spine (> of

two vertebrae)
52 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Thompson, 1995; Ruppert and Muth, 1997). Fredenberg

(1992) used the criteria and reported that despite cases in

which hemorrhages were observed without corresponding

identification of vertebral damage, and vice versa, severity

ratings for damage to the spine and hemorrhages in

associated tissues were reasonably similar, and severe

injuries were nearly always detected as such by both

criteria. Although the procedures originally recommended

by Reynolds specified only lateral-view X-rays,

Thompson et al. (1997a) suggested use of dorsal- as well

as lateral-view X-rays to facilitate interpretation of the

nature and severity of spinal injuries (Fig. 16). Fredenberg

(1992) observed that less severe hemorrhages are often

only visible on one side of the spine and suggested that

necropsy procedures should include fillets of both sides.

Bent or curved backs, bleeding, or brands may be

obvious signs of internal injuries, but such external signs

of spinal and related internal injuries are often absent.

Injured fish often look fine and appear to behave normally. When external manifestations of present or past

(healed) injuries are present, they usually indicate that

internal injuries are or were relatively severe (Kocovsky

et al., 1997).

Brands, sometimes referred to as bruises or burn

marks, are particularly obvious indications of injury

(Fig. 2). They can result from direct contact with or proximity to the electrode, but also appear on fish netted some

distance from an electrode (Lamarque, 1990). Although

Lamarque (1990) noted that some brands may be true

burns from direct contact with an electrode, he, Emery

(1984), Fredenberg (1992), and Sharber and Black (1999)

suggested that most brands are discolorations of the skin

due to the dilation of skin melanophores, possibly as a

result of sympathetic nerve damage or stimulation.

Reynolds (1996) agreed that at least blotchy, irregularshaped marks are probably temporary intensifications of

dermal pigment, but suggested that some dark marks,

particularly the anterior-pointing chevron-shaped marks,

are hemorrhages in or under the skin caused by ruptured

capillaries. Blood also might seep from deep internal hemorrhages along myosepta and appear under the skin. Over

30 years ago, Horak and Klein (1967) recognized internal

hemorrhages and possible vertebral injuries as a cause

for such marks. Lamarque (1990) suggested that if a large

part of the body became dark, a total rupture of the spinal

column was probable. Although marks resulting from hemorrhages under the skin are best described as bruises,

the term brand is more widely accepted to cover dark

discolorations regardless of cause and is used accordingly in this report.

Most brands, especially pigmental brands, tend to

be ephemeral. They rapidly dissipate after death

(Fredenberg, 1992) and vanish within 4 days, perhaps

much sooner, on living specimens (Holmes et al., 1990).

However, Hudy (1985) observed brands, probably resulting from hemorrhages, remaining on some fish for 15 days

after they were electrofished.

Brands, especially those resulting from subdermal

hemorrhage, effectively approximate the location of damaged vertebrae or associated tissues (Lamarque, 1990;

Fredenberg, 1992), but their absence does not indicate a

lack of spinal injuries. In one sample of 152 electrofished

rainbow trout, Fredenberg (1992) reported that 26% had

brands, and all but one of those branded fish were found

upon X-ray analysis or necropsy to have associated spinal injuries. However, among the unbranded fish in the

sample, another 37% were determined to have spinal or

related tissue damage, bringing the total with such injuries to 63%. Among injured fish, the incidence of severe

injuries was much greater among branded than unbranded

fish (64% vs. 17%). Horak and Klein (1967) found brands

on 39% of the hatchery-reared rainbow trout they

electrofished; extrapolating from Fredenberg’s (1992)

observations, many more of their fish probably had spinal injures. Krueger (personal communication) observed

that over 50% of rainbow trout and brown trout that he

electrofished for contaminants analysis had brands posterior to the dorsal fin. Many of the trout he subsequently

dissected had damaged spinal columns, and most of these

also had brands. McMichael and Olson (unpublished

manuscript, 1991) also reported a positive relation between the incidence of brands and spinal injuries for

hatchery rainbow trout subjected to electrofishing fields.

Except when particularly severe, recent spinal and

related internal injuries often can only be detected or

positively verified by X-ray and necropsy (Sharber and

Carothers, 1988, 1990). Although Grisak (1966) reported

up to 43% spinal injuries (class 1 only) or hemorrhages

(class 1 and 2) among electrofished non-salmonid fishes

based on X-rays and necropsy, he observed no brands,

even though all were reportedly tetanized and some had

been in contact with the anode. Kocovsky et al. (1997)

found accumulated incidences of externally detectable

spinal injuries (old, healed injuries) in up to 23% of

salmonids sampled annually by electrofishing, but also

found upon X-ray examination that nearly half (44%) of

the fish without external signs of injury also had spinal

injuries. Although Fredenberg (personal communication

to McMichael, 1993) suggested that necropsies may be

up to a third less effective than X-rays for detection of

less-severe or less-obvious vertebral damage, necropsies

may be necessary to support or help interpret X-ray

analyses. Necropsies are also necessary to detect softtissue injury and hemorrhages, which, as discussed earlier,

might or might not be directly associated with obvious

spinal damage. Hollender and Carline (1994) concluded
 SNYDER 53

that accurate assessment of spinal injuries and muscular

hemorrhages should be based on both X-rays and necropsy.

Based on their own experiments and observations,

McCrimmon and Bidgood (1965) stated that unless Xrays are also taken prior to electric-field exposure, vertebral damage caused by electric fields might be difficult to

distinguish from previous anomalies. They documented

such prior anomalies in up to 16% of rainbow trout

electrofished from Great Lakes tributaries in Ontario. All

anomalies were compacted segments of the spine usually

involving four to nine vertebrae between the dorsal and

pelvic fins, but no significant curvature or misalignment

was noted. The affected vertebrae were typically 60 to

75% shorter than normal vertebrae and, at least in fish

that were dissected, fused and immobile. Sharber and

Carothers (1988) stated that McCrimmon and Bidgood

(1965) could not determine the cause of the abnormalities, but McCrimmon and Bidgood (1965) had concluded

that these anomalies were probably of natural origin (genetic or developmental) and definitely not electrofishing

injuries.

Other researchers have also documented the incidence of natural or non-electrofishing spinal injuries or

anomalies. Gill and Fisk (1966) X-rayed nearly 20,000 fish

and documented “natural” (genetic or environmental)

vertebral abnormalities in 0 to 11% of the fish in samples

of wild adult pink, sockeye, and chum salmon. Gabriel

(1944, as cited by McCrimmon and Bidgood, 1965) similarly documented vertebral abnormalities in 2 to 3% of

mummichog examined from natural populations. As in

McCrimmon and Bidgood’s (1965) trout, most of these

abnormalities were compressed and fused vertebrae;

misalignments, if any, were not reported. Zeigenfuss (1995)

X-rayed 209 wild salmonids ( = 21–32 cm TL) that were

trapped from three Colorado reservoirs and presumably

never exposed to electrofishing fields. He observed spinal anomalies in 72% of brook trout from one high mountain reservoir, in 6% of brook trout and 14% of rainbow

trout from another reservoir, and in none of kokanee from

the third reservoir. Zeigenfuss (1995) described the

anomalies as mostly severe spinal compressions (probably similar to that illustrated in Fig. 19A). Meismer (1999)

found that many hatchery rainbow trout used to assess

injurious effects of electric fields had externally obvious

shortened caudal peduncles caused by compression and

fusion of several caudal vertebrae. These anomalies, which

he suspected to be congenital defects, included a covering of calcified tissue and were readily distinguishable

from new electrical-field injuries.

The distinction between natural spinal anomalies and

old and new spinal injuries can sometimes be disturbingly

subjective. Sharber and Carothers (1988, 1990) observed

that naturally occurring (e.g., genetic, developmental)

spinal anomalies appeared dense (compressed) and fused

in X-rays and that electrofishing-induced damage was

distinguished by separation or notable misalignment of

vertebrae. They also implied, with photographs of X-rays

(reproduced here as Fig. 19), that old injuries could be

distinguished from natural anomalies and recent injuries

but did not discuss criteria. Fredenberg (1992), Dalbey

et al. (1996) and Thompson et al. (1997a) more specifically

noted that in X-rays, old electrofishing injuries were

evidenced by heavy calcification and fusion and, as

suggested by Sharber and Carothers (1988, 1990), were

usually distinguished by vertebral misalignment from

natural anomalies. However, electrofishing induced

injuries usually include, and are often predominated by,

compressions of the spine without misalignment

(designated as class 1 injuries) and in these cases may be

particularly difficult to distinguish from similar-appearing,

old or new injuries or anomalies by other causes.

Despite difficulties in distinguishing between some

natural spinal anomalies and old and new spinal injuries,

such determinations are critical to interpretation of the

results of an investigation. For fish electrofished (60-Hz,

half-sine PDC) from three Colorado Rivers and examined

by X-rays, Thompson et al. (1997a) reported frequencies

of 9 to 19% for old vertebral injuries and genetic abnormalities among rainbow trout and 8 to 33% among brown

trout versus frequencies of 6 to 64% and 18 to 52%, respectively, for new spinal injuries. Among X-rayed and

necropsied fishes collected from the Missouri River,

Montana, by electrofishing and other techniques, Grisak

(1996) reported that 4% of goldeye, 3% of flathead chub,

24% of river carpsucker, 6% of shorthead redhorse, and

11% of longnose sucker had spinal anomalies that he

attributed to congenital deformities or old injuries. Based

on close external examination, Kocovsky et al. (1997) assessed the accumulation of old spinal injuries among three

salmonids and longnose sucker electrofished annually

from three Colorado streams. For the salmonids, they reported annual increases in injury incidences with cumulative totals up to 23%. For longnose sucker, incidences

of externally detected spinal injuries ranged up to 13%

but varied among years suggesting a notable change or

turnover in the population rather than progressive accumulation of electrofishing injuries.

Interpretation of the nature and cause of spinal

anomalies or injuries must be made with care. As

documented by Gabriel (1944, as cited by McCrimmon

and Bidgood, 1965), McCrimmon and Bidgood (1965), and

Gill and Fisk (1966), natural occurrences of spinal

anomalies, especially compressions, may be common in

some wild or cultured populations. Also, lordosis (dorsoventral bends or misalignments), scoliosis (lateral bends

or misalignments), or vertebral compressions can result

0
54 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Fig. 19. X-rays of a natural spinal anomaly (A), Fig. 19 Fig. 19 an old spinal injury (B), and a recent electrofishing-caused spinal injury

(C) in rainbow trout (Oncorhynchus mykiss). (Reproduced with permission from Fig. 1 in Sharber and Carothers, 1988.)
 SNYDER 55

from abnormal development, nutritional deficiencies,

pollutants, or injury caused by accidents, parasites, or

predators.

Spinal injuries also can be caused by other sampling

gear (Holmes et al., 1990) or careless handling by field

personnel. Apparently, even fresh hemorrhages that are

relatively minor cannot always be attributed to

electrofishing. Fredenberg (1992) found some lateral, intervertebral, and especially subvertebral hemorrhages in

control fish. Likewise for Thompson et al. (1997a) who

reported a rather high incidence of such injuries (16%)

among brown trout controls captured in gill nets. Comparable evaluation of the incidence of spinal injuries and

anomalies among “control” fish that are not electrofished

is recommended to determine background levels of such

occurrences and assist in the interpretation of injuries

and anomalies found in electrofished specimens. For controlled experiments in which individual fish can be identified, biologists should consider pretreatment X-rays as

suggested by McCrimmon and Bidgood (1965).

Even with X-rays, some vertebral damage, particularly hairline fractures, might go undetected or be hidden

by potentially less severe injuries. Based on re-examination of 38 fish with X-rays 335 days after being

electrofished and initially X-rayed, Dalbey et al. (1996)

found that healing of spinal injuries resulted in significant calcification around and perhaps fusion of damaged

vertebrae. Reclassification of these nearly year-old injuries based on the extent of healed tissues dramatically

increased the proportion of severe, class-3 injuries over

less severe class 1 and 2 injuries from 16 to 68% of all

spinal injuries. They suggested that severity of injuries

to vertebrae had been initially underestimated with many

hairline fractures apparently overlooked or hidden by

spinal compressions.

Relation Between Injury and Mortality

Taylor et al. (1957) concluded, based on limited

evidence, that the primary cause of electrofishing mortality

is physiological and only occasionally due to physical

injuries. Many subsequent investigators, especially since

the late 1980’s, have reported high incidences of

electrofishing-caused spinal injuries and associated

hemorrhages in field operations or experiments but almost

never mentioned the occurrence (or absence) of

mortalities. Obviously, very severely injured fish would

be expected to die. However, among the few papers

specifically comparing incidences of immediate or shortterm mortality and physical injury, none have reported an

especially strong correlation between injuries and

mortalities. Spencer (1967a), for example, reported that

many bluegill killed by electricity in his experiments had

no spinal injuries, whereas many of the survivors did.

Based on another experiment with a small number of

channel catfish, Spencer (1967a) concluded that many

spinal injuries heal completely. After 45 days, even catfish

with externally obvious spinal deformities survived and

appeared to swim normally. Hudy (1985) similarly found

that among trout with electrofishing-induced injuries,

nearly 90% survived, although over half the injured

survivors continued to exhibit abnormal swimming

behavior or brands 15 days after the electrofishing event.

Although McMichael (1993) reported only one death

among over 120 hatchery rainbow trout exposed to DC

and PDC fields and held for 7 days, he found that 25% of

the treated fish had incurred vertebral injuries or

hemorrhages, including 17% with broken backs. Habera

et al. (1996) reported that 9% of 227 electrofished rainbow

trout died within 7 days (13 of 20 mortalities were

unrecovered fish assumed to be dead), but unlike a few

survivors that were examined, none of the seven

mortalities available for examination had incurred spinal

injuries or associated hemorrhages. Fredenberg (1992)

commonly observed old, healed spinal injuries in X-rays

of trout collected in Montana—a further testament to the

survivability of many fish with spinal injuries.

Factors Affecting Electrofishing

Injury and Mortality

Electrical-field factors considered in the literature to

affect the incidence of electrofishing-induced mortality

and spinal injuries (including associated hemorrhages)

include type of current, intensity, duration, orientation

(relative to the fish), and for AC and PDC, waveform characteristics such as pulse or wave frequency, shape, and

width. Related biological factors of concern include species, size, and condition.

Comparisons of results among and sometimes within

publications and reports discussed in this section, as

throughout the review, are difficult and often suspect

because of differing or inadequately described biological,

field, or experimental conditions, including electrical

parameters. Even when electrical output and field

intensity were reported for currents other than straight

DC, authors frequently failed to indicate whether values

represented peak or mean (rms in AC) measurements.

Occasional mortalities have often been accepted as a

normal consequence of electrofishing operations (and

most other collection techniques) and as such may not

be reported. Also, unless biologists specifically looked

for and documented bruises or other external or behavioral

signs of injury, absence of such information does not

necessarily mean injuries did not occur. Even if externally

obvious injuries were adequately documented, most

internal injuries, as emphasized earlier, could only have

been determined by X-ray analysis and necropsy. Despite
56 INFORMATION AND TECHNOLOGY REPORT--2003-0002

these limitations, the impact of some electrical and

biological factors on electrofishing mortalities and injuries

is reasonably clear, if not well understood.

Type of Current

Among types of current, most electrofishing

authorities consider AC to be the most harmful to fish,

DC the least fatiguing and injurious, and PDC somewhere

between. Comparative field and laboratory investigations

tend to support this generalization, but there are

exceptions, and for each type of current, including AC,

there are reports of no or insignificant mortality or injury

(Appendix B). With sufficient field intensity and duration

of exposure, any current can be lethal and, under certain

conditions, even DC can injure substantial numbers of

fish. The extent of mortality or injury caused by the

different types of electrical currents (AC, DC, or PDC)

varies considerably with how the currents are used

(electrofishing techniques or procedures), electrical

parameters (e.g., field size and intensity, pulse or cyclic

frequency), biological factors (species, size, condition),

and environmental conditions (e.g., water conductivity,

temperature, basin configuration and dimensions).

Generally, immediate or short-term mortalities reported

for PDC are as low or nearly as low as for DC (often

none), but this does not appear to be the case with regard

to spinal and associated injuries when using moderate to

high-frequency PDCs. Although the incidence of such

injuries detected for constant low-frequency PDCs (especially <20 Hz) and some specially designed PDC pulse

trains (e.g., Coffelt’s complex-pulse system, CPS) usually

approaches or approximates the low levels observed for

DC, that for constant moderate to high-frequency PDCs

often approaches or approximates the substantially higher

levels reported for AC. Because of the availability and

commercial promotion of CPS as a less injurious form of

PDC, it is treated in the literature and in this report as a

distinct type of current, but its effects on fish might not

be representative of other pulse-train configurations.

As discussed earlier under major responses of fish

in electric fields, anodic taxis in PDC appears to differ

from that in DC (Lamarque, 1990; Fredenberg, personal

communication). Assuming the mechanisms involved and

aspects of the current inducing taxis also differ, perhaps

these differences are also responsible, at least in part, for

the greater incidence of spinal injuries often observed

under moderate to higher-frequency PDCs. Lamarque

(1990) noted that injuries caused by DC occur mostly

when fish lie motionless and tetanized near the cathode

or when the current abruptly ceases or is reestablished.

In the latter case (as when the current is repeatedly

switched on and off), he suggested that DC momentarily

acts like PDC.

Mortality. Amazingly little published data is available

to support AC’s reputation as the most lethal

electrofishing current. Only investigations by Pratt (1955),

Taylor et al. (1957), and DeMont (1971) compared

incidences of mortality among fish subjected to AC with

those similarly subjected to DC or constant-frequency PDC.

Taylor et al. (1957) conducted the only investigation

comparing electrofishing mortality among all three types

of current. In laboratory experiments (conductivity calculated as 1,494 μS/cm, but unusually high and uncertain; 16–18° C) with voltage gradients greater than

required for narcosis, they exposed rainbow trout (20–23

cm) to homogeneous fields of 60-Hz AC (>0.3 Vp/cm), 12-

to 20-Hz, triangular-wave PDCs (>1.5 Vp/cm at 12 Hz to

>0.3 Vp/cm at 120 Hz with 33–88% duty cycles), and DC

(>0.4 V/cm). Mortality was 4% for AC, just 0.3% overall

for the PDCs and zero for DC. They also reported no

mortality among larger (33-cm) and smaller (5-cm) rainbow trout similarly exposed to DC at field intensities

greater than required for narcosis (>0.3 V/cm and >0.5 V/

cm, respectively). Similar trends in mortality caused by

these currents were observed when electrofishing in natural streams.

Pratt (1955) and DeMont (1971) also found fish more

susceptible to mortality when exposed to AC than DC. In

hatchery raceways (308 μS/cm), Pratt reported mortalities

of 4% for rainbow trout ( = 19 cm), 10% for brook trout

( = 25 cm), and 20% for brown trout ( = 20 cm)

electrofished with 110-V AC but only 2%, 0%, and 4%,

respectively, with 230-V DC. In a series of electrocution

experiments (200 μS/cm), DeMont (1971) found that substantially lower voltage gradients were required to kill

50% of threespine stickleback with AC (4.8 V/cm, probably rms) than with DC (14 V/cm); however, exposure

times were twice as long for AC than for DC trials (20 s vs.

10 s), thereby confounding the comparison.

Unlike the preceding biologists, Spencer (1967b)

found no consistent differences in lethal effects of 115-V

AC and 115-V DC. In a series of concrete-pond

experiments to assess the usefulness of electrofishing

for killing intermediate-size (~8–10 cm) bluegill to thin

populations, Spencer confined batches of test fish to a

0.3 by 0.9 m screened enclosure and, using the same

electrodes, exposed them for 1 to 120 s to 230-V, 180-Hz

AC (three phase); 115-V, 60-Hz AC (single phase); or 115-

V DC (water resistivity reported as 6,000 ohms, which, if

properly interpreted, calculates to a conductivity of 167

μS/cm). Survival was monitored for 24 h (prior experiments

had revealed that only a very small percentage died after

24 h), and the number of dead fish was recorded at 5 min,

1 h, 2 h, 4 h, and 24 h. Percent mortalities generally

increased with exposure time and, beyond 1 s, were always

much greater for the 230-V AC (1–58%) than for either

115-V AC (0–19%) or 115-V DC (0–29%). For the 115-V

0

0 0
 SNYDER 57

currents, percent mortality was sometimes greater for AC

than DC and sometimes vice versa. Mortality was only

substantially greater for AC than DC at 30-s exposures

(3% vs. 0%) and 60-s exposures (9% vs. 4%), whereas it

was substantially greater for DC than AC only at the 120-

s exposure (29% vs. 19%). Interestingly, if the AC output

is a mean (rms) rather than peak value, peak output and

field intensity would have been about 41% greater than

for DC. Trials for 230-V AC were extended with exposure

times up to 300 s and resulted in up to 75% mortality.

About 80% or more of the 230-V AC mortalities and 50%

or more of the 115-V AC and DC mortalities occurred within

the first 2 h after exposure. For exposures of 90 s or

greater, over 90% of 230-V AC mortalities and over 50%

of 115-V AC and DC mortalities occurred within the first

hour (over 65% of 230-V AC mortalities for 90-s or longer

exposures occurred within 5 min–no comparable data for

other currents).

All remaining reports of AC mortality were noncomparative observations and, except for the report of

26% mortality by Hauck (1949, discussed earlier), none

disclosed immediate or very short-term mortalities greater

than 3%. Hudy (1985) observed less than 1% immediate

mortality among 1,125 hatchery rainbow trout (16–26 cm)

and 1,125 brook trout (12–24 cm) stunned with 250 to

300-Hz AC in a concrete raceway (350–760 V output;

10 μS/cm; 5.5° C) and reported that mortality increased to

no more than 3% during the next 15 days. Schneider (1992)

reported very little or no immediate or very short-term

mortality (4 days) among yellow perch, bluegill,

pumpkinseed, green sunfish, lake chubsucker, and golden

shiner collected from Michigan lakes and ponds (66–520

μS/cm) using 3-phase AC. Output voltage was adjusted

such that large fish recovered within 30 s but was high

enough to stun small fish as well. Habera et al. (1996)

reported 1% immediate and 3% 24-h mortality among

rainbow trout (5–23 cm TL) electrofished with 60-Hz AC

in a three-pass depletion population estimate (14 μS/cm;

15° C). No mortality was observed among control fish

captured by angling.

Like Taylor et al. (1957), Edwards and Higgins (1973)

and Dalbey et al. (1996, also Dalbey, 1994) compared DC

and PDC and observed very little or no immediate mortality among exposed fish. In homogeneous field experiments to determine stun thresholds for channel catfish

and bluegill in DC and 11 variations of PDC, Edwards and

Higgins (1973) reported that the fish recovered instantly

or within a few minutes and that very few deaths during

the next 10 days could be traced directly to treatment in

electric fields. Dalbey et al. (1996) similarly reported no

immediate mortality for rainbow trout (15–39 cm FL) captured with DC, 60-Hz PDC, and a hybrid of the two.

Other investigators reported substantially greater

mortalities for fish subjected to DC or PDC. Lamarque

(1967a,b, 1990) reported immediate or very short-term

mortalities of 6% and 17% for trout exposed for 20 s at a

distance of 20 cm from the anode to two forms of DC

(probably smooth and rippled) and 50 to 93% for those

similarly exposed to four forms of constant-frequency

PDC. However, peak field intensities were not reported

and might have been substantially higher for the PDCs.

Among rainbow trout (33–60 cm TL) and Colorado

pikeminnow (30–39 cm TL) exposed to homogeneous 15-

and 60-Hz, square-wave PDC, CPS, or DC for 5 s at various response threshold levels (530 μS/cm; 18° C),

Meismer (1999) reported immediate mortality only among

rainbow trout (10%) subjected to the highest intensity of

60-Hz PDC tested, 1 Vp/cm, a level sufficient to assure full

tetany. In a separate laboratory comparison of effects on

the same species by exposure to 10 s of 60-Hz, squarewave and 60-Hz, quarter-sine-wave PDCs (generated by

Cofflet’s VVP-15 and Smith-Root’s GPP 5.0 electrofishers,

respectively) at 1.5 Vp/cm, Meismer (1999) again reported

immediate mortality only for rainbow trout (30%) exposed

to the 60-Hz, square-wave current. Meyer and Miller (1990)

reported no immediate mortality among rainbow and

brown trout (29–54 cm TL) electrofished with 60-Hz PDC

or CPS, but 3% delayed mortality (all within 2 days) among

trout collected with 60-Hz PDC and held in a live net for 7

days. For rainbow trout (24–51 cm FL) collected during

boat-electrofishing field trials (30 μS/cm; 7° C) to compare two currents at a time, Taube (1992) reported 5-day

mortalities of 3% for DC versus 15% for 25-Hz PDC (75%

duty cycle) for one set of trials but 11% for DC versus no

mortality for CPS in another set of trials. In a non-comparative investigation using 450- to 650-V DC, Eloranta

(1990) reported acute mortalities greater than 50% for

burbot collected in the littoral zones of a lake (40–60 μS/

cm) and less than 11% for most other species; mortality

was greatest when operating at over 600 V.

Injury. Taube (1992) conducted the only published

investigation comparing frequency of spinal injuries and

associated inter-muscular hemorrhages among all three

types of current. In controlled laboratory experiments

(100–121 μS/cm, 9–13° C), he reported that incidence of

spinal injury among large adult rainbow trout ( = 39–48

cm FL) was least for DC and CPS at 28 and 21%, respectively, intermediate for 60-Hz and two 30-Hz PDCs at 42–

50%, and greatest for AC at 67%. Incidence of hemorrhages

was also least for DC at 28% but similarly high, 42 to 46%,

for all other currents. For each current, fish were exposed

for 5 s in homogeneous fields at either of two levels of

intensity, one just above the threshold for stun and the

other much higher. However, differences in the number of

injured fish at the two levels of intensity were not significant, and the data were combined for this comparison

among current types. Most vertebral injuries were

misalignments (class 2), and the remainder were spinal

0
58 INFORMATION AND TECHNOLOGY REPORT--2003-0002

(43–64 μS/cm, 8–11° C) and one moderate-conductivity

stream (440 μS/cm, 11° C) in Pennsylvania with 250 to

300-Hz AC and 60-Hz PDC and subsequently examined

them by X-ray and necropsy. Overall, they found hemorrhages or spinal damage in 26% (14–41%) of trout collected with AC and similar numbers, 22% (9–43%), among

those collected with PDC. They also examined trout angled

in those same streams as controls. They reported no hemorrhages or vertebral injuries among angled fish except in

the moderate-conductivity stream where the incidence of

vertebral damage, 12%, was comparable to the 14% observed for trout collected in the same stream with AC and

9% with PDC (least incidences of electrofishing injury

among the four streams). Although the moderate-conductivity stream had been electrofished a year before,

Hollender and Carline (1994) observed that most of the

injuries in angled fish appeared to be more recent.

Hollender and Carline (1994) electrofished brook trout

(9–24 cm TL) in three small, low-conductivity streams

Among remaining reports of incidences of injury

caused by exposure to AC fields, all of which are noncomparative, only Hauck (1949) and Spencer (1967a) reported substantial impacts. As discussed earlier

(beginning of section on spinal and related injuries), Hauck

(1949) described a variety of injuries that probably afflicted more than the 26% of rainbow trout that died within

2 to 5 days (Reynolds and Kolz in Reynolds et al., 1988).

Spencer (1967a) observed spinal injuries in 6 of 10 channel catfish exposed to 230-V, 3-phase AC (presumably

180 Hz). The remaining investigators of AC impacts reported relatively few electrofishing injuries and suggested

that, at least under similar circumstances, AC

electrofishing is an acceptable technique for monitoring

or assessing fish populations.

Among various coolwater and warmwater fish he

electrofished with AC from various Michigan lakes and

ponds (66–520 μS/cm, 0–28° C) and monitored for delayed mortality, Schneider (1992) reported externally obvious injuries only among yellow perch—50% had

accumulations of bright-red blood in the sinus venosus

near the base of the gills. The blood dispersed within a

day, and all fish survived and appeared in good condition at the end of their respective holding periods (1–40

days). Schneider (1992) also emphasized that he rarely

observed external indications of injuries among thousands of fish he had collected with AC, but he did not Xray or dissect any of the fish.

In addition to a low incidence of mortality (1% immediate, 3% after 15 days), Hudy (1985) reported that after

15 days, less than 3% of 2,250 rainbow trout (16–26 cm)

and brook trout (12–24 cm) electrofished in hatchery raceways (10 μS/cm, 6° C) with AC displayed externally visible physical or behavioral abnormalities (brands or erratic

swimming). Based on X-rays, he detected an overall incidence of fractured or dislocated vertebrae in about 4% of

the fish—21% of the mortalities, 77% of the abnormal

compressions (class 1) with no detected vertebral fractures (class 3). Over half of the internal hemorrhages were

severe (class 3).

Spencer (1967a) conducted the only other published

comparison of AC and DC-caused incidences of spinal

injury. He reported frequencies of injured bluegill as 0 to

3% for 115-V DC, 3 to 7% for 115-V AC, and 9 to 16% for

230-V AC. However, if reported AC voltage outputs were

rms rather than peak, the comparison between 115-V currents is confounded by a 41% greater peak output voltage for AC.

In addition to Taube (1992), three other investigations

have compared incidences of injuries caused by AC and

PDC. McCrimmon and Bidgood (1965) reported no skeletal

damage attributable to either current, Walker et al. (1994)

reported substantially greater numbers of injuries for AC,

and Hollender and Carline (1994) reported equally high

frequencies of injuries for both currents.

McCrimmon and Bidgood (1965) exposed hatchery

rainbow trout (11–27 cm TL) to either 60-Hz AC or 120-Hz,

half-sine PDC (475–550 μS/cm, 11–13° C) and

electrofished wild rainbow trout (6–59 cm TL) from Ontario

streams with the same currents. All fish were X-rayed

(the hatchery fish before and after exposure), and some

were dissected. Among the hatchery fish, spinal anomalies

were detected in 4% of the fish before exposure, but no

new injuries were detected after exposure. Up to 16% of

the wild fish, depending on the stream, had spinal

compressions, but among a subsample also examined by

necropsy, the compressed vertebrae were fused, immobile,

and, therefore, not considered electrofishing injuries.

In a series of tests in aquaria (200–230 μS/cm; 12–

14° C) to assess optimal currents, field intensities, and

exposure times for narcosis without externally obvious

injury, Walker et al. (1994) exposed juvenile northern pike

(13–19 cm SL) to homogeneous fields of 50-Hz, sine-wave

AC; 50-Hz, triangular-wave AC (0.4 to 2.1 Vrms/cm each);

and 50-Hz PDC (36% duty cycle, 0.4–2.1 Vp/cm) for 10 to

60 s. Of the fish exposed to sine-wave AC, 24% remained

on their sides for 16 to 24 h after exposure and had cutaneous hemorrhages (brands) along the entire body; some

also had bent spines. They observed similar results for

fish exposed to triangular-wave AC; 33% were injured

and half of these remained on their sides throughout the

24-h observation period. However, with the triangular

waveform, the hemorrhages occurred in the paired and

median fins rather than along the body. In contrast to

both AC currents, no externally obvious injuries resulted

from exposure to 50-Hz PDC. More extensive tank-trials

(410 μS/cm, 11° C) with adult northern pike (45–97 cm SL)

subjected to 50-Hz PDC also resulted in no externally

obvious injuries. No fish were X-rayed or necropsied.
 SNYDER 59

survivors, 1% of subsampled, normal-appearing survivors, and none of the control fish. Hudy (1985) observed

that some fish in each treatment had fused vertebrae but

assumed that these anomalies were not caused by

electrofishing (he made no mention of compressed,

unfused vertebrae).

Habera et al. (1996) assessed injury (and short-term

mortality) among rainbow trout (5–23 cm TL) electrofished

with 60-Hz AC in a three-pass depletion population estimate in a southern Appalachian stream (14 μS/cm, 15° C).

No spinal injuries or hemorrhages were detected among

mortalities (3%) or 12 angled controls examined by X-ray

or necropsy, but among a subsample of survivors, 3%

incurred class-2 spinal injuries and another 3% class-2

hemorrhages (6% combined). The injured fish were greater

than 10 cm TL (12–17 cm) and collected only in second

and third passes (fish taken during these passes may

have been shocked but uncaptured in the preceding pass

or passes). No external injuries (e.g., brands) or erratic

swimming behavior were observed among survivors not

X-rayed and necropsied.

Fredenberg (1992, personal communication), Taube

(1992), and Meismer (1999) compared the injurious effects of DC with constant-frequency PDCs and the pulse

train CPS in several investigations. Simple (constant-frequency) PDCs were usually more harmful than DC or CPS.

Comparing electrofishing injuries in rainbow trout

(23–54 cm TL) collected from a wide range of Montana

rivers and streams (33–900 μS/cm, 4–18° C), Fredenberg

(1992) reported vertebral injuries in 5 to 18% of the trout

collected with DC, 4 to 43% with CPS, and 13 to 68%

(usually greater than 41%) for 60-Hz, square- and halfsine-wave PDCs. Incidences of inter-musculature hemorrhages were 0 to 25%, 25 to 77%, and 57 to 91%,

respectively. Overall, spinal injuries or hemorrhages were

found in up to 30% of the fish taken with DC and up to

98% of those collected with PDC (combined figures for

CPS not reported). A hybrid of DC and 60-Hz PDC (Fig. 5J)

resulted in spinal injury among 30% and hemorrhages

among 72% of the collected rainbow trout. Many of the

reported injuries in these collections were minor or class

one.

In another investigation, Fredenberg (personal communication) also compared the incidence of spinal injuries, determined by necropsy, among adult ( = 38–42 cm

TL) rainbow trout, mountain whitefish, and suckers (white

and longnose) collected by three currents purported to

be least damaging to fish: DC, 15-Hz PDC, and CPS. The

fish were collected by boat electrofishing (cable electrodes) in late October from the Missouri River in Montana (450 μS/cm, 10° C). Despite a maximum (interpreted

as peak) output voltage twice that for 15-Hz PDC and

four times that for DC, he found CPS consistently least

damaging. Among rainbow trout, CPS caused vertebral

damage in just 2% and associated hemorrhages in 4% of

the fish and DC just 6 and 2%, respectively. However, 15-

Hz PDC caused spinal injuries in 20% and hemorrhages

in 22% of the rainbow trout. Few, if any, injuries to the

spine, just 0 to 2%, were observed for mountain whitefish

or suckers regardless of the current. Incidences of hemorrhages (all minor, class 1) among mountain whitefish

were again least for CPS and DC, just 2 and 6%, respectively, and greatest at 18% for 15-Hz PDC. In a reversal of

the above results for DC and 15-Hz PDC, incidence of

hemorrhages among suckers was greatest at 18% with

DC, intermediate at 10% with 15-Hz PDC, and least at 4%

with CPS.

Taube (1992) compared incidences of spinal injury

among large rainbow trout ( = 38–42 mm FL) stunned in

controlled experiments with heterogenous fields of DC,

four variations of constant-frequency, square-wave PDC,

and CPS. He found CPS to be significantly less harmful

than the other currents, including DC. Output voltage

was the same for all currents except CPS for which output

was necessarily doubled to stun the fish. The trout were

individually placed at the distal end of the exposure area

in a raceway and chased towards the anode into an effective portion of the field where they were shocked

(stunned) for 5 s. X-rays revealed spinal injuries in 8% of

the fish subjected to CPS, 17% for DC, and 25 to 67% for

various PDCs (20–60 Hz, 25–75% duty cycle). For comparable currents, these results were similar to those for his

homogeneous-field experiments discussed above for comparison with AC. Most spinal injuries in both sets of experiments were recorded as misalignments (class 2) with

some compressions only (class 1) and no fractures

(class 3).

In one-on-one boat-electrofishing field trials in an

Alaskan stream (30 μS/cm, 7° C), Taube (1992) also reported that rainbow trout ( = 32–40 mm FL) had spinal

injury rates of 47% in DC versus 13% in CPS (difference

not statistically significant), but on another day, 0% in

DC versus 57% in 25-Hz PDC (75% duty cycle; difference

statistically significant). He offered no explanation for

the unusually high incidence of spinal injury in DC during the trial with CPS.

Combining treatments at various field intensity levels

from the thresholds for twitch to 1 Vp/cm, which was

sufficient to assure full tetany (n = 80 per species and

current), Meismer (1999) found the percentage of Colorado

pikeminnow (30–39 cm TL) with spinal injuries was

significantly greater than controls (no injury) when

exposed to 15-Hz, square-wave PDC (11%), but not DC

(6%), CPS (6%), or 60-Hz, square-wave PDC (3%). For

similarly treated rainbow trout (33–60 cm TL), the number

of fish with spinal injuries was greater for exposures to

CPS (5%) than DC (3%) or 15- or 60-Hz, square-wave PDC

(each 1%), but differences between currents or relative to

0

0

0
60 INFORMATION AND TECHNOLOGY REPORT--2003-0002

controls were insignificant. All spinal injuries were minor

(class 1) except for a slight misalignment in one Colorado

pikeminnow exposed to DC. For Colorado pikeminnow,

internal soft-tissue injuries, all minor (class 1) hemorrhages

in or near the musculature of the spine, were detected in

just a few specimens (1–3% for each current, none

significantly different from controls). Hemorrhages were

not assessed in the rainbow trout. Externally obvious

injuries, all of which were brands (dark brown to black

stripes or blotches, often in a zebra-like pattern), were

common on rainbow trout regardless of treatment current,

but significantly more so for those exposed to 60-Hz PDC

(46% vs. 16–21% for the other currents). In stark contrast,

Meismer (1999) observed no external injuries, brands or

otherwise, on Colorado pikeminnow. For all treatment

exposures (530 μS/cm; 18° C), field intensity was gradually

increased to the desired response threshold or voltage

gradient level and held there for 5 s.

If the muscular convulsions responsible for spinal

injuries are induced by sudden changes in voltage, as

suspected, then the spinal injuries documented in

Meismer’s (1999) experiments using DC must have occurred when the current was switched off at the end of

exposure, or in the case of some rainbow trout observed

by Meismer (1999), when they leaped frantically out of

the water. Otherwise, it appears that such convulsions

can occur as well, or instead, under constant current, as

suggested by the observation of twitches in DC as the

intensity was gradually increased to or beyond the threshold for that response. The question might be resolved by

comparable experiments in which DC field intensity is

also reduced gradually back to zero rather than suddenly

switched off.

Fredenberg (1992), McMichael (1993), Dalbey et al.

(1996), and Grisak (1966) compared frequency of injuries

for fish subjected to DC and PDC exclusive of CPS.

Fredenberg (1992), as in comparisons above with CPS,

and Dalbey et al. (1996) also compared results for DC and

60-Hz PDC with results for a hybrid of the two currents

(Fig. 5J). With one exception (Grisak, 1966), these investigators reported that DC caused far fewer injuries than 30

to 90-Hz PDC and that frequencies of injury for the hybrid current were intermediate but generally much closer

to that for the PDCs.

Fredenberg (1992) necropsied and compared incidences of injury for brown trout (30–56 cm TL)

electrofished with DC, 60-Hz PDC, and a hybrid of the

two currents. Incidences of spinal injury and hemorrhages

were 8 and 6%, respectively (10% combined), for brown

trout electrofished with DC, 36 and 56%, respectively, for

60-Hz PDC, and 32 and 28%, respectively, for the hybrid

current. However, in dramatic contrast to the frequency

of injuries reported for brown trout, and earlier for rainbow trout, Arctic grayling (37–45 cm TL) examined by

Fredenberg (1992) suffered no similar injuries when captured with DC and only a 4% incidence of hemorrhages

when collected with the hybrid current (no data for 60-Hz

PDC).

Based on X-rays, Dalbey et al. (1996, also Dalbey,

1994), like most other researchers, reported a substantially lower percentage of injured fish among wild rainbow trout (15–39 cm FL) captured with DC (12%) than

with 60-Hz PDC (54%) or a hybrid of the two currents

(40%). For all captures, they used the same mobile electrode system (Vincent, 1971) with a triangular anode

thrown from a fiberglass boat and a peak output of 400 V

(260 μs/cm, 13–16° C). Peak voltage gradients around the

anode should have been identical regardless of the waveform used, but reported oscilloscope measurements were

strangely much higher for DC than PDC or the hybrid

current at least out to 1.2 m from the anode where reported values were about 4.7, 1.2 and 0.8 Vp/cm, respectively. At a distance of 2.4 m from the anode, measured

peak voltage gradients were nearly the same, about 0.5 to

0.7 Vp/cm. Perhaps the DC voltage gradient at 1.2 m from

the anode was measured perpendicular to a point of the

triangle and the others perpendicular to the flat aspect of

the anode between corners.

In a hatchery experiment, McMichael (1993) also

found substantially fewer injuries among necropsied rainbow trout (14–48 cm FL) electrofished with DC than

square-wave PDCs. For trout electrofished twice, 7 days

apart, they reported 3% brands, 4% spinal injuries, and

4% hemorrhages among trout taken with DC at 300 V

output, and 8, 14, and 17%, respectively, with DC at 400 V.

In contrast, they reported 4, 22, and 35%, respectively,

for trout taken with 30-Hz PDC at 300 V and 58, 35, and

53%, respectively, with 90-Hz PDC at 300 V output. However, if output voltages for the PDCs were mean rather

than peak values, their reported duty cycles of 12.5%

(McMichael, personal communication) would have resulted peak output voltages of 2,400 V and the PDC fields

would have been six to eight times more intense than the

DC fields, thereby confounding comparison between

types of current.

Grisak (1996) studied the effects of electrofishing on

four non-salmonid fishes in the Missouri River, Montana.

Unlike many other investigators, he reported that when

data were combined for all species X-rayed and

necropsied, more spinal injuries were caused by DC than

40-Hz PDC (25% duty cycle), 14% and 5% respectively,

and the same frequency of muscular hemorrhages, 14%

(about half class 1 and half class 2) for each current (only

3% of the injured fish had both spinal damage and

hemorrhages). However, this generalization is misleading.

Among the four species reported upon, only goldeye

(11–37 cm TL), the predominate species captured, was

injured using DC and for this species the incidence of
 SNYDER 61

spinal injury was far greater for DC than 40-Hz PDC, 21%

and 4%, respectively. In contrast, incidences of

hemorrhages among goldeye were considerably fewer for

DC than PDC, 21% and 39%, respectively, and

percentages with either spinal injuries or hemorrhages,

or both, were also less for DC than PDC, 32% and 43%,

respectively. Grisak (1996) suggested that goldeye’s

usually high incidence of spinal injury when captured

with DC might have been associated with its especially

strong taxic response to DC or resultant proximity to or

contact with the anode. Among the other three species

collected and examined, all reported injuries were caused

by 40-Hz PDC: flathead chub (8% spinal; 11–24 cm TL),

shorthead redhorse (5% hemorrhage; 18–49 cm TL), and

river carpsucker (18% spinal, 9% hemorrhage, 27%

combined; 30–58 cm TL). All fish succumbed to tetany

but revived within minutes of capture and no brands were

observed with either current. Among control fish collected

by other means, Grisak (1996) reported only one fresh

internal injury, a spinal compression.

Several investigators compared frequencies of injury among fishes captured or exposed to simple (constant-frequency) PDCs with those for fish captured or

exposed to CPS. As in most above comparisons of CPS

with DC and PDC, Meyer and Miller (1990), Sharber et al.

(1994) and Ruppert and Muth (1997) found CPS less harmful than 30-Hz or greater PDC, but Meyer and Miller (unpublished manuscript, 1991) reported similar spinal injury

rates for brown trout taken by the two currents and higher

percentages for rainbow trout taken with CPS than 40-Hz

PDC.

Based on X-rays of single-pass, field-collected fish

from the Laramie River (600–610 μS/cm) in late April and

early May 1990, Meyer and Miller (1990) reported that

78% of rainbow trout (30–36 cm TL) and 82% of brown

trout (28–54 cm TL) taken with 60-Hz PDC incurred spinal

injuries, whereas, 50% of the rainbow trout (30–41 cm TL)

and 25% of the brown trout (13–59 cm TL) were similarly

injured when taken with CPS. Sample sizes were small,

but during the previous September in the Wind River

(299 μS/cm; September), comparable injury rates (60% for

rainbow trout, 26–43 cm TL, and 86% for brown trout, 17–

51 cm TL) were also observed for 60-Hz PDC among a

much larger number of previously uncaptured trout taken

during the fourth pass of a population estimate effort.

However, some of those injuries might have occurred

during earlier passes and the electrofishing control box

was seriously out of calibration (output voltage and actual PDC characteristics questionable) and might have

been partially responsible for the high incidence of spinal injuries (CPS was not used in the latter effort).

In September 1990, Meyer and Miller (unpublished

manuscript, 1991; Wyoming Fish and Game Department,

1991) repeated the comparison of current-induced spinal

injuries but used 40-Hz rather than 60-Hz PDC and collected the fish from the Wind River (340–350 μS/cm; 7–

8° C). The frequency of spinal injuries was much lower

than the prior spring in the Laramie River and, with one

exception, was similar for both 40-Hz PDC and CPS. Spinal injuries were observed among none of the X-rayed

rainbow trout (16–31 cm TL) and 15% of the brown trout

(17–41 cm TL) taken with 40-Hz PDC as compared with

12% and 14% of the fish, respectively (both species 16–

39 cm TL), taken with CPS. On the same day, during the

fourth pass of a population estimate, captures of trout

not previously captured using 40-Hz PDC in a downstream

segment of the same river also revealed far fewer injuries

(35% for rainbow trout, 14–40 cm TL, and 26% for brown

trout, 17–38 cm TL) than during the prior year, but injuries were notably greater than for upstream single-pass

captures. Some of the fourth-pass injuries probably occurred earlier in September during the first three passes

for that population estimate.

Sharber et al. (1994) compared the frequency of spinal injuries revealed by X-rays of adult rainbow trout

(>30 cm TL) captured with 15-, 30-, 60-, and 512-Hz, squarewave PDC and CPS at night in the Colorado River below

Glen Canyon Dam (600–800 μS/cm, 9–11° C). They found

that for PDC injury rates increased with pulse frequency

from 3 to 24, 43, and 62%, respectively, and that the percentage of injured fish for CPS, 8% (range 7–9%), was

less than for 30-Hz, but not 15-Hz PDC. Although 15-Hz

PDC generated fewer injuries than CPS, it is presumably

less effective for attracting and capturing fish (Sharber,

personal communication).

Ruppert (1996) and Ruppert and Muth (1997) compared the frequency of vertebral injuries and muscular

hemorrhages near the spine in juvenile bonytail (5–8 cm

TL) exposed in laboratory experiments (940 μ, 15° C) to 10

s of homogeneous CPS and 30-, 60-, and 80-Hz, squarewave PDC at predetermined voltage-gradient thresholds

for taxis, narcosis, and tetany. No vertebral damage was

detected, but spinal hemorrhages, all class 2 with an average of three per fish, were observed in 13% of all treatment fish (range of 3–27% for individual treatments, none

in controls). Combining field-intensity treatments, mean

frequency of hemorrhages was least for CPS, 8%, and

ranged from 10 to 19% for the constant-frequency PDCs.

Among juvenile humpback chub (5–10 cm TL), similarly

subjected to CPS, no spinal injuries were detected but

20% had muscular hemorrhages (insufficient specimens

for comparable trials with PDCs).

As noted above, McCrimmon and Bidgood (1965)

reported no spinal injuries among hatchery or wild rainbow

trout exposed to 120-Hz, half-sine, PDC. Fredenberg (1992)

also reported no spinal injuries or hemorrhages among

walleye and sauger collected with 60-Hz PDC. Similarly,

Dwyer and White (1995) reported no spinal trauma among
62 INFORMATION AND TECHNOLOGY REPORT--2003-0002

hatchery rainbow trout (about 33 cm TL), including four

mortalities, individually exposed to 10 s of homogeneous

250-Hz, half-sine PDC at 3.5 to 3.9 Vp/cm (0.9–1.0 Vm/cm;

270–340 μS/cm, 8° C). Mitton and McDonald (1994a)

exposed rainbow trout (30–600 g) in tanks to 20 to 40 s of

60-Hz PDC with output voltages of 200 to 600 V (240–270

μS/cm, 7 and 15–20° C), and detected spinal injuries with

X-rays only in a few of the largest fish. They concluded

that PDC electrofishing does not normally produce any

skeletal damage. These reports of no injury using

constant-frequency PDC are in stark contrast to an everincreasing number of field studies in which substantial

numbers of fish were injured when subjected to PDC.

Many of the comparative studies noted above revealed substantial incidences of spinal injuries and hemorrhages for fish captured with or exposed to

constant-frequency PDC. Among investigations comparing or reporting on only PDCs, Sharber and Carothers

(1988, 1990), based on X-rays and necropsy, reported spinal and associated soft-tissue injuries in 44 to 67% of 209

large rainbow trout (30–56 cm TL) that were boat (raft)

electrofished at night in the Colorado River below Lake

Powell (450–600 μS/cm, 10–11° C) in two successive years

using 60-Hz, square, quarter-sine, and exponential PDCs.

Spinal injuries were not observed in 12 nonelectrofished

hatchery trout of similar size. Reynolds and Kolz (in

Reynolds et al., 1988) calculated approximate voltage gradients of about 8.6 Vp/cm at the surface of the spherical

anode, 0.5 Vp/cm at 0.5 m, and about 0.15 Vp/cm at 1 m

from the anode.

Reynolds and Kolz (in Reynolds et al., 1988) reported

that recent studies by the Alaska Department of Fish and

Game corroborated Sharber and Carothers’ (1988) findings. In a Kenai River investigation (50 μS/cm at 7° C), 11

of 22 large rainbow trout (>40 cm FL) suffered spinal injuries. The Alaskan investigators noted that injuries seemed

more likely among trout captured within 0.5 m of the anode where voltage gradients were greater than 1 V/cm.

In addition to the experiments discussed above,

Meismer (1999) observed brands on most rainbow trout

exposed for 10 s in homogeneous, 1.5 Vp/cm fields of 60-

Hz , square-wave and 60-Hz, quarter-sine-wave PDCs (95%

and 50%, respectively). However, the incidence of spinal

injuries was insignificant (15% and 5%, respectively). In

contrast, no external or internal injuries were detected in

Colorado pikeminnow similarly exposed to either PDC.

Field Intensity

It is well documented that beyond threshold levels,

the lethality of electrofishing fields generally increases

with field intensity (i.e., voltage gradient, current density,

or power density). However, as with other responses to

field intensity, the lethal physiological effects of field

intensity appear to be a function of the voltage differential

it causes across the fish and that differential also depends

on the orientation of the fish relative to the lines of current.

Unlike mortality, the relation between electrofishinginduced injuries and field intensity, beyond some

threshold level, remains unclear. Like severe stress,

fatigue, and mortality, spinal and related injuries have

long been attributed to intense, tetany-causing currents,

especially in AC fields. But contrary to this long-held

belief, such injuries are not restricted to the higher

intensities required for tetany or even narcosis. Spinal

injuries, and the myoclonic jerks assumed to be their

principal cause, might not even be possible while a fish is

in a state of narcosis (petit mal) or full tetany (grand mal).

Recent studies clearly document that spinal injuries are

just as likely to occur at or above the threshold for twitch

in the zone of perception as in more intense portions of

the field, regardless of the type of current. Accordingly,

and unlike severe stress, fatigue, and mortality due

primarily to apnea (respiratory failure) and muscular

tension (tetany), measures to reduce the intense zone of

tetany around an electrode might not have much impact

on the frequency of spinal injuries. As discussed below,

there is evidence both for and against increasing

incidences, and perhaps severity, of spinal injuries as

field intensity increases beyond the threshold for injury.

Since the threshold for seizures sometimes causing spinal

injuries (threshold for twitch), occurs in the zone of

perception, the number of fish sustaining spinal injuries

and escaping the field and capture might be significant,

perhaps as great as among the fish that are caught.

Mortality. The effect of field intensity on mortality

was dramatically demonstrated by Lamarque (1967a,b,

1990). Trout (probably brown trout) were exposed for 20 s

at distances of 20 cm and 50 cm from the anode using a

variety of DC and PDC currents. At 20 cm, where field

intensities were much higher, mortalities resulted with all

six currents tested (6–93%, Appendix B), but at 50 cm,

mortalities were observed for only one of these currents

(90 Hz, half-sine PDC) and for that current were far fewer

than at 20 cm (27% vs. 89%).

However, the importance of electrical-field intensity

as a cause of fish mortality was documented much earlier

by other researchers. For homogeneous 60-Hz AC fields

(39 μS/cm based on a reported water resistivity of 10,000

ohm/in3

 (interpreted as ohm-in); 12° C), McMillan (1929)

documented the increase in mortality with increased field

intensity for YOY (young-of-the-year) chinook salmon

( = 7.9 cm FL). For fish exposed for 1 min, mortality rose

from none at 0.5 to 0.6 Vm/cm (<threshold for stun) to 10%

at 0.7 Vm/cm, 39% at 0.8 Vm/cm, and 57% at 1.0 Vm/cm. For

fish exposed for 5 min, mortality rose from none at 0.2 to

0
 SNYDER 63

0.5 Vm/cm to 62 to 69% at 0.6 to 0.7 Vm/cm and 67 to 79% at

0.8 Vm/cm.

In brief summary of the responses of sardines and

topsmelt to DC and a variety of PDCs in sea water (>50,000

μS/cm), Groody et al. (1950) noted that current densities

greater than 5.4 mA/cm2 “seemed to have a detrimental

effect on the fish in the form of temporary paralysis or

death, depending on duration of the current and the size

of the fish” and that this was especially true for nonpulsating current (DC). As discussed earlier under “Comparison of Currents for Electrofishing Purposes,” Groody

et al. (1950) concluded that among tested currents, 3- to

4-Hz, square-wave PDCs with duty cycles of 67 to 75%

were not only most effective in producing taxis but least

injurious (interpreted as least lethal). Unfortunately, the

details of their experiments and results were apparently

not published.

Using very-low-frequency, square-wave PDC (2-Hz,

20-ms pulses) in fresh water, Collins et al. (1954) reported

that mortality for four size-groups of (YOY) chinook

salmon (4–12 cm TL) increased in direct proportion to

increases in voltage gradient (and current density) from a

threshold at 3.5 to 4 Vp/cm to 57 to 78% at 15 Vp/cm (48

μS/cm, 10–20° C). In these homogeneous-field experiments, fish were held parallel to the lines of current for 30

s. In a similar set of experiments but with voltage gradient

held constant at 4 V/cm and water conductivity increased

incrementally from 50 to 500 μS/cm (thereby increasing

current density), they found that mortality generally increased with current density from a threshold level (first

incidence) at a current density between 0.17 and 0.41 mA/

cm2 to 8 to 75% at 1.9 mA/cm2, but results varied greatly

within and among groups and temperature ranges (10–

19.9° C and 20–25° C).

In a set of heterogenous 15- and 30-Hz, 8.3-ms, squareand half-sine-wave PDC experiments in a raceway (15–

17° C) with YOY coho salmon (6–10 cm SL) forced through

sequentially activated electrode arrays, Pugh (1962) reported mixed results but generally greater mortality in the

more intense electric fields. Trials were run with peak

output voltages of 165 or 250 V at water conductivities of

67, 100, 200, and 1,000 μS/cm. Overall, short-term (24-h)

mortality ranged from 1 to 18% ( = 8%) at 250 V versus

1 to 8% ( = 5%) at 165 V. Immediate mortality among

controls ranged from 2 to 4%. With output held constant

at 250 V and data averaged for the two lower conductivity levels, mortality increased with increasing current density (increased conductivity). However, no similarly

consistent differences were observed when output was

held at 165 V. Test and control fish were held for 30 days

to assess delayed mortality; although mortality ranged

from 2 to 29% and 5 to 15%, respectively, no significant

differences were detected (5% level).

As described under “Type of Current,” Spencer

(1967b) compared the lethal effects of 230-V AC to 115-

V AC on bluegill (8–10 cm) exposed for 1 to 120 s. He

reported far greater 24-h mortalities for 230-V AC than for

115-V AC, up to 58% and 19%, respectively. However, the

two currents differed in more than intensity—the 230-

V AC was a three-phase, 180-Hz current, whereas the 115-

V AC was a single-phase, 60-Hz current. Spencer (1967b)

also exposed bluegill to 60 s of the 230-V AC in another

set of experiments for which water conductivity was

increased in several steps from 100 to 1,000 μS/cm (thereby

also increasing current and power density; water

conductivities calculated from reported water resistivities

of 10,000 to 1,000 ohms [interpreted as ohm-cm]). In these

experiments, 24-h mortality increased progressively with

water conductivity and field intensity from 4 to 56%.

In one set of data for which Whaley (1975) subjected

bluegill and fantail to 45 s of 9-Hz PDC (154 μS/cm, 10° C),

mortality increased progressively from about 21 to 44%

and 19 to 37%, respectively, as voltage gradient increased

from 3.1 to 5.0 V/cm. In another set of data, also incorporating exposures 30, 60, 120, and 180 s (apparently a different set of experiments since 45-s exposure did not

correspond to the aforementioned), a similar pattern of

progressively increased mortality with voltage gradient

for both species was apparent only for exposures greater

than 45 s. Especially for exposures beyond 45 s, the effect of voltage gradient on mortality increased with duration of exposure. For exposures of 180 s at 5 V/cm, mortality

was about 95% for bluegill and 100% for fantail darter.

Whaley (1975) observed no indication of an interaction

between the effect of field intensity and fish length.

Much more recently and contrary to the above research, Hudy (1985) reported very low mortalities (0.5–

1.8%) with no statistically significant differences among

voltage levels (350-V, 700-V, and 760-V output) for 2,250

hatchery rainbow trout (16–26 cm) and brook trout (12–

24 cm) subjected to very high-frequency AC (250–300 Hz;

10 μS/cm, 6° C). However, Sharber (in Sharber and Hudy,

1986) suggested that very low water conductivity and

the type of electrodes used (radio antennae and 25- and

50-cm hoops) probably resulted in relatively small, lowcurrent-density fields (except very close to the electrodes)

and that few mortalities would be expected in such weak

fields. Low water temperature (6° C) might also have minimized mortality.

Among rainbow trout (33–60 cm TL) and Colorado

pikeminnow (30–39 cm TL) exposed to homogeneous 15-

and 60-Hz, square-wave PDC, CPS, or DC gradually

increasing to and held for 5 s at threshold levels for twitch,

taxis, or narcosis, or 1 Vp/cm to assure full tetany (530 μS/

cm; 18° C), Meismer (1999) reported immediate mortality

(10%) only at the highest intensity level and then only

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64 INFORMATION AND TECHNOLOGY REPORT--2003-0002

for rainbow trout subjected to 60-Hz PDC. In a separate

experiment under the same conditions, Meismer (1999)

reported still higher immediate mortality, 30%, for rainbow

trout exposed directly (rather than gradually) to the same

current at an even higher intensity level of 1.5 Vp/cm for

10 s.

As with other responses to a specific field intensity,

the orientation of fish in an electric field is a critical factor

in determining whether the field is strong enough to cause

mortality. Collins et al. (1954) demonstrated its importance

in a homogeneous-field experiment with 7-cm fish held

either parallel or perpendicular to the lines of current at 6

V/cm. Fish held parallel to the lines of current experienced

42 V along their bodies (fish length x voltage gradient)

and suffered 77% mortality, whereas fish held

perpendicular to the field experienced only about 6 V

across the body (about 1 cm in width) and all survived.

Exploratory experiments revealed no differences in

mortality whether fish faced the anode or the cathode.

Collins et al. (1954) concluded from these experiments that

the effective, mortality-producing factor in field intensity

is the total voltage differential across the fish (head-totail voltage, or side-to-side voltage if perpendicular to

the lines of current).

Whaley et al. (1978; also Whaley, 1975) tested the

survival of bluegill (9–17 cm) and fantail darter (3–8 cm)

held parallel to the lines of current in homogeneous PDC

fields (up to 16 Hz) and reported as much as 75 to 95%

mortality when exposed for 2 or 3 min at 4 V/cm (154 μS/

cm, 10° C). Recognizing that their test fish were always

subjected to the maximum voltage differential available,

they suggested that under natural conditions in a stream,

the percentage of fish killed directly by electrofishing

should be notably less because fish would be randomly

located in a heterogenous field, primarily aligned with

water current, and therefore, subjected to varying headto-tail voltages.

Various factors other than orientation were reported

to compound the effect of field intensity on mortality.

Nearly all biologists testing the factor found that mortality at a particular field intensity (above the threshold)

increased with time of exposure (McMillan, 1929; Collins

et al., 1954; Spencer, 1967b; Whaley, 1975; Whaley et al.,

1978). When voltage gradient was held constant, mortality also generally increased with water temperature

(Collins et al., 1954) and, in PDC, with pulse frequency

(Collins et al., 1954; Whaley, 1975; Whaley et al., 1978).

In an experiment with disconcerting results,

Zeigenfuss (1995) subjected several hundred 15- to 35-

cm-TL hatchery-reared rainbow trout to homogeneous

fields of 60-Hz, square-wave PDC for 2 s at 1.6 V/cm (probably peak) in spring of one year (283 μS/cm, 10–12° C)

and 3.2 V/cm the next spring (226 μS/cm, 8° C). Contrary

to expectations, immediate and short-term mortality (within

24-h) was greater at the lower field intensity (9%) than

the higher field intensity (2%), but short-term mortality

among measured and tagged controls was not reported

for comparison. Zeigenfuss (1995) suggested that warmer

water temperatures were probably responsible for the substantially higher immediate and short-term mortality during the first-year trial. He observed that mortality varied

throughout the day but was lowest among those shocked

in the morning and increased among those shocked in

the afternoon (presumably as water temperature increased). During the second-year trial, water temperatures

remained low and mortality was low and constant. There

was no apparent relationship between incidences of mortality and incidences of spinal injury. Although injured

fish had a higher incidence of mortality than shocked

uninjured fish, the difference was not significant. Also,

there was no correlation between length and mortality of

shocked fish.

Injury. Experiments by Spencer (1967a), Roach (1992),

and Mitton and McDonald (1994a) suggest that the incidence of spinal injuries increases with field intensity. In

controlled pond experiments, Spencer (1967a) consistently

found two to three times more spinal injuries in bluegill

subjected to more intense AC fields—9 to 16% at 230 V

and 3.1 A versus 3 to 7% at 115 V and 2.0 A. However, two

different forms of AC were used, a three-phase, 180-Hz

current for the former and a single-phase, 60-Hz current

for the latter. Similarly, Roach (1992) observed greater

incidences of vertebral injuries among adult northern pike

(38–74 cm FL) exposed for 5 s to homogeneous fields of

30 or 60-Hz PDC (50% duty cycles) at 400 Vp than at 100

Vp output (109–132 μS/cm; 11–16° C). For 30-Hz PDC,

incidence of spinal injury was 10% at an output of 400 Vp

(measured voltage gradient of 0.98 Vm/cm) versus 5% at

an output of 100 Vp (0.25 Vm/cm). For 60-Hz, incidence of

spinal injury was 12% at 400 Vp (1.76 Vm/cm) versus 8% at

100 Vp (0.44 Vm/cm). (Note that voltage gradients reported

by Roach, 1992, are inconsistent with current or output

parameters—assuming the same wave shape, same output voltage, and a constant distance between electrodes,

PDCs with the same duty cycle should produce the same

mean or peak voltage gradients, regardless of frequency;

with square-waves, a 50% duty cycle, and plate electrodes

positioned at the ends of the 91-cm-long exposure tank,

mean field intensity should have been about 2.2 Vm/cm

for output of 400 Vp and 0.56 Vm/cm for a peak output of

100 Vp). Mitton and McDonald (1994a) exposed 65 rainbow trout in 50-, 100-, and 600-g size groups each to 20 or

40 s of 60-Hz PDC (5-ms) with outputs of 200, 400, or 600

V (240–270 μS/cm; 7° C). Of all fish tested, only two of the
 SNYDER 65

15 largest (600-g) specimens (13%) exposed at the highest field intensity (600 V output) experienced spinal injuries, both class 2 involving three vertebrae.

In contrast to the above observations, several investigators have reported no significant relationship between incidence of injury and field intensity. Taube (1992),

using equipment similar to that used by Roach (1992),

exposed large adult rainbow trout ( = 39–48 cm FL) for

5 s to low-intensity (100 V output, at or above threshold

level for stun) and high-intensity (400-V output) homogeneous fields of AC, DC, 30-Hz and 60-Hz PDC, and CPS

(100–121 μS/cm; 9–13° C). But unlike Roach (1992), Taube

(1992) found no significant differences in the frequency

of spinal injury or hemorrhages attributable to differences

in tested field intensities for any current (incidences of

injury ranged from 17 to 75% depending on the type of

current). [Taube, 1992, did not indicate whether reported

output voltages for PDC and CPS were peak or mean

values, nor how he measured voltage gradients, but there

were considerable discrepancies between measured values (e.g., for DC, about 0.5 V/cm for 100 V output and 0.9

V/cm for 400 V output) and what might have been expected based on output voltage divided by maximum distance between electrodes (e.g., about 1.1 V/cm and 4.4 V/

cm, respectively). However, if voltage gradient was measured while each fish occupied the tank, their considerable length and volume relative to the exposure tank

probably affected voltage gradient throughout much of

the tank. Voltage gradients for currents other than DC

were reported as mean values.] For northern pike (also

the subject of Roach’s 1992 experiments), Walker et al.

(1994), found no correlation between incidence of externally obvious injury (including failure to swim upright

within 16 to 24 h of exposure) and treatment field intensities between 0.4 to 2.1 Vrms/cm for either sine-wave or

triangular-wave 50-Hz AC, regardless of exposure time

(10, 30, or 60 s; 200–230 μS/cm, 12–14° C). Nor did they

report any such injuries for a comparable series of experiments using 50-Hz PDC. However, Walker et al. (1994) did

not X-ray or necropsy fish for detection of internal injuries. Hudy (1985) reported low percentages of spinal-injury (0.8–2.4%) for hatchery rainbow trout and brook trout

exposed to 250 to 300-Hz AC, but with no significant differences among tested output voltages (350-V, 700-V, and

760-V). As noted above for mortality, Sharber (in Sharber

and Hudy, 1986) suggested that Hudy’s (1985) fish might

not have been subjected to the high field intensities implied by his output voltages. However, field intensities

were obviously high enough to induce at least narcosis.

Just as confounding is the report of no injury by

McCrimmon and Bidgood (1965) for hatchery-reared rainbow trout (11–27 cm). McCrimmon and Bidgood (1965)

exposed their trout two to four times for 3 to 10 s each in

60-Hz AC fields averaging 0.8 and 1.5 V/cm and 120-Hz,

half-sine PDC fields averaging 0.7 and 1.4 V/cm. In this

case, water conductivities and temperatures were moderate (475–550 μS/cm, 11–13° C), and at least the higher

voltage gradients should have been sufficient to induce

tetany, although such was not specifically mentioned.

Like his results for immediate and short-term mortality

(discussed above), Zeigenfuss (1995) reported a

substantially greater incidence of spinal injury among

fish subjected to the lower of two field intensities. Based

on lateral-view X-rays, he detected a 70% incidence of

spinal injuries for rainbow trout exposed to 2 s of 60-Hz,

square-wave PDC at a homogeneous 1.6 V/cm, but only

40% for those similarly exposed to a field with twice the

field intensity (3.2 V/cm). However, nearly all that

difference was accounted for in the least-severe class of

spinal injuries (class 1, compressions); in both high and

low-intensity fields, only 4 or 5% of the fish incurred

class-2 injuries and no more than 2% incurred class-3

injuries. No injuries were detected among control fish.

Upon necropsy, Zeigenfuss (1995) found minor

hemorrhaging in subsamples of both control and shocked

fish. These were mostly thin spikes extending from

between vertebrae into the muscle and suspected to be

artifacts of freezing because many were not associated

with obvious vertebral damage. The only fish with severe,

class-3, hemorrhages were shocked fish with class-2

spinal injuries from the lower-intensity treatment. Among

fish from the higher-intensity trials, all presumably

electrofishing-induced hemorrhages were class 1 and

none were aligned with vertebral injuries.

In laboratory experiments, Meismer (1999) clearly

proved that at least minor (class 1) spinal injuries are just

as likely to occur at the threshold for twitch near the

outermost margins of the perceived electric field as within

the effective portion of that field (zones of taxis, narcosis,

and tetany). Meismer (1999) exposed rainbow trout (33–

60 cm TL) and Colorado pikeminnow (30–39 cm TL) to

homogeneous fields of various currents (15- or 60-Hz,

square-wave PDC, CPS, or DC; 530 μS/cm, 18° C) by

gradually increasing field intensity from zero to the

observed threshold for twitch, taxis, or narcosis or 1 Vp/

cm (sufficient to assure full tetany), then holding the fish

at that treatment level for 5 s. Combining data for the

different currents, the percentages of rainbow trout with

spinal injuries were 4% for twitch, 1% for taxis, 1% for

narcosis, and 4% for tetany. For Colorado pikeminnow,

they were 11, 4, 5, and 6%, respectively. Of both species,

only the number of Colorado pikeminnow exposed up to

and at the threshold for twitch (11%) was significantly

greater than for controls (no injuries). All spinal injuries

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66 INFORMATION AND TECHNOLOGY REPORT--2003-0002

were minor except for a slight misalignment (class 2) in

one Colorado pikeminnow exposed in DC at the threshold

for narcosis. Although exposures at each higher response

level included ramping intensity through all lower

thresholds, the data do not suggest cumulative increases

in spinal injuries (all or most injuries could have occurred

at the lowest threshold). Incidences of detected

hemorrhages at or near the spine were few and insignificant

for Colorado pikeminnow (1%, 0, 3%, and 3%,

respectively); hemorrhages were not assessed for rainbow

trout. In contrast to spinal injuries, the incidence of brands

significantly increased with treatment field intensity from

4% for twitch, to 19% for taxis, 34% for narcosis, and 46%

for tetany. This trend was evident for all treatment currents

except DC.

In another set of experiments, Meismer (1999) exposed both species directly to a still higher field intensity

of 1.5 Vp/cm in either of two waveforms of 60-Hz PDC

(square and quarter-sine) for 10 s and reported an even

higher incidence of branding (73%) for rainbow trout than

discussed in the preceding paragraph. Although the overall incidence of spinal injury for rainbow trout in this

experiment was 10%, it was not significantly different from

the control or incidences of injuries for lower intensity

treatments in Meismer’s (1999) previously discussed experiments. Like the previously discussed experiments, no

brands were observed on Colorado pikeminnow, but unlike those lower intensity treatments, neither were spinal

injuries or associated hemorrhages. Again, hemorrhages

were not able to be assessed for the trout.

For most other investigations comparing frequency

of spinal injuries for fish subjected to different levels of

field intensity, results were mixed or inconclusive. For

example, Sharber et al. (1994) conducted field experiments

in the Colorado River (600–800 μS/cm, 9–11° C) and concluded that differences in field intensity near the anodes

had no significant effect on the percentage of large rainbow trout with X-ray-detected spinal injuries. However,

the results in that regard should probably be considered

inconclusive. Electric fields were generated by 12 Ap of

60-Hz, 4-ms, square-wave PDC between a spherical 30-cm

cathode and one of three different types and sizes of

anodes. The anode assumed to have the lowest nearfield intensity (that near the surface of the anode), a 1-m

Wisconsin ring with ten 20-cm droppers, required an output of 215 Vp, and resulted in injuries to 43% of the large

trout collected. A 30-cm sphere anode was assumed to

represent an intermediate near-field intensity, required

315 Vp, and also resulted in injuries to 43% of the fish. A

1.2-m-long by 1-cm-diameter steel cable was assumed to

represent a high near-field intensity, required 380 Vp, and

resulted in injury to 65% of the fish. Despite capture and

examination of fewer fish using the cable electrode (23

vs. 60 for the dropper ring and 116 for the sphere), the

latter result suggests that high-intensity fields near an

electrode can cause substantially more injuries. However,

because fish appeared to be attracted to the lower end of

the cable electrode and were stunned farther below the

water surface than with the other electrodes, Sharber et al.

(1994) noted that the fish were more difficult to net and

suggested that the higher incidence of trout injuries for

the cable electrode might have been due to longer exposure time rather than the higher intensity of the field near

the electrode’s surface.

The electrical resistance of an electrode (or electrode

array) and field intensity next to its surface are inversely

proportional to its exposed surface area, whereas the

shape and distribution of field intensity beyond the

surface but within a few multiples of the principal

dimension depend on the shape, size, and orientation of

the anode. Reynolds and Kolz (1995) calculated the

surface areas of the anodes used by Sharber et al. (1994)

and suggested that the sphere rather than the Wisconsin

ring in their investigation had the least resistance and

least intense electric field near its surface, but their

calculations assumed a smooth surface for the cable anode

and the droppers of the Wisconsin-ring anode. The

rough, convoluted surface of stranded or braided cable

that was probably used has a considerably greater surface

area. Also, the output voltages required to maintain a

current of 12 Ap between the anode and cathode support

Sharber et al.’s (1994) assumption that anode resistance

and near-electrode-surface field intensity was least for

the Wisconsin ring and greatest for the cable. Even so,

because most fish were probably captured some distance

from the anodes and because Sharber et al. (1994) failed

to document the magnitude and distribution field intensity

around each anode with actual voltage-gradient

measurements, relative field intensity somewhat beyond

anode surfaces and that to which the fish were actually

exposed remains uncertain (Reynolds and Kolz, 1995;

Sharber et al., 1995).

Regardless of the limitations and interpretation of

the above experiment, Sharber et al. (1994) reported that

in this and a related experiment, as well as experiments

reported by Sharber and Carothers (1988), few if any captured specimens exhibited symptoms of tetany despite

high incidences of injury. Based on this remark, anecdotal observations, and circumstantial evidence, such as

brands on fish netted relatively far from the electrodes,

field intensities high enough to induce tetany are not

required to cause spinal injuries and such injuries can

occur anywhere within at least the effective portion of

the field, and possibly outside the threshold for taxis

(Sharber and Carothers, 1988; Sharber and Carothers, in

Reynolds et al., 1988; Meyer and Miller, 1991, unpublished

manuscript, 1991; Sharber et al., 1994). Lamarque (1990)

noted that a single pulse and sometimes a low voltage
 SNYDER 67

can be sufficient to cause the violent contractions resulting in such injuries (but he did not elaborate) and Meismer

(1999), as discussed above, proved in homogeneous field

experiments that spinal injuries are just as likely to occur

at the threshold for twitch as in more intense portions of

the field.

As in the comparison of anode systems by Sharber

et al. (1994) above, Thompson (1995) and Thompson et al.

(1997a) compared the injurious effects of two anode

systems with significantly different near-field intensity

and reported mixed results. However, in this case, the

authors actually measured peak field intensity for each

event at 15 cm from the anodes—2.3 to 7.4 V/cm for the

single throwable anode of a boat electrofishing system

and 0.5 to 1.2 V/cm for the multiple anode array (four or

five hand-held anodes) of a shore-based wading system.

Using 60-Hz, half-sine-wave PDC, approximately 50

rainbow trout (13–51 cm TL) and 50 brown trout (10–49

cm TL) were collected, frozen, and later X-rayed and

necropsied for each electrofishing system from each of

three Colorado rivers (90–270 μS/cm, 1–7° C). Incidences

of spinal injury and associated hemorrhages in rainbow

trout were greater for the higher-intensity throwable anode

system (18–64% spinal injury and 28– 65% hemorrhage,

32–76% combined) than the multiple anode wading

system (6–40% spinal injury and 13–49% hemorrhage,

13–58% combined). For brown trout (10–49 cm TL)

incidences of hemorrhages were also greater for the

throwable anode than the multiple anode system (24–

45% and 13–30%, respectively) but incidences of spinal

injuries were similar for both high and low-intensity

anodes (18–52% and 27–38%, respectively). Combined

incidences of spinal injury and hemorrhage for brown

trout were 36–61% and 25–51%, respectively. In addition

to considerable overlap in incidences of injury between

the higher and lower near-field-intensity systems, rank

for incidence of injury within each system usually, but

not always, correlated with rank for corresponding nearfield intensity for rainbow trout but not for brown trout.

Brown trout often incurred the least incidences of spinal

injuries in the most intense fields. Despite relatively high

rates of internal injury, Thompson et al. (1997a) noted

that very few of the fish exhibited external signs of injury.

Spinal injuries were not only most numerous but also

most severe for rainbow trout collected with the moreintense, throwable-anode system (10–27% for class 1, 8–

30% for class 2, and 0–13% for class 3). For rainbow trout

collected with the less-intense, multiple-anode, wading

system and brown trout collected with by either system,

most spinal injuries were class 1 (6–32%), half as many

were class 2 (0–20%) and very few were class 3 (0–2%).

Control rainbow trout and brown trout, which were

collected by angling or gill net from river segments not

recently electrofished and by dip net from rearing

raceways, had 7–11% spinal injuries (one class 2 and the

rest class 1). Among control fish collected by angling

and dip net, only one hemorrhage (a class 3 wound) was

detected, but among controls taken by gill net (all rainbow

trout), 16% had hemorrhages (one class 3 wound and the

rest class 1).

Ruppert (1996) and Ruppert and Muth (1997)

compared the frequency of vertebral injuries and muscular

hemorrhages near the spine in juvenile bonytail (5–8 cm

TL) exposed to 10 s of homogeneous CPS and 30-, 60-,

and 80-Hz, square-wave PDC (2.6-, 4-, 4-, and 5-ms pulses)

at field intensities corresponding to predetermined

thresholds for taxis (0.8 Vp/cm for CPS and 0.4–0.5 Vp/cm

for the others), narcosis (1.0 and 0.6–0.7 Vp/cm,

respectively), and tetany (1.4 and 1.0–1.1 Vp/cm,

respectively; water 950 μS/cm and 15° C). The anticipated

responses were achieved in all treatment fish. Based on

low-power microscopic examination of the spines in

filleted fish, no vertebral damage was detected for any

treatment fish or controls, but spinal hemorrhages, all

class 2 with an average of three per fish, were observed in

13% of all treatment fish (range of 3–27% for individual

treatments, none in controls). Combining electrical-current

treatments, frequency of spinal hemorrhages was greatest

for tetany-level treatments (18%), least for narcosis (11%),

and intermediate for taxis (13%). However, rank for

frequency of hemorrhages among response intensity

levels varied with individual treatment currents. The

highest mean incidence of hemorrhages occurred at the

taxis level for 80-Hz PDC (23%), the narcosis level for CPS

(10%), and tetany level for 30- and 60-Hz PDC (20 and

27%, respectively). Although higher field intensities tend

to induce a higher frequency of spinal hemorrhages, this

obviously is not always the case. Sometimes, as many or

more injuries can occur in fish subjected to field intensities

no higher than those needed to induce taxis (perhaps

even lower).

Voltage differentials across fish must exceed some

minimum value (threshold) before muscular seizures and

spinal injuries are likely to occur. Thus, orientation of fish

when first exposed to the effective portion of the field (or

later) is probably as significant a factor as it is for other

responses and mortality. However, in preliminary

laboratory experiments, Sharber (personal

communication) found the situation for spinal injuries to

be opposite that discussed above for mortality (Collins

et al., 1954). Using 60-Hz, 4-ms, square-wave PDC to

produce a homogeneous field, he recorded injuries in over

30% of trout held perpendicular to the electric current

(head-to-tail voltage least) but in only 3% of trout held

parallel to the current (greatest heat-to-tail voltage

differential). For trout held perpendicular to the current,

reduction of pulse frequency to 15-Hz (5-ms) also reduced

injuries to 3%. These results further indicate that high
68 INFORMATION AND TECHNOLOGY REPORT--2003-0002

field intensities and head-to-tail voltage differentials might

not be critical factors in electrofishing injuries, at least

when using PDC. The results also correlate well with early

observations by Haskell et al. (1954) that upon circuit

closure, muscular bending of fish toward the anode was

greatest when fish were oriented perpendicular to the

current and almost nil when they were oriented parallel to

the current. The matter deserves confirmation and further

investigation. Evidence that spinal-related injuries are

usually far fewer in DC than PDC (see discussion under

“Type of Current”) indicates that some attributes of PDC

fields in addition to field intensity must be responsible.

Duration of Exposure

Concentration (in this case, field intensity—

discussed above) and duration of exposure are the most

critical factors in determining the effects of most chemicals

and physical parameters on the physiology, behavior, and

survival of organisms. So it is with the lethal and sublethal

stressful effects of electric fields. Increased mortality with

increased exposure time has been well documented,

especially at field intensities sufficient to induce tetany.

But beyond a necessary minimum threshold, and although

addressed in only a couple investigations, duration of

exposure in AC and DC fields does not appear to have an

important effect on spinal and related injuries. However,

this might not be the case for PDC (or pulsed AC).

Observations of a direct relation between pulse frequency

and injuries (discussed later) suggest that duration of

exposure also should be important because longer

exposures would subject fish to more pulses.

As noted above under “Field Intensity,” McMillan

(1929) and Collins et al. (1954) reported increases in

mortality of YOY chinook salmon with increases in

duration of exposure. Using 60-Hz AC, McMillan

documented mortality rates of 10 to 57% for 1 min

exposures at 0.7 to 1.0 Vm/cm versus 62 to 79% for 5 min

exposures at 0.6 to 0.8 Vm/cm. Using 2-Hz, square-wave

PDC (20-ms pulses, 4% duty cycle), Collins et al. (1954)

held treatment fish ( = 7 cm TL) parallel to the lines of

current at three homogeneous levels of field intensity

(water about 50 μS/cm, 10 to 20° C). At 1 V/cm, a 15-min

exposure resulted in 2% mortality and a 20-min exposure

in 17% mortality. At 2 V/cm, the exposure threshold for

mortality was 5 min when 6% of the fish succumbed;

longer exposures of 7.5, 10, and 15 min resulted in 12, 32,

and 30% mortality, respectively. At 4 V/cm, mortality

increased progressively with exposure time from 1% for

0.5 min to 52% for 10 min; it then dropped to 39% and

48% for 11- and 12-min trials and rose again to 77% for 13

min, the longest exposure tested. In another experiment

with voltage gradient held constant at 2 V/cm in 50 μS/cm

water (100 μA/cm2

), Collins et al. (1954) reported that

mortality increased progressively from 2 to 4% for 3- to

3.5-min exposures to 59% for a 10-min exposure. With

water conductivity increased to 85 μS/cm (0.17 μA/cm2),

exposure times of 2, 3, 4, 5, 7.5, and 10 min resulted in 4, 6,

44, 37, 58, and 48% mortality, respectively. They further

reported that the effect of duration of exposure on

mortality in PDC increased directly with size of fish, water

temperature, and pulse frequency but not pulse duration.

Like Collins et al. (1954), Whaley et al. (1978; also

Whaley, 1975) also found that mortality increased with

duration of exposure for PDC. Based on tests of fantail

darter (3–8 cm) and bluegill (9–17 cm) held parallel to the

lines of current in a homogeneous field at 4 V/cm (154 μS/

cm, 10° C), they reported that mortality was low to negligible for exposures up to 15 s and that thereafter recovery time and mortality increased progressively with longer

exposures. Mortality was greater than 35% for 2-min exposures and greater than 50% for 3-min exposures. The

effect of exposure time was compounded by increases in

pulse rate such that greatest mortalities (75–95%) were

recorded for the longest exposures (2 and 3 min) at the

highest pulse rate tested (16 Hz). Of the two factors, they

concluded that exposure time had the greater impact on

mortality. Referencing unpublished data by O. Maughan

and C. Schreck, Whaley et al. (1978) noted that mortality

also increased with exposure time for fathead minnow

and bluegill subjected to electric fields for up to 4.5 min

(160 V output; form of current and field intensity not

noted).

As discussed under “Type of Current,” Spencer

(1967b) conducted a series of experiments to assess the

usefulness of electrofishing for killing intermediate-size

(~8–10 cm) bluegill to thin populations. In doing so, he

subjected confined batches of test bluegill to 1 to 120 s of

230-V AC, 115-V AC, and 115-V DC (heterogenous fields

in a concrete pond) and monitored survival for 24 h. In

each case, mortality increased progressively with exposure time (with a few minor deviations)—from 1 to 58%

for 230-V AC, 0 to 19% for 115-V AC, and 0 to 29% for 115-

V DC. Trials for 230-V AC were extended up to 300 s and

mortality continued to increase to 75%. About 80% or

more of all 230-V-AC mortalities and 50% or more of mortalities for the other currents occurred within 2 h after

exposure. For exposures of at least 90 s, over 90% of 230-

V-AC mortalities occurred within the first hour and over

65% within the first 5 min after exposure. Over 50% of

mortalities for the other currents occurred within the first

hour (no 5-min data).

Although Spencer (1967b) and others discussed

above found time of exposure to be a critical factor in

electrofishing mortality, Spencer (1967a) found no significant relation between duration of exposure and incidence of spinal injury for bluegill exposed to the same

currents and range of exposure times. Accordingly, he

0
 SNYDER 69

concluded that these injuries occur immediately when

fish are first exposed to the electric field.

Walker et al. (1994) also reported no correlation between injury and exposure time (10, 30, or 60 s), although

they observed externally obvious injury (or failure to swim

upright within 16–24 h) among 24% and 33% of juvenile

northern pike subjected to 50-Hz, sine- and triangularwave ACs, respectively. As noted under “Type of Current,” they observed no such injuries in a comparable

series of experiments using 50-Hz PDC. However, no fish

in these experiments were X-rayed or necropsied for internal injuries not reflected in external damage or abnormal swimming behavior. No additional reports were found

on the effect of exposure time on incidences of spinal or

related injuries.

Exposure times utilized in these and other experiments on the effects of electric fields on fish vary from a

second to several minutes or longer. In electrofishing practice, 20 s is a long exposure; in rivers, most fish are subjected to electrofishing fields for less than 10 to 15 s

(Bestgen, personal communication). However, fish may

be subjected to much longer exposures when they encounter electrical barriers or guiding devices or when electricity is used, like a chemical anesthetic, to induce

narcosis for handling or experimental purposes (see earlier discussion under “Zones of Narcosis and Tetany”).

For example, using DC at a homogeneous intensity level

just sufficient to maintain narcosis with fish still actively

breathing (0.25 V/cm, 13–21° C, 450 μS/cm), Kynard and

Lonsdale (1975) exposed yearling rainbow trout (~12 cm)

for 1,2, 4, or 6 h but reported mortalities (7%) only for the

6-h trials. Recovery to normal swimming and feeding was

almost instantaneous for fish exposed up to 2 h but required up to half a day for some fish exposed for 6 h.

Waveform, Pulse Shape

The effect of waveform or pulse shape on AC or

PDC-electrofishing mortality or injury has been poorly

studied and remains inconclusive. Exponential, and halfsine (rectified AC) PDC and square-wave PDC, in separate studies, have been implicated as particularly lethal

and quarter-sine PDC and square and half-sine PDCs,

again in separate studies, as particularly injurious. A comparison of sine-wave and triangular-wave AC revealed

no significant differences in incidence of externally obvious injuries in exposed fish but notable differences in the

nature and perhaps severity of those injuries.

Vibert (1967b), in agreement with Halsband (1967),

claimed that exponential (i.e., capacitor or condenserdischarge) waveforms have the greatest physiological

effect on fish and are therefore among the best waveforms

for electrofishing. But according to his associate,

Lamarque (1967a), use of exponential waveforms in

electrofishing was based on the false assumption that

best results always are obtained with high voltage or

tetanizing currents. Lamarque (1967a) observed that

exponential waveforms can kill an eel in 30 s and concluded

that with their steep initial slopes and short pulse

durations exponential waveforms are the worst form of

PDC. Lamarque (1967a,b, 1990) further documented the

adverse effects of exponential, as well as half-sine,

waveforms in tests conducted with an assortment of gear

in a stream. For trout exposed to electric fields for 20 s

while being held about 20 cm from and facing a 40-cm ring

anode, he reported mortalities of 86 to 93% when using

80-Hz, exponential waveforms (33% and 50% duty-cycle)

and 89% when using a 90-Hz, 400-V, half-sine waveform

(rectified AC, probably full-wave because no duty cycle

was reported). For other waveforms, mortalities were

substantially less—50% mortality for a 5-Hz, 400-V, 33%

duty-cycle, square waveform; 17% for a 400-V, rippled

DC (partially smoothed, rectified AC); and only 6% for

500-V DC. Testing fish in the same currents at 50 cm from

the anode, he recorded mortalities for only the half-sine

PDC waveform (27%). Lamarque (1967a) suggested that

the high mortalities for the exponential and sine-wave

PDCs tested might be attributed to their high frequencies.

If output voltage was mean rather than peak, the more

lethal currents were also more intense than the DC

currents, and as discussed earlier, intensity is a very

important factor in electrofishing mortality.

In a one-on-one laboratory comparison, Meismer

(1999) found 60-Hz, square-wave PDC generated by

Cofflet’s VVP-15 electrofisher was much more lethal to

adult rainbow trout (30% immediate mortality) than 60-

Hz, quarter-sine-wave PDC generated by Smith-Root’s

GPP 5.0 electrofisher (no immediate mortality). The fish

were exposed at 1.5 Vp/cm for 10 s (530 μS/cm; 18° C).

Large subadult Colorado pikeminnow were similarly tested

but experienced no immediate mortality.

Although Lamarque (1967a,b, 1990) concluded that

exponential and half-sine waveforms were among the most

lethal of PDC waveforms, Sharber and Carothers (1988,

1990), did not report any immediate mortalities for large

rainbow trout (30–56 cm TL) collected from the Colorado

River using similar waveforms. However, they did find

that 60-Hz, quarter-sine PDC was more injurious than either 60-Hz, exponential or square-wave PDCs—67% of

the fish had X-ray-detected spinal injuries versus 44%

and 44%, respectively. Both the quarter-sine and square

waveforms had a 25% duty cycle (4-ms pulses) and were

output at 260 Vp. Quarter-sine-wave PDC also damaged

significantly more vertebrae per fish ( = 9.5) than exponential PDC ( = 6.6), but the number of vertebrae damaged by square-wave PDC ( = 8.2) was not statistically

different from either. Spinal injuries were not observed in

12 nonelectrofished trout of similar size from a hatchery.

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70 INFORMATION AND TECHNOLOGY REPORT--2003-0002

In a similar comparison of electrofishing injuries,

Fredenberg (1992) reported very high injury rates for rainbow trout and brown trout collected from several drainages in Montana using both 60-Hz, square-wave (~8-ms

pulses) and half-sine-wave (both half and fully rectified

AC, ~8- and ~16-ms pulses) PDCs—78 to 98% and 62 to

90% of the fish, respectively. Fewer fish were injured using square-wave CPS (pulse train; 15-Hz packets or bursts

of three 240-Hz pulses) or a hybrid DC-PDC waveform

(top half of 60-Hz, half-sine pulses over a half-voltage DC

baseline)—31 to 54% and 44 to 64%, respectively. These

percentages are based on combined data for X-rays and

necropsy (Sharber and Carothers 1988 and 1990 considered only vertebral damage based on X-rays) and do not

take into account substantial differences in severity of

the injuries. The data include minor hemorrhages likely to

have been discounted or overlooked by others

(Fredenberg, personal communication).

In addition to the differences in mortality discussed

above for a one-on-one laboratory comparison, Meismer

(1999) found 60-Hz, square-wave PDC somewhat more

harmful than 60-Hz, quarter-sine-wave PDC for adult rainbow trout. When exposed for 10 s to the square-wave

PDC at 1.5Vp/cm, 95% of the trout suffered brands, but

when similarly exposed to the quarter-sine PDC, only 50%

suffered brands. Meismer (1999) also reported that 15%

of the rainbow trout exposed to the square-wave current

and 5% of those exposed to the quarter-sine current experienced minor spinal injuries, but neither incidence of

injury was significantly different from controls (no injury). Large subadult Colorado pikeminnow were similarly tested with both waveforms, but experienced no

detectable external or internal injuries.

The typical waveform for AC currents is a sine wave,

but others are possible. Walker et al. (1994) compared the

narcotic and externally obvious injurious effects of 50-

Hz, sine- and triangular-wave ACs, at various field intensities and exposure times in juvenile northern pike.

Differences in the incidence of injuries between these

currents were unpredictable, but there were relatively consistent differences in the nature and severity of those

injuries. Fish injured when exposed to sine-wave AC (24%

of the fish) had cutaneous hemorrhages (brands) along

their entire body length and all failed to swim upright

within 16 to 24 h, whereas those injured when exposed to

triangular-wave AC (33% of the fish) had hemorrhages in

their paired and median fins, and half of those (17%) failed

to recover upright swimming within 16 to 24 h.

Pulse Frequency

Pulse frequency appears to be a primary factor

affecting PDC-caused spinal injuries and may be a

significant, but probably secondary, factor in

electrofishing mortalities. If, as strongly suspected, most

spinal injuries and related hemorrhages in electric fields

are caused by sudden changes in electrical potential, as

when currents are switched on and off, then it should not

be surprising that investigators have generally reported

increasing incidences of such injuries with increasing PDC

frequency. Only a couple of studies suggested no

relationship. If field intensity and exposure time are

maintained above the threshold for lethal effects, there is

some evidence that mortality can also be greater when

using higher-frequency PDCs.

As mentioned in the preceding section (Waveform,

Pulse Shape), Lamarque (1967a) suggested that high mortalities observed after 20-s exposures near the anode using exponential and half-sine PDCs (86–93%) might have

been caused by the high pulse frequencies of those currents, 80 and 90 Hz, respectively. But even for 5-Hz,

square-wave PDC, he reported 50% mortality. With regard to injuries in PDCs, Lamarque (1990) suggested that

extent of injury depends mainly on pulse frequency and

pulse duration. He concluded that “the worst currents

are those with a pulse duration of 2–5 ms at 5–200 Hz.”

Yet these are precisely the PDC ranges most used in recent decades, including currents designed to reduce the

occurrence of spinal injuries.

Collins et al. (1954) and Whaley et al. (1978; also

Whaley, 1975) provided two of only three other reports

of the effects of PDC pulse frequency on electrical-field

mortality. In both cases, pulse frequencies were limited to

no more than 16 Hz. Collins et al. (1954) reported that

mortality among YOY chinook salmon exposed for 30 s to

homogeneous fields of square-wave PDC increased with

pulse frequency from none for 5-cm fish at 3 Hz to a

maximum of 75% for 11-cm fish at 15 Hz (4 V/cm, 768 μW/

cm3

, 20-ms pulses, 48 μS/cm). Whaley et al. (1978; also

Whaley, 1975) found that mortality of fantail darter and

bluegill exposed for 60 to 180 s in homogeneous PDC

also increased with pulse rate (and exposure)—from 20

to 69% at 2 Hz and 32 to 77% at 9 Hz to 62 to 95% at 16 Hz

(4 V/cm, 154 μS/cm, 10° C). Noting that Northrop (1967)

had reported poor taxis for frequencies of 10 Hz or less,

Whaley et al. (1978) concluded that their “. . . data showed

relatively high mortality of fantail darters and bluegills in

the pulse frequency defined as giving good electrotactic

response.” Collins et al. (1954) reported that mortality rates

were compounded by both increased exposure times and

increased size. Whaley et al. (1978) also reported that

mortality was compounded by exposure time but did not

observe a size-of-fish effect. Noting that pulse duration,

and therefore the total energy applied per unit time, did

not appear to influence the incidence of mortality, Collins

et al. (1954) concluded that “change in potential” and the
 SNYDER 71

rate at which such occurs (i.e., pulse frequency, switching

current on and off) significantly affected the extent of

electrofishing mortality.

In a comparison of the harmful effects of 15- and 60-

Hz, square-wave PDC on fish (530 μS/cm; 18° C), Meismer

(1999) reported that, after gradually increasing homogeneous field intensity to beyond the threshold for full

tetany and holding it for 5 s at 1 Vp/cm, rainbow trout

experienced an immediate, but insignificant, mortality of

10% in the 60-Hz current, but none in the 15-Hz current.

All trout survived similar treatments in these currents at

lower field intensities (thresholds for twitch, taxis and

narcosis). Meismer (1999) similarly tested subadult Colorado pikeminnow with these PDCs and both species with

CPS and DC but observed no other cases of immediate

mortality.

Combining data for the various field intensities in

these same experiments, Meismer (1999) found that although rainbow trout suffered insignificantly few spinal

injuries in any current, they were much more susceptible

to brands when exposed to 60-Hz PDC (46%) than 15-Hz

PDC (16%), or either of the other two currents tested (19–

21%). In contrast and contrary to general expectations,

he found Colorado pikeminnow, which suffered no brands

in any treatment, more susceptible to spinal injury when

exposed to 15-Hz PDC (11%) than 60-Hz PDC (5%) or the

other currents tested (6% each). However, all spinal injuries but one were minor (class 1) and only the percentage

of fish with spinal injuries exposed to the 15-Hz PDC was

significantly different from controls (no injury).

Northrop (1962, 1967) reported that all brown trout

(20–25 cm) he subjected to 33- and 100-Hz, square-wave

PDCs (20- and 5-ms pulses, respectively) recovered within

a few minutes, swam, and reacted normally to external

stimuli, but that those subjected to the 100-Hz current

were immediately narcotized (tetanized), precluding the

taxis observed at 33-Hz, and had bloody vents. He attributed this internal bleeding to violent uncoordinated muscle

spasms caused by the higher-frequency current. Unlike

Northrop (1962, 1967), most subsequent investigations

of electrofishing injuries relative to PDC frequency were

based on X-ray analysis or necropsy.

McMichael (1993) and McMichael and Olson (unpublished manuscript 1991—exclusive source of brand

data) reported substantially higher incidences of brands,

vertebral damage, and associated hemorrhages among

hatchery-reared rainbow trout (14–48 cm FL) exposed

twice to 90-Hz, square-wave PDC (58%, 35%, and 53%,

respectively) than those similarly exposed to 30-Hz,

square-wave PDC (4%, 22%, and 35%, respectively). No

injuries were detected among unexposed controls. Electric fields were produced in separate raceway pens using

a backpack electrofisher with an output of 300 V and duty

cycle of 12.5% (McMichael, personal communication).

After initial exposure, the fish were processed (anesthetized, measured, weighed, scale samples removed, and

tagged) and monitored for 7 days. Following the 7-day

monitoring period, they were recaptured with the same

currents, iced, and necropsied within 2 h.

Field experiments in the Colorado River by Sharber

et al. (1994) also support a direct relation between spinal

injuries to large rainbow trout and pulse frequency for

square-wave PDCs (600–800 μS/cm, 9–11° C; spherical

electrodes). Based on X-rays alone, they reported spinal

injuries among 3% of the fish taken at 15 Hz, 24% at 30 Hz,

43% at 60 Hz, and 62% at 512 Hz (pulse width was 4-ms at

all frequencies except 512 Hz, where it was 0.2-ms). A

laboratory test in homogeneous 15-Hz PDC at 0.5 V/cm

also resulted in injury to 3% of the fish (Sharber, personal

communication).

Although Taube (1992) also documented very high

incidences of spinal injuries and related hemorrhages for

large rainbow trout exposed to homogeneous square-wave

PDCs, and his data suggest a tendency for more injuries

at higher frequencies, he reported no significant difference in injury rates between currents of 30 Hz (33–58%

spinal injuries, same for hemorrhages) and 60-Hz (42–

58% spinal injuries, 33–50% for hemorrhages). Likewise,

for trout exposed to heterogeneous fields of 20-Hz (25–

58%), 30-Hz (33%), and 60-Hz, square-wave PDC (67%).

Contrary to the above reports of high incidences of

spinal injury to rainbow trout subjected to moderate and

high-frequency PDCs, McCrimmon and Bidgood (1965)

and Dwyer and White (1995), as discussed earlier under

“Type of Current,” reported no electrically induced spinal injuries for rainbow trout (11–27 cm and ~33 cm, respectively) exposed to 120- and 250-Hz, half-sine PDC,

respectively. Also as discussed in that earlier section,

Mitton and McDonald (1994a) detected very few spinal

injuries among rainbow trout (~600 g) they exposed to

60-Hz PDC for 20 to 30 s. However, these were non-comparative investigations with respect to PDC frequencies.

Although much less obvious and serious than injuries usually observed in trout (Roach, personal communication), Roach (1992) observed a higher percentage of

vertebral injury in northern pike (36–74 cm FL) exposed

to homogeneous fields of 120-Hz PDC (28%) than 30- and

60-Hz PDCs—28% versus 5–12%. However, the results

are confounded by field intensities, water conductivities,

and water temperatures that also differed (300–600 V, 0.93

Vm/cm, 1,017–1,090 μS/cm, 10–13° C for the 120-Hz PDC

vs. 100 or 400 Vp, 109– 32 μS/cm, 0.25–1.76 V/cm, 11–16°

C for the 30 and 60-Hz PDCs). Over an output range of 50

to 300 V, Roach (1992) also observed spinal-related hemorrhages among 15% of pike exposed to 60-Hz PDC but

none among those exposed to 30-Hz PDC.
72 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Remaining investigations for non-salmonids failed

to demonstrate a significant relationship between pulse

frequency and spinal injuries or hemorrhages. Newman

(1992; unpublished manuscript, 1991) reported spinal injuries for 25% of walleye (18–48 cm TL) collected in lakes

using 120-Hz PDC and 31% for those collected in rivers

using 30-Hz PDC. However, output voltage (mean or peak

not specified) was 200 V at 120 Hz versus 310 V at 30 Hz

and water temperatures were also warmer for the 30-Hz

collections (26° C vs. 22° C). For juvenile bonytail (5–8

cm TL) exposed to 10 s of homogeneous 30-, 60-, and 80-

Hz, square-wave PDC (4-, 4-, and 5-ms pulses) at voltagegradient thresholds for taxis, narcosis, and tetany (940

μS/cm, 15° C), Ruppert (1996) and Ruppert and Muth (1997)

observed no vertebral damage via low-power microscopic

examination but reported a non-significant tendency for

increased incidences of spinal hemorrhages with increased pulse frequency. Combining field-intensity treatments, frequency of spinal hemorrhages (all class 2 with

an average of three per fish) averaged 10% (3–20%) at 30-

Hz, 14% (7–27%) at 60-Hz, and 19% (13–23%) at 80-Hz.

However, the differences were not significant and the

overall tendency for hemorrhages did not hold for taxis

and tetany-intensity treatments. At the intensity level

required for taxis, incidence of hemorrhages was the same

for 30- and 60-Hz PDCs (7%) and at the tetany level, incidence of hemorrhages was the same for 30- and 80-Hz

PDCs (20%) and greatest for 60-Hz PDC (27%).

Perhaps, as suggested by Collins et al. (1954) for

mortality, convulsions resulting in spinal injury occur

predominantly as the current or pulse is “switched on.”

This might explain why fewer spinal injuries generally

occur at lower frequencies in PDC and perhaps why even

in straight DC (no pulses) some spinal injury has been

observed (DC momentarily acting like PDC when switched

on and off—Lamarque, 1990). Indeed, Haskell et al. (1954)

documented that in sufficiently strong fields, fish responded to each circuit closure with a muscular seizure

that resulted in a bending of the body towards the anode.

Interestingly, and counter to the concept of greater headto-tail voltages yielding stronger responses, Haskell et al.

(1954) found that the more nearly perpendicular the fish

was to the lines of current, the stronger the bending response. Fish in line with the current exhibited little, if any,

bending of the body. Perhaps the convulsions resulting

in these bends occur on both sides of the body but are

proportionally stronger on the side facing the anode and

essentially equal when the fish is parallel to the current.

Pulse Trains

Pulse trains are a complex variation of PDC frequency

usually consisting of a short series of higher-frequency

pulses (referred as trains, packets, or bursts) delivered at

a lower secondary frequency. For example, Coffelt

Manufacturing’s CPS (complex pulse system) consists of

trains of three very rapid 240-Hz, 2.6-ms pulses delivered

at a secondary frequency of 15 Hz (resulting in a 12%

duty cycle). Most PDCs are simple and characterized by

constant pulse frequency, intensity, shape, and width,

but many pulse trains and other complex variations are

possible and some have become commercially available—

CPS and Smith-Root’s P.O.W. (programmable output

waveforms including custom pulse trains) and “sweeping” PDC waveforms (pulse frequency or width gradually

reduced to a specified level over a 10-s interval). However, of these, only the CPS pulse train has been included

in reported investigations of lethal or injurious effects.

Whether other pulse trains or sweeping waveforms are

more, less, or comparably injurious to fish remains to be

documented.

Only two very limited investigations compared the

immediate or short-term lethal effect of CPS with one or

more other currents. For large rainbow trout electrofished

in a one-on-one comparison from an Alaskan stream,

Taube (1992) reported no deaths for CPS and 11% mortality for DC. However, as discussed above under “Type of

Current,” Taube (1992) also reported only 3% mortality

for DC versus 15% for 25-Hz PDC in another one-on-one

comparison on another day. Also for adult rainbow trout,

as well as large subadult Colorado pikeminnow, Meismer

(1999) compared the adverse effects of CPS with those of

DC and 15- and 60-Hz, square-wave PDC at various field

intensities from the threshold for twitch to a level sufficient to assure tetany, 1 Vp/cm. He reported no immediate

mortalities for any CPS or other treatment except rainbow

trout exposed to the 60-Hz PDC at the highest intensity

level (10% mortality, but not significantly different from

controls which suffered no mortality).

In general, CPS has been found to be as effective as

low-frequency PDCs, and sometimes DC, for minimizing

spinal injuries. However, as for low-frequency PDCs (<20

Hz), some, but not all, biologists have found the current

less effective for taxis and capture of fish than higherfrequency PDCs (see earlier discussion on “Comparison

of Currents for Electrofishing Purposes”).

Meyer and Miller (1990, 1991, unpublished

manuscript, 1991; Wyoming Game and Fish Department,

1990, 1991) reported CPS to be less injurious than 60-Hz,

square-wave PDC but, depending on species, comparable

to or even in more injurious than 40-Hz, square-wave PDC.

Among fish collected and X-rayed from the Laramie River

in spring 1990 using 60-Hz PDC, they reported that 78%

of the rainbow trout (30–36 cm TL) and 82% of the brown

trout (28–54 cm TL) had incurred spinal injuries, whereas,

50% of the rainbow trout (30–41 cm TL) and 25% of the

brown trout (13–59 cm TL) collected with CPS were

similarly injured. Among fish collected from the Wind River
 SNYDER 73

the following fall with 40-Hz PDC, none of the X-rayed

rainbow trout (16–31 cm TL) and 15% of the brown trout

(17–41 cm TL) incurred spinal injuries whereas with CPS,

12% and 14% of the fish, respectively (both species 16–

39 cm TL), were injured. Output voltages were about 20

to 25% higher for CPS (460–470 V) than 40-Hz PDC (370–

390 V) to maintain comparable sampling efficiency.

Fredenberg (1992) compared injuries among trout

electrofished with CPS and other currents in Montana.

Combining damage to the spine with associated hemorrhages (including minor ones that may have been discounted or overlooked by others—Fredenberg, personal

communication), he reported that CPS caused notably

fewer injuries (31 to 54% of captured fish) than 60-Hz,

square-wave or half-sine-wave PDCs (62 to 98%), and

somewhat fewer injuries than a hybrid DC-PDC waveform

(Fig. 5J; 44 to 64%), but more injuries than DC (7 to 30%).

In another investigation, Fredenberg (personal communication) compared incidences of injury for selected

species (white and longnose sucker, rainbow trout, and

mountain whitefish) collected in the Missouri River (October 1990, 10° C, about 450 μS/cm) using CPS and other

currents often recommended to minimize electrofishing

injuries. He reported fewer total injuries using CPS (400 V,

22.4 A) than DC (110 V, 5 A) and 15-Hz PDC (200 V,

17.5 A)—2 to 6% (0–2% for vertebral damage only) versus 8 to 18% (0–6% vertebrae only) for DC and 10 to 42%

(2–20% vertebrae only) for 15-Hz PDC.

For large rainbow trout exposed to homogeneous or

heterogeneous fields under laboratory or hatchery conditions, or heterogeneous fields in an Alaskan stream,

Taube (1992) also found incidences of spinal injuries

caused by CPS to be lowest among tested currents. In

homogeneous trials, Taube (1992) reported spinal-injury

frequencies of 17 to 25% for CPS, 22 to 33% for DC, 25 to

50% for 30-Hz PDC, 42 to 58% for 60-Hz PDC, and 58 to

75% for AC. However, incidences of associated hemorrhages, except for DC (28%) were similarly high for all

currents tested, including CPS (42–46%). In heterogeneous trials, Taube reported spinal-injury frequencies of

8% for CPS, 17% for DC, 25 to 58% for 20-Hz PDC, 33%

for 30-Hz PDC, and 67% for 60-Hz PDC (output voltage

was the same for all currents except CPS, for which it was

doubled to elicit comparable responses). In the same oneon-one instream comparison discussed above regarding

lethal effects, he reported spinal injuries for only 13% of

the fish captured with CPS versus 47% for those captured with DC. Again, however, DC performed quite differently on another day in a one-on-one comparison with

25-Hz PDC–no spinal injuries versus 57%, respectively.

Unfortunately, in these one-on-one boat electrofishing

trials, catch rate was 56 to 68% lower for CPS than DC or

25-Hz PDC (relative peak output or field strength for the

tested currents was not reported).

Like many of the above discussed investigators,

Sharber et al. (1994) also documented fewer spinal injuries

for large rainbow trout (>30 cm) collected with CPS, this

time from the Colorado River in a comparison with simple

square-wave PDCs with frequencies of 30 to 512 Hz.

Incidence of injury with CPS was just 8% versus 24 to

62% for the simple PDCs. For further comparison, and as

discussed above under “Waveform, Pulse Shape,” 60-

Hz, square-wave, quarter-sine, and exponential PDCs

field-tested by Sharber and Carothers (1988, 1990), also

in the Colorado River, resulted in injuries to 44 to 67% of

the trout collected. As noted above under “Pulse

Frequencies,” Sharber et al. (1994) also reported an even

lower incidence of injuries, just 3%, for 15-Hz PDC (6%

duty cycle), but according to Sharber (personal

communication), taxis at this frequency was

unsatisfactory for effective electrofishing. Sharber

(personal communication) noted that for a similar power

output, CPS was also less effective than 60-Hz PDC, but

that by increasing voltage for CPS by about 20%, a

comparable response level could be obtained. In hatchery

experiments, Sharber (personal communication) reported

spinal injuries in 6% of trout exposed to CPS versus an

average of 18% for those exposed to 60-Hz, square-wave

PDC.

Combining data for the various field intensities in

the same experiments discussed above with reference to

immediate mortality, Meismer (1999) found that the incidence of brands on adult rainbow trout subjected to CPS

(21%) was comparable to that for those subjected to DC

(19%) or 15-Hz PDC (16%), but much less than that for

those exposed to 60-Hz PDC (46%). However, if only treatments at the tetany level of field intensity are considered,

incidences of branding were 60, 10, 40, and 75%, respectively. Again combining data for the various field intensities, incidences of spinal injuries for rainbow trout were

insignificantly low for all treatment currents, but highest

for CPS (5, 3, 1, and 1%, respectively). For comparable

experiments with Colorado pikeminnow, Meismer (1999)

reported no brands for any treatment and an incidence of

spinal injury for CPS (6%) comparable to that for DC (6%)

and 60-Hz PDC (5%) but somewhat less than for 15-Hz

PDC (11%). Of these results, only the incidence of spinal

injuries for Colorado pikeminnow exposed to 15-Hz PDC

was significantly greater than that for controls (no injuries). All spinal injuries but one slight misalignment were

minor. Associated hemorrhages were also assessed for

Colorado pikeminnow (but not rainbow trout), but such

injuries were insignificantly low for all treatment currents.

As discussed under “Pulse Frequency” above,

Ruppert (1996) and Ruppert and Muth (1997) reported no

vertebral damage for juvenile bonytail (5–8 cm TL) exposed to 10 s of homogeneous CPS or 30-, 60-, and 80-Hz

PDCs at predetermined voltage-gradient thresholds for
74 INFORMATION AND TECHNOLOGY REPORT--2003-0002

taxis, narcosis, and tetany but variable results with respect to spinal or muscular hemorrhages (all class 2).

Combining intensity-level treatments, CPS caused fewer

hemorrhages than the PDCs (means of 8% means 10–

19%, respectively). However, this trend was not always

the case within intensity-level treatments–at the taxis

level, incidence of spinal hemorrhages for CPS was similar to that for both 30- and 60-Hz PDCs (7%); at the narcosis level incidence of hemorrhages was again similar to

that for 60-Hz PDC (10%) but greater than that for 30-Hz

PDC (3%).

Muth and Ruppert (1996) have found that spawning

razorback sucker are quite susceptible to electrofishinginduced spinal injuries and associated internal hemorrhages, but much less so with CPS than 60-Hz,

square-wave PDC (4-ms pulses) at the same peak field

intensity. They individually exposed several captive ripe

males (50–55 cm TL) and females (55–60 cm TL) per treatment, for 10 s to homogeneous fields of the currents,

each at a peak intensity of 1 V/cm (610 μS/cm; 20° C).

Tetany was induced in all fish, but for those exposed to

CPS, it was apparently incomplete since the fish continued to quiver during exposure (possibly the pseudo-swimming response). All fish expelled gametes during treatment,

at least several hundred eggs by each female. No external

hemorrhages or brands were observed, but subsequent

necropsy and X-ray analysis revealed spinal injuries or

associated internal hemorrhages in 50% of the fish (two

males and two females) exposed to 60-Hz PDC and 14%

(one female) of those exposed to CPS. Of the fish injured

by 60-Hz PDC, the two males had class-3 hemorrhages

above the spine slightly anterior to the dorsal fin, one

female had a class-3 fracture slightly posterior to the dorsal fin and a class-2 spinal injury just beyond the anal fin,

both with associated class-1 hemorrhages, and the other

female had a class-2 hemorrhage above the spine slightly

posterior to the origin of the dorsal fin. The female injured with CPS also had a class-2 hemorrhage also just

behind the origin of the dorsal fin. No spinal injuries or

hemorrhages were detected in control fish and no internal organ damage was observed for either treatment or

control fish.

Sharber et al. (1994) suggested that despite the high

frequency of pulses within each CPS train (intended to

improve taxis), the reduction in the amount of electricity

per unit time resulting from the spacing of those trains

likely lessened the severity of myoclonic jerks and thereby

the incidence of spinal injury relative to high-frequency

PDCs. Reynolds and Kolz (1995) interpreted this “amount

of electricity per unit time” as equivalent to duty cycle or

percentage of “on time” per pulse or pulse train cycle,

but Sharber et al. (1995) subsequently clarified “amount

of electricity per unit time” to be that only during rapid

changes in voltage at the beginning and end of pulses

and not the full portion of time current was switched per

unit time.

This is consistent with the observation discussed

above under “Pulse Frequency” that spinal injuries probably occur with sudden changes in voltage differential as

when current of sufficient intensity is switched on or off.

However, other factors also appear to be involved in at

least some pulse trains since CPS, with its three, quick,

square-wave pulses delivered 15 times per second, effectively puts out a total of 45 pulses per second but generally results in injury rates comparable to or less than simple

15-Hz PDCs. Perhaps with respect to production of sudden muscular contractions resulting in spinal injuries,

the effect of the three very rapid, 240-Hz pulses in each

pulse train is physiologically comparable to a single pulse

(see discussion of temporal summation of electrical stimuli

under “Response to Electric Fields”).

Pulse Duration, Duty Cycle

Neither of these interrelated factors have been adequately investigated to assess their effects on either

electrofishing mortality or injury. The little evidence that

does exist suggests no effect on mortality and a possible

tendency for fewer spinal injuries using currents with

longer pulses or greater duty cycles.

Collins et al. (1954) reported that under the conditions of their experiments, PDC pulse duration was not a

lethal factor and that there was no direct relation between

mortality and total energy applied per unit time (duty

cycle). In controlled experiments on juvenile chinook

salmon (5–11 cm TL) with homogeneous fields of 8-Hz,

square-wave PDC, they found that fish exposed to a pulse

duration of 20 ms (16% duty cycle) had the same mortality as those exposed to a pulse duration of 80 ms (64%

duty cycle).

Lamarque (1990), suggested that pulse duration (as

well as frequency) has a major effect on extent of injury

and that pulse durations of 2 to 5 ms characterize some of

the worst PDCs. Although pulse durations in this range

are commonly used in PDC electrofishing, only one reviewed investigation, Taube (1992), addressed the effects

of pulse duration or duty cycle on spinal injuries. However the experiments were limited and the overall results

inconclusive. Some of Taube’s (1992) results suggest a

tendency towards fewer injuries among PDCs with longer

pulse widths and higher duty cycles but other results

suggest no relationship. Comparing incidence of spinal

injury in large trout exposed to 5 s of heterogeneous 20-

Hz PDC, he reported spinal injuries in 25% of the fish

when using a duty cycle of 75% and pulse width of 38 ms

and 58% of the fish when using a duty cycle of 25% and

pulse width of 13 ms, but sample sizes were very small.

When large rainbow trout were exposed instead for 5 s to
 SNYDER 75

homogeneous fields of 30-Hz, square-wave PDCs with

duty cycles of 50% or 72% (pulse widths of 17 or 24 ms

respectively), incidences of spinal injuries and hemorrhages ranged from 33 to 58% without significant differences.

In commenting on the paper by Sharber et al. (1994;

discussed above under “Pulse Frequency” and “Pulse

Trains”), Reynolds and Kolz (1995) noted that when comparing results for tested 15-, 30-, and 60-Hz PDCs, it is

possible to conclude that injury rates (3, 24, and 42%,

respectively) increased with increasing duty cycle (6, 12,

and 24%, respectively) as well as pulse frequency. But

because injury rates among the three currents Sharber

et al. (1994) tested with duty cycles between 10 to 12%

were radically different (8% for CPS, 24% for 30-Hz PDC,

and 62% for 512-Hz PDC), Reynolds and Kolz (1995) concluded that pulse frequency and the nature of the waveform (i.e., complex vs. simple PDC) are more important in

this regard than duty cycle.

Voltage Spikes

As discussed earlier under “Electrofishing Currents

and Waveforms,” some electrofishing systems create positive voltage spikes (well beyond nominal peak voltage)

at the leading (and) or trailing edges of pulses (or continuous current when it is switched on and/or off) and

sometimes negative spikes or dips at the trailing end of

pulses. Although a single, limited-scope investigation

suggests that these waveform anomalies have little or no

impact with respect to electrofishing injuries and mortality, the matter has not been adequately investigated and

the effects on fish remain uncertain.

Sharber (personal communication) suggested that

although the voltage of such spikes can be much higher

than the designed peak voltage of the pulse, thereby dramatically increasing the magnitude of sudden voltage

change at the leading or trailing edge of the pulse, the

spikes are probably too short in duration to have a significant physiological or behavioral effect. In the only

behavioral experiment on the effects of voltage spikes

found for this review, Haskell et al. (1954) noted no significant improvement in the behavioral responses of fish

subjected to a 1-Hz, square waveform (80% duty cycle)

with a high initial peak (interpreted here as a spike) over

that of fish subjected to a similar waveform without the

high initial peak.

Hill and Willis (1994) conducted the only investigation of the adverse effects of a spiked PDC waveform.

They used both a spiked square waveform described by

Van Zee et al. (1996) and a similar unspiked waveform to

electrofish hundreds of 20-cm-TL or larger largemouth

bass in reservoirs of moderate to high conductivity (400

to 1,700 μS/cm) and temperature (16–25° C) and reported

no immediate mortality and few brands for either waveform (fish were not X-rayed or necropsied).

The biological effects of voltage spikes, or lack

thereof, remain inadequately documented. If voltage

spikes are found to affect the incidence of spinal injury or

have other harmful effects, it should be possible to electronically filter them out of the applied current (Novotny,

personal communication).

Species

Evidence to date strongly indicates that trout, char,

and probably salmon (subfamily Salmoninae) are generally more susceptible to brands, spinal injuries, associated hemorrhages, and probably mortality during

electrofishing than most other fishes (Appendix B;

Fredenberg, 1992—occurrences of spinal injury and/or

hemorrhages reported as high as 98% for rainbow and

cutthroat trout; Miskimmin and Paul, 1997a—concise review of injurious and lethal effects by species with tabulated summaries for 11 of 15 species of interest to the

Province of Alberta; Paul and Miskimmin, 1997—review

of effects and efficiency of electrofishing emphasizing

selected species of interest to Alberta). In northern and

upland regions of the north temperate zone, Salmoninae

also are among the most frequently targeted species in

electrofishing investigations. Data on the harmful effects

of electrofishing on fishes other than the Salmoninae are

limited and seldom comparable, but among species included in such reports and under at least some environmental and electrical-field conditions, burbot and sculpins

(Cottidae) are particularly sensitive to electrofishing mortality and goldeye, some suckers (Catostomidae), channel catfish, largemouth bass, walleye, and possibly

paddlefish are most sensitive to electrofishing-induced

spinal injuries and associated hemorrhages. As discussed

under “Effects Other Than Spinal and Related Injuries,”

mountain whitefish are at least sometimes especially susceptible to bleeding at the gills when subjected to

electrofishing fields.

Mortality. Salmoninae appear to be more sensitive

to electrofishing mortality than other fish taxa, but available data are few and seldom comparable. Only Meismer

(1999) directly compared the lethal effects of selected

currents at various field intensities for a salmonid, rainbow trout (33–60 cm TL), and a non-salmonid, Colorado

pikeminnow (30–39 cm TL). Both species were similarly

exposed to homogeneous 15- or 60-Hz, square-wave PDC,

CPS, or DC with field intensity gradually increased to and

held for 5 s at various levels from the threshold for twitch

to 1 Vp/cm, a level sufficient to assure full tetany (530 μS/

cm; 18° C). Only rainbow trout subjected to the highest

intensity of 60-Hz PDC experienced immediate mortality

(10%), but that mortality was reported to be insignificant
76 INFORMATION AND TECHNOLOGY REPORT--2003-0002

relative to controls (no mortality). In a separate laboratory comparison of the same two species with 10 s of 60-

Hz, square-wave PDC or 60-Hz, quarter-sine wave PDC

abruptly applied at a still higher field intensity (1.5 Vp/

cm), Meismer (1999) again reported immediate mortality

only for rainbow trout exposed to the 60-Hz, square-wave

PDC, but this time at a significant 30%.

Only two investigations reported on relative susceptibility among species of Salmoninae) to electrofishing

mortality. Pratt (1955) reported greater mortality for hatchery brown trout than brook trout or rainbow trout when

exposed to either AC or DC. Although mortalities and

injuries were very low among trout electrofished with AC,

Hudy (1985) reported significantly greater mortality among

rainbow trout than brook trout (but greater numbers of

surviving fish with abnormalities, including spinal injuries, among brook trout).

Among non-salmonid taxa, there are several reports

of very high immediate or short-term electrofishing mortality. Under rather extreme circumstances, Whaley et al.

(1978; also Whaley, 1975) reported as much as 75 to 95%

mortality for bluegill and fantail darter exposed for up to 3

min to PDC in laboratory experiments. However, some

field operations can be just as lethal for certain species.

Sculpins, according to Gowan (personal communication),

are highly susceptible to extended tetany with flared

opercules and subsequent mortality when captured in

shallow riffles with outputs of 300 V or greater. Eloranta

(1990) found burbot to be the most sensitive species to

DC electrofishing mortality in the littoral zone of a lake in

Finland. He reported that mortality for burbot was usually less than 25% but occasionally up to 50% when temperatures were high, whereas for other species, mortality

was usually under 11%.

In contrast, most other investigators addressing the

matter reported little or no electrofishing mortality among

non-salmonids. Sorensen (1994) subjected spawning goldfish (32 female and 24 males) to 15 s of 100-Hz, squarewave PDC in an aquarium and reported recovery times

greater than 10 min but no short-term mortality or brands.

Cowdell and Valdez (1994) electrofished roundtail chub

(22–40 cm TL) from the Colorado River with 40-Hz, squarewave PDC to check for electrofishing injuries and reported

that all fish recovered quickly, typically in less than 60 s,

with no immediate mortalities. Ruppert (1996) and Ruppert

and Muth (1997) reported no mortalities among several

hundred juvenile bonytail and humpback chub exposed

for 10 s to homogeneous CPS or 30-, 60-, or 80-Hz PDC at

intensities sufficient to induce taxis, narcosis or tetany.

Walker et al. (1994) subjected juvenile northern pike to 10

to 60 s exposures of homogeneous AC and PDC at 0.4 to

2.1 V/cm (rms for AC, peak for PDC) and reported no

mortalities within 16 to 24 h. However, 24 to 33% of the

fish exposed to AC, but not PDC, had externally obvious

cutaneous injuries along the entire length of the body

(brands) or in the paired and median fins, and most of

these (17–24%) failed to recover upright swimming within

the 16 to 24-h monitoring period (survival beyond that

time was not reported but most, if not all, of those fish

would probably have died). Bardygula-Nonn et al. (1995)

reported just 0 to 5% immediate to short-term (3-day)

mortality for four centrarchids exposed to 30-, 60-, or 120-

Hz PDC–zero for pumpkinseed, 5% for bluegill, 1% for

smallmouth bass, and 1% for largemouth bass. Zeigenfuss

(1995) reported no immediate mortality for several

warmwater species collected with 60- to 80-Hz PDC, Xrayed for injury, and released back to Colorado Reservoirs (see discussion below regarding injuries). All fish

immediately swam away upon release except some walleye which settled to the bottom for less than an hour

before swimming away; no fish were retained and monitored for delayed mortality.

Injury. Relatively few investigations have directly

compared the susceptibility of different species to

electrofishing-induced spinal injuries and muscular hemorrhages, especially with or among non-salmonids. Frequencies of injuries reported for specific species are highly

variable among and often within investigations and sometimes appear to be contradictory. Differences in rates and

degree of injury, especially between investigations, are

often difficult to attribute to species, fish size or condition, environment (including water conductivity and temperature), field intensity, or other current or field

characteristics. For example, many recent studies report

very high percentages of electrofished rainbow trout with

spinal injuries and hemorrhages (Appendix B), but

McCrimmon and Bidgood (1965) reported no skeletal damage attributable to either AC or PDC fields among 80

hatchery rainbow trout (11–26 cm TL) that were experimentally exposed in the laboratory or among 291 wild

rainbow trout (6–59 cm TL) that were electrofished in

Ontario streams tributary to the Great Lakes. All fish were

X-rayed (the hatchery fish before and after exposure),

and some were dissected. Dwyer and White (1995) also

reported no spinal injuries among 44 X-rayed rainbow

trout, including 4 mortalities, exposed to high-frequency

PDC (20 were examined 35 days after treatment, the rest

were frozen immediately or within 24 h of treatment).

Still, most existing data support Salmoninae as the

fish taxa most susceptible to electrofishing injury. In one

investigation, Fredenberg (personal communication)

found spinal injuries in 2 to 20% of rainbow trout captured with DC, 15-Hz PDC, or CPS, but only 0 to 2% of

mountain whitefish, white sucker, or longnose sucker

captured with the same currents. When specimens with

only hemorrhages along the spine or associated musculature (all minor) were added to these figures, the percentages of injured fish increased to 6 to 42% for rainbow
 SNYDER 77

trout, 2 to 29% for mountain whitefish, and 4 to 18% for

the suckers. In addition to his investigation of impacts

on rainbow trout, Zeigenfuss (1995) X-rayed and released

several warmwater species collected in three Colorado

reservoirs and concluded that warmwater species (see

details below) are less vulnerable to spinal injuries than

rainbow trout. Also, for contaminants analysis, Krueger

(personal communication) dissected several electrofished

species, including common carp, suckers, walleye, northern pike, and black basses, but recalled only seeing substantial numbers of spinal injuries among trout.

Only Kocovsky et al. (1997) and Meismer (1999) reported greater frequencies of electrofishing injuries for

non-salmonid than salmonid species. In a comparative 3-

year field study also referenced below with regard to

salmonids only, Kocovsky et al. (1997) reported greater

percentages of old, externally detectable spinal injuries

in longnose sucker (7–13%) than in rainbow trout or brook

trout (2–6%) or, for two of three years, brown trout (3–

12%). In laboratory experiments discussed above with

regard to lethal effects, Meismer (1999) also directly compared the injurious effects of the selected currents (DC,

15- or 60-Hz PDC, or CPS) and field intensities (gradually

increased from zero to the thresholds for twitch, taxis, or

narcosis, or to 1 Vp/cm to assure full tetany) on rainbow

trout (33–60 cm TL) and Colorado pikeminnow (30–39 cm

TL). When data for all treatments were combined (n =

320/species), 26% of the rainbow trout suffered brands

and just 3% suffered spinal injuries, whereas none of the

Colorado pikeminnow suffered brands but 7% suffered

spinal injuries. All spinal injuries were minor (class 1)

except for a slight misalignment in one Colorado

pikeminnow. In a separate comparison of the effects of

the same two species abruptly exposed to 10 s of 60-Hz,

square-wave PDC or 60-Hz, quarter-sine-wave PDC at a

still higher field intensity (1.5 Vp/cm), Meismer (1999) reported even higher percentages of rainbow trout with

brands (73% for combined treatments; n = 40/species)

and 10% with spinal injuries, all minor. No Colorado

pikeminnow in these higher-intensity, abrupt-exposure

treatments suffered brands or spinal injuries.

Among the Salmoninae, particularly rainbow, brook,

and brown trout, there is no consistent ranking regarding

susceptibility to electrofishing injury. Although Hudy

(1985) observed few injuries among trout captured with

AC, he reported significantly greater numbers of surviving fish with abnormalities, including spinal injuries,

among brook trout than among rainbow trout (but, as

noted above, greater mortality among rainbow trout).

Fredenberg (1992) generally found rainbow trout (and

probably cutthroat trout) more susceptible to spinal and

related injuries than brown trout. Data reported by Meyer

and Miller (1991, unpublished manuscript, 1991; Wyoming Game and Fish Department, 1991) indicated the same

for fish in stream sections electrofished four times in succession with 40-Hz, square-wave PDC but the reverse for

stream sections electrofished only once. However, differences between species in the latter reports were not statistically significant (Meyer, personal communication),

and when stream sections were fished only once with

Coffelt’s CPS current, incidences of injuries were similar

for rainbow trout and brown trout. Kocovsky et al. (1997),

using three-pass depletion electrofishing with 100-Hz

PDC, found that incidence of externally detectable (old)

spinal injuries increased progressively for three successive years in an annually sampled stream segment and

that the frequency of these injuries was greater for brown

trout (3–12%) than rainbow or brook trout (2–6%). Comparing incidences of spinal injuries and hemorrhages for

rainbow and brown trout electrofished with 60-Hz, halfsine PDC in three Colorado Rivers, Thompson (1995) and

Thompson et al. (1997a) reported highest and lowest percentages for rainbow trout (e.g., 6 and 64% for spinal

injury, 13 and 76% combined with muscular hemorrhages),

but rank in susceptibility relative to brown trout varied

with river, electrofishing technique (boat with throwable

anode vs. shore-based wading with multiple anodes), and

field intensity near the anodes.

Several investigations, in addition to a couple mentioned above, compared incidences of electrofishing injuries among species other than Salmoninae. Spencer

(1967a) reported up to 16% spinal injury among experimentally electrofished bluegill but almost none among

largemouth bass. Clady (1970, according to Schneider,

1992) reported some injury to smallmouth bass and white

sucker with 560-volt AC gear, but Schneider (1992) did

not specify whether these were spinal or other injuries or

compare percentages. Holmes et al. (1990) documented

12.5% spinal injury for northern pike, zero to 18% (but

less severe) injury for Arctic grayling, and no injury for

humpback whitefish and least cisco. Fredenberg (1992)

reported only one minor injury for Arctic grayling and no

injuries among small numbers of sauger. In a cursory investigation of the injurious effects of DC and PDC,

Gardner (1992, according to Grisak, 1996) found only hemorrhages, no spinal injuries, among shocked and control

smallmouth bass and channel catfish and only one injury

among shocked paddlefish he X-rayed and necropsied.

Bardygula-Nonn et al. (1995) investigated the lethal

(discussed above) and injurious effects of 30-, 60-, and

120-Hz PDC on bluegill, pumpkinseed, smallmouth bass,

and largemouth bass collected from lakes with

conductivities of 122 to 789 μS/cm. Fish were monitored

for 3 days after capture. Fish that died, fish with severe

external injuries, and 25% of the normal appearing

survivors for each species were X-rayed and necropsied.

Brands (external ecchymoses, as described by the

authors) were found on most fish that died (0–5%
78 INFORMATION AND TECHNOLOGY REPORT--2003-0002

depending on species), but few others. Spinal injuries

were observed only in one bluegill mortality and all six

largemouth bass that died and were limited to partial

separations (subluxations) or misalignments; vertebral

fractures and presumably compressions were not

detected. Internal soft-tissue damage and hemorrhages

were found in fish that died within 3 days and in 14% of

the smallmouth bass and 2% of the largemouth bass

survivors that were sacrificed and necropsied; none were

observed in bluegill or pumpkinseed survivors that were

similarly examined.

Zeigenfuss (1995) X-rayed and released several

warmwater species collected in three Colorado reservoirs

during June or July by boat electrofishing with 60- to 80-

Hz PDC. Among these fish, walleye and largemouth bass

appeared to be most vulnerable with incidences of spinal

injury ranging from 0 to 21% (18% overall) and 0 to 33%

(12% overall), respectively. Other species for which spinal injuries were observed included common carp (8%

injured), bluegill (1 of 6 fish), and yellow perch (1 of 12).

No injuries were observed among one white sucker, two

freshwater drum, six white crappie, and one black crappie. All spinal injures documented by Zeigenfuss (1995)

for these warmwater species were limited to compressions

(class 1); no misalignments or more serious injuries were

observed.

As discussed earlier under “Nature of the Injuries”

and “Type of Current—Injury,” Grisak (1996) compared

the injurious effects of DC and 40-Hz PDC for several

species of warmwater fishes electrofished from the Missouri River in Montana. All fish succumbed to tetany but

revived within minutes of capture and no brands were

observed. Goldeye were the predominate species taken

with each current, the only species injured under DC (perhaps because of unusually strong taxis), and the species

suffering the greatest incidence of hemorrhages under

PDC. Among goldeye, incidences of spinal injuries, muscular hemorrhages, and either or both types of injury were

21, 21 and 32%, respectively, for DC, and 4, 39, and 43%,

respectively, for PDC. Electrofishing injuries for other

species were observed only among those collected with

PDC. River carpsucker (30–58 cm TL) were the next most

susceptible species with 18% experiencing spinal injuries and 9% experiencing hemorrhages (27% total, none

with both types of injury). Among the remaining fishes

collected with both currents (but injured only by PDC),

8% of flathead chub (11–24 cm TL) incurred spinal damage and 5% of shorthead redhorse (18–49 cm TL) incurred

hemorrhages. Small numbers of longnose sucker (including fish 43–49 cm TL) and freshwater drum (30–43 cm TL)

were captured only with PDC, but no injuries were detected. Several adult sicklefin chub and sturgeon chub

captured by trawl were placed in a plastic bucket with

holes and subjected to 10 s of up to 0.18 V/cm about a

meter from the anode. Grisak (1996) reported no obvious

spinal damage or hemorrhages for either species but noted

that the results should be considered inconclusive because injuries in such small fish are difficult to detect. All

vertebral injuries in captured fish were class 1, except for

a solitary class-2 injury in a river carpsucker, and involved

3 to 9 vertebrae. Hemorrhages were half class 1 and half

class 2. Among controls collected by other means and

also X-rayed and necropsied, Grisak (1996) reported only

one fish (a goldeye) with a fresh internal injury, a spinal

compression.

In laboratory experiments, Ruppert (1996) and

Ruppert and Muth (1997) reported hemorrhages associated with the spine for 7% of juvenile bonytail and 20%

of juvenile humpback chub exposed to homogeneous CPS

at tetany-level intensity, but the difference was not statistically significant. For juvenile bonytail similarly subjected to 30-, 60-, or 80-Hz PDC at homogeneous field

intensities sufficient to induce taxis, narcosis, or tetany,

incidences of such hemorrhages ranged from 3 to 27%.

No immediate mortalities, external injuries, or vertebral

damage (based on dissection and low-power microscopic

examination) were observed for either species. Among

controls, only one humpback chub had a muscular hemorrhage at or near the spine.

Remaining reports of electrofishing injuries for fishes

other than Salmoninae were based on single-species

investigations or observations. Spencer (1967a) reported

substantial occurrences of injury for channel catfish (at

least 6 of 10) electrofished in a pond with 3-phase AC.

Newman (1992, unpublished manuscript, 1991) reported

up to 31% injury for walleye. Roach (1992) reported

injuries in 5 to 28% of northern pike exposed to

homogeneous 30- to 120-Hz PDC; however, he noted

(Roach, personal communication) that the injuries were

much less obvious and less serious than those usually

observed in trout. Among 40 roundtail chub taken from

the Colorado River with 40-Hz PDC, Cowdell and Valdez

(1994) reported rapid recovery, no abnormal swimming

behavior, no brands or external hemorrhaging, no signs

of spinal injury based on lateral X-rays, and only 5% with

internal hemorrhages based on fillets along the spine

(classes 2 and 3). They also noted that external signs of

injury (including brands, abnormal swimming, and

bleeding at the gills) were rarely observed in native

cyprinids they had electrofished from the Colorado River

in other investigations.

Among teleosts in North America, catfishes (order

Siluriformes, mostly Ictaluridae) may be relatively unique

in their sensitivity and reaction to electric fields (Morris

and Novak, 1968; Corcoran, 1979). Their lateral-line system

includes electroreceptors (Peters and Buwalda, 1972;

Kramer, 1990), which may account for their ease of capture

with extremely simple and low voltage devices, some of
 SNYDER 79

which are illegal in certain states (McSwain, 1988). In

support of these observations, Jesien and Hocutt (1990)

found 50% tetany voltage-gradient thresholds for channel

catfish to be generally much lower than reported for other

species at comparable water conductivities. However,

Edwards and Higgins (1973) reported stun thresholds for

22- and 28-cm channel catfish that differed little from those

for 15-cm bluegill and were slightly greater than for 41-cm

bowfin when using 10- to 200-Hz, square-wave PDC; 100-

Hz, exponential PDC; and a 100-Hz, square-wave pulse

train delivered at 25 Hz. Stun thresholds for the catfish

were substantially lower than for the smaller bluegill only

when using DC (but still slightly greater than for the

bowfin) and were actually much higher than for either the

smaller bluegill and larger bowfin when using 25-Hz,

exponential PDC. As noted earlier under “Comparison of

Currents for Electrofishing Purposes,” Pugh and Schramm

(1998) found flathead catfish and blue catfish generally

much more susceptible to capture using 15-Hz than 60-

Hz PDC, but channel catfish generally more susceptible

to 60-Hz PDC. Aside from Spencer’s (1967a) observations

of high incidence of spinal injuries for channel catfish

(noted above) and Edwards and Higgins’ (1973)

observation that channel catfish recovered quickly from

electrical immobilization with few mortalities, the adverse

effects of electrofishing on catfish have not been studied.

Likewise for any relationship between susceptibility to

electric fields and the presence of special electroreceptors.

The Chondrostei, sturgeon, and paddlefish also have

electroreceptors. Whether these fish are also more sensitive to electric fields than most other species has not

been reported. Fredenberg (1992) reported no injuries

among small numbers of electrofished shovelnose sturgeon. Berg (1982, according to Grisak, 1996) who visually

surveyed over a 1,000 paddlefish after electrical agitation

in the Missouri River, Montana, reported only two mortalities, both with ruptured notochords (voltages may

have been excessively high and pulse frequencies were

as high as 120 to 160 Hz). Gardner (1992, also according

to Grisak, 1996) reported only one spinal injury among

paddlefish he experimentally exposed to PDC. However,

according to Pfeifer (personal communication), paddlefish electrofished with PDC in the Yellowstone and Missouri Rivers were highly susceptible to spinal injuries

despite their cartilaginous endoskeletons and lack of vertebral centra. Necropsy of those fish revealed, as per Berg

(1982) above, that their notochords were badly ruptured.

Size

As discussed earlier (see end of section on

“Response Thresholds”), fish generally become more

sensitive to electric fields (i.e., respond at lower fieldintensity thresholds or, in heterogenous fields, at a greater

distance from the electrode) as size increases, at least up

to some point beyond which size appears to no longer

matter. Accordingly, since electrofishing mortality is at

least partially dependent on field intensity and spinal

injuries appear to occur with sudden changes in voltage

differential beyond some threshold level, larger fish might

be expected to be more susceptible to electrofishing

mortality and injury than smaller fish, but experimental

and field data either fail to support these relations or do

so inconsistently. With respect to mortality, this relation

might only exist with increasing exposure time, and some

researchers have even reported greater electrofishing

mortality for smaller fish. With respect to spinal injuries,

the anticipated relation has been supported by some

experimental and field research but not by others.

For fish of a particular species, similarly oriented in

an electric field (e.g., parallel to the lines of flux), Collins

et al. (1954) and Whaley (1975) concluded that the increase in mortality attributed to field intensity appears to

be unaffected by fish length whereas that attributed to

exposure time increases with fish length. Collins et al.

(1954), exposed fingerling chinook salmon in four size

groups of about 5, 7, 9 and 11 cm TL to 30 s of homogeneous 2-Hz PDC (48 μS/cm, 10–20° C) with fish held parallel to the lines of current and reported that similar field

intensities (about 12.5 to 15 V/cm) were required to kill

50% of the fish in each size group. However, as a corollary, there is a direct relation between fish size and the

total voltage across the fish required to kill that fish (headto-tail voltage differential = voltage gradient x fish length).

Plotting their data for corresponding head-to-tail voltages, Collins et al. (1954) found that to kill 50% of the

fish, 60 V was required across 5-cm salmon and 140 V

across 11-cm salmon. Whaley (1975) subjected similarsize subgroups of 3- to 8-cm fantail darters and 9- to 17-

cm bluegills to 2-, 9-, and 16-Hz PDC at 3 to 5 V/cm for 5 to

180 s (154 μS/cm, 10° C) and also reported that increased

fish length further increased mortality as exposure time

was increased but not as field strength was increased. In

fields of fixed intensity, Collins et al. (1954) determined

that increased fish length also further increased mortality

as either pulse frequency or water temperature was increased. However, Whaley (1975) and Whaley et al. (1978)

reported no significant effect of fish length on mortality

attributed to pulse frequency.

Contrary to expectations based on the above

discussed work, Habera et al. (1996) and Bardygula-Nonn

et al. (1995) actually found greater electrofishing mortality

among fish under rather than over 10 cm TL. Habera et al.

(1996) used three-pass depletion AC in a very-low

conductivity stream and reported 15 to 23% mortality in

rainbow trout measuring 5 to 9.9 cm TL and 2 to 9%

mortality in trout measuring 10 to 23 cm TL. Thirteen of

the 20 mortalities on which these figures were based had
80 INFORMATION AND TECHNOLOGY REPORT--2003-0002

not been recovered and were assumed to have died. If

they escaped or were preyed upon rather than having

died, electrofishing mortalities would have been reduced

to 5 to 10% and 2 to 4%, respectively. Bardygula-Nonn

et al. (1995) reported 5.3% mortality among 568 bluegill

electrofished in lakes with 30-, 60-, or 120-Hz PDC but

that mortality was proportionately greater among

specimens less than 10 cm TL than among larger

specimens.

Unlike mortality, many researchers have documented

a positive relation between increasing fish size and incidences of spinal injuries and hemorrhages in tissues near

the spine. McMichael et al. (1991; also McMichael and

Olson, unpublished manuscript, 1991) subjected 14- to

48-cm-FL rainbow trout to DC and 30- and 90-Hz PDC in

hatchery raceways and reported a significant positive

correlation between fish length and occurrence of spinal

injuries and major hemorrhages (but not minor hemorrhages). Similarly, Hollender and Carline (1994) reported

that the incidence of injury among AC and PDCelectrofished brook trout increased with size from 14%

for fish 9 to 13 cm to 26% for fish 13 to 17 cm and 42% for

fish 18 to 24 cm. Habera et al. (1996) found that, contrary

to lethal effects, rainbow trout greater than 10 cm TL incurred significantly more spinal injuries or hemorrhages

than smaller fish (0–15% for fish 10–23 cm TL and 0% for

fish 5–9.9 cm). Thompson (1995) and Thompson et al.

(1997a) extensively modeled the relationship between size

and incidences of spinal damage or hemorrhages for rainbow trout (13–51 cm TL) and brown trout (10–49 cm TL)

electrofished with 60-Hz PDC from three Colorado rivers

and concluded that in most cases longer fish had a higher

probability of injury.

Combining data for wild rainbow trout captured with

DC, 60-Hz PDC, and a hybrid of the two currents (Fig. 5J),

Dalbey et al. (1996, also Dalbey, 1994) also found a tendency for increased incidences of spinal injury with increased size from 27% for 15- to 20-cm trout to 45% for 33-

to 35-cm fish. Within this size range, incidences of spinal

compressions only (class 1 spinal injury, Table 3) generally decreased with increasing size from 23% for 15- to 18-

cm trout to 0% for 33- to 35-cm fish, whereas incidences

of spinal misalignment and compression (class 2) generally increased from 5% for 15- to 18-cm trout to 30% for

30- to 33-cm fish but then dropped to 11% for 33- to 35-cm

fish, and incidences of vertebral fractures or complete

separation of two or more vertebrae (class 3) increased

from zero for 15- to 18-cm specimens to 8 to 11% for 25- to

33-cm fish, then rose to 35% for 33- to 35-cm fish.

Data supporting the relation for non-salmonid fishes

are more limited. Among northern pike 36 to 74 cm FL that

were subjected to similar electric fields, Roach (1992) found

that those fish experiencing spinal injuries were

significantly larger ( = 57 cm) than those that were not

injured ( = 51 cm). Newman (unpublished manuscript,

1991) noted that size might be a factor for walleye, but his

sample size (30 specimens, 18–48 cm) was too small and

variable to be conclusive.

Other researchers have reported no relation or inconsistent relations between fish size and incidences of

injury. Zeigenfuss (1995) compared injury rates among

five size classes of rainbow trout subjected to 60-Hz PDC

and in his first-year trial found that the smallest group

(15–27 cm) had significantly fewer spinal injuries (~52%,

Zeigenfuss, 1995–Fig. 1) than the four larger-size groups

(27–35 cm; ~65–75%) for which differences in mortality

were not significant. In a second-year trial with higher

field intensity, Zeigenfuss (1995) reported that spinalinjury rates were nearly equal for all size groups. Similarly, in extensive surveys of spinal injuries among

salmonids, neither Meyer and Miller (1991, unpublished

manuscript, 1991; also Meyer, personal communication)

nor Fredenberg (1992) found an overall relation between

the percentage of injured fish and size. Zeigenfuss (1995)

also X-rayed and released several warmwater species collected in three Colorado reservoirs during June or July by

boat electrofishing with 60- to 80-Hz PDC. Based on the

capture of fish averaging 15 to 45 cm TL, he reported that

there was no evidence that larger warmwater fish were

generally more vulnerable to injuries than smaller fish.

Among species collected in greater numbers, mean length

of injured fish was greater than for uninjured fish only for

largemouth bass.

Condition

The physical condition of fish subjected to electric

fields can affect their susceptibility to electrofishing injury

and mortality, but assessment of this factor is based

mostly on suppositions and casual observations rather

than specific experiments and data. It is logical to expect

that fish in poor health, or an otherwise highly stressed

condition (as when habitat approaches upper limit

temperature or lower limit oxygen conditions), might be

less alert and sensitive to electric fields, thereby

responding less strongly and reducing chances for spinal

injury, but they also would be less able to withstand the

stresses of tetany and apnea during narcosis, thereby

increasing probability of death. Thompson et al. (1997a)

observed higher incidences of injury among populations

of rainbow trout with generally higher condition factors

and suggested that better-condition wild fish may be more

likely to be injured because of more powerful muscular

contractions. However, whether in poor condition or

otherwise normal, fish with weakened or brittle bones,

particularly vertebrae, may be especially susceptible to

spinal injuries. Stewart (1967, as cited by Lamarque, 1990)

suggested that spawning fish, particularly salmon, may

0

0
 SNYDER 81

be especially susceptible to spinal injuries due to skeletal

decalcification; likewise for fish with diets deficient in

magnesium and calcium (Lamarque, 1990). Over-wintering

fish may be less likely to suffer either spinal injuries or

mortality due to thermally reduced metabolism and

slowed responses, but like most of the above, this

hypothesis has not been experimentally tested.

Repeated Exposures

If there are significant adverse impacts on fish resulting from single events, lethal or otherwise, the effects

of multiple electrofishing events should at least be cumulative. Hypothetically, fish that do not fully recover between events may be more susceptible to harmful effects

in subsequent exposures, thereby compounding those

impacts. Conversely, they may be physiologically unable

to respond as strongly to subsequent exposures, thereby

reducing expected cumulative effects. If electrofishing

events are sufficiently spread to allow full physiological

recovery, fish may learn from the experience and be more

apt to escape less effective portions of the field in subsequent events, although they may still be injured by the

exposure. However, investigations of lethal effects (only

two) have demonstrated no short-term differences in

mortality over controls and suggest that stress of repeated

handling may have a greater impact on delayed mortality.

On the other hand, and as might be expected, investigators of injurious effects have documented cumulative increases in the incidence of injuries among fishes

inhabiting repeatedly sampled waters, not only during

multiple-pass sessions, but in successive sessions or

years of sampling. In doing so, they also documented

past injuries among fish that either were missed by netters

or escaped at the fringes of the effective field during

earlier passes, sessions, or years. Stress and injury caused

by repeated exposures to electric fields might also affect

short and long-term growth and condition of fish (see

later discussion on “Effects on Growth and Condition”).

Barrett and Grossman (1988) studied the effects of

repeated electrofishing events on survival of mottled sculpin

(4–8 cm SL) and reported no significant differences between

treatment and control fish. They exposed mottled sculpin

for 30 s to DC fields weekly, five times over a 4-week period,

in an outdoor artificial stream (low conductivity, 12–14° C).

Controls were initially collected by kick-netting and both

control and treatment fish were handled after each

electrofishing event. Cumulative mortalities increased

progressively with exposure-handling events and time and

by the end of the experiment ranged from 35 to 60% for

treatment fish and 45 to 50% for controls. Barrett and

Grossman (1988) concluded that repeated-handling stress

had a greater impact on cumulative mortality of mottled

sculpin than repeated DC electrofishing.

Eloranta (1990) exposed 3- to 30-cm burbot, ruffe,

and bullhead to 20 s of 550-V DC at about 15 to 20 cm from

the anode on each of 10 consecutive days to assess the

lethal effects of repeated exposures. During those 10 days,

he, like Barrett and Grossman (1988), observed no differences in short-term mortality between the experimental

groups and controls and concluded that delayed effects

were minimal.

Meyer and Miller (1991, unpublished manuscript,

1991) reported nearly four times as many spinal injuries (30% vs. 8% based on X-rays) among previously

uncaptured rainbow and brown trout (14–40 cm TL)

collected during the last pass of a four-pass, 2-week

population estimate than among trout collected in a

single-pass operation in an upstream portion of the

same stream (340–350 μS/cm; 7–8° C). They concluded

that many of the unmarked (not previously captured)

trout taken in the fourth pass had suffered some injuries during prior passes.

Habera et al. (1996) assessed injury among rainbow

trout (5–23 cm TL) electrofished with 60-Hz AC in a

three-pass depletion population estimate in a southern

Appalachian stream (14 μS/cm, 15° C). No spinal injuries

or hemorrhages were detected among mortalities (3%)

or 12 angled controls examined by X-ray or necropsy,

but among a subsample of electrofished survivors, 3%

incurred class-2 spinal injuries and another 3% class-2

hemorrhages (6% combined). The injured fish were

greater than 10 cm TL (12–17 cm) and collected only in

second and third passes (fish taken during these

passes may have been shocked but not captured in

the preceding pass or passes). No external injuries (e.g.,

brands or erratic swimming behavior) were observed

among survivors not X-rayed and necropsied.

Kocovsky et al. (1997) evaluated the injurious effects

of annual three-pass-depletion electrofishing (for population estimates) on salmonids and longnose sucker in

three small, low-conductivity (34–63 μS/cm), streams. Prior

to this 3-year investigation, study reaches had been annually electrofished for 5 years, and biologists noted that

a significant proportion of the fish had spinal deformations and related anomalies. Electrofishing, as in the past,

was conducted by wading with backpack units (handoperated anode and trailing cathode screen) using 100-

Hz, square-wave PDC with a 50% duty cycle. As controls

for two streams, Kocovsky et al. (1997) also single-pass

sampled stream segments that were not believed to have

been previously electrofished. Sampling was conducted

in mid-to-late summer with maximum water temperatures

of 12 to 18° C. Over 8,000 yearling and older fish were

examined visually and by touch for externally evident

anomalies suspected to represent healed spinal injuries

from prior years of three-pass sampling (and probably

the more severe of new injuries). For brook trout in two
82 INFORMATION AND TECHNOLOGY REPORT--2003-0002

streams, they found no significant difference in incidence

of externally detectable injuries between streams but significant differences among years and between three-pass

study and single-pass control reaches. Incidences of

detected injuries in three-pass segments progressively

increased from one year to the next—5% and 4% in the

first study year, 12% and 11% in the second year, and

14% and 23% in the third year. Incidences of injuries for

single-pass control segments of the two streams were

much lower and remained low in all three years—zero in

the first year, 2% and 0% in the second year, and 1% and

3% in the third year. In the third stream, detected injuries

in three-pass electrofishing study segments also increased annually from 2 to 6% for brook trout and rainbow trout and 3 to 12% for brown trout, but not for

longnose sucker in which externally detected injuries first

increased from 9 to 13%, then fell to 7% in the third year.

They also found that externally detectable injuries represent only a relatively small proportion of total spinal injuries; 44% of 114 captures not showing external signs of

old spinal injuries had spinal injuries that could only be

detected by X-ray (or necropsy). Kocovsky et al. (1997)

concluded that electrofishing-induced spinal injuries in

salmonids and longnose suckers can accumulate over

time in stream segments that are sampled annually by

intensive electrofishing.

Summary

Factors considered in the literature to affect

electrofishing injuries and mortalities include type of current, field intensity, duration of exposure, orientation of

fish relative to lines of current, and for AC and PDC, waveform characteristics such as shape, wave or pulse frequency, and pulse width; also, fish species, size, and

condition. However, data regarding the effects of these

factors are sometimes sparse, difficult to compare, and

often questionable.

Available data generally support the contention that

of the three types of electrofishing currents, AC is most

harmful, DC least, and PDC usually somewhere between

depending on the frequency and complexity of pulses.

Although there are reports of no mortality or injury for

each type of current, when such adverse effects do occur

and comparisons are possible, AC tends to be more lethal

than either DC or PDC, and AC and moderate to highfrequency PDC tend to cause more spinal injuries and

hemorrhages than DC, low-frequency PDC, or the CPS

pulse train (a complex PDC). The extent of mortality or

injury caused by each of these currents varies considerably with how they are used, other electrical parameters,

biological factors, and environmental conditions. With

enough field intensity and duration of exposure, any type

of current can be lethal, and under certain conditions

even DC can injure substantial numbers of fish.

As for most chemical substances and physical

parameters affecting living organisms, concentration (in

this case, field intensity) and duration of exposure are the

primary factors determining the physiological

stressfulness and lethality of electrofishing currents on

fish. Beyond lethal threshold levels, increases in electricalfield intensity or duration of exposure typically result in

increased mortality. However, it is not field intensity itself,

but the magnitude of voltage differential it generates

across fish (usually head-to-tail voltage) or specifically

affected nerves or tissues that causes electrofishing

mortalities and most sublethal physiological effects and

behavioral responses. That voltage differential is a

function of both field intensity and orientation of the fish

relative to the lines of current.

Unlike their crucial roles in electrofishing mortality,

field intensity beyond requisite threshold levels has an

unclear but certainly not critical effect on electrofishinginduced injuries, and exposure time does not appear to

be important except when using PDC. Spinal injuries and

associated hemorrhages can occur in fish located anywhere in the field at or above the intensity threshold for

twitch in the zone of perception. Among fish injured in

the zone of perception, as many are likely to escape as

move into the effective portion of the field for capture.

The principal cause of spinal injuries appears to be

muscular convulsions (myoclonic jerks or seizures) induced by sudden changes in field intensity or, more specifically, in voltage differential across the fish or affected

tissues at or above the relatively low threshold in magnitude of change for twitch. Such sudden changes occur

when current is switched on and off or pulsed, when fish

leap frantically out of and back into the electrified water,

and when netted fish are removed from or dipped in and

out of the field. Accordingly, duration of exposure in DC

should have no effect on incidences of spinal injuries

while fish remain in the water, but in PDC, longer exposures subject fish to more pulses and thereby increase

potential for spinal injury. However, neither muscular convulsions as the principal cause of spinal injuries in fish

nor sudden changes in voltage differential as the principal cause of the convulsions have been experimentally

documented. Also, the latter seemingly is contradicted

by the observation of twitches during uninterrupted DC

and occasional documentation of as many spinal injuries

(at least minor ones) in DC with just two sudden change

events (when the current is switched on and later off) as

in some simple or complex PDCs with numerous sudden

changes in voltage differential.
 SNYDER 83

Increases in spinal injuries with exposure time might

be expected as well for AC with its cyclic changes in

voltage differential and direction (effectively alternating

half-sine pulses), but limited experimental evidence suggests otherwise. Perhaps the changes in AC voltage are

not sufficiently sudden (if so, the same would apply to

half-sine PDC), or the change in direction precludes possible consecutive-pulse summation effects that might

sometimes be necessary to achieve the threshold magnitude of change in voltage differential.

Whether the probability or degree of spinal injuries

and hemorrhages increases with field intensity or not,

fish in a state of narcosis (petit mal) or tetany (grand mal)

may no longer be subject to the sudden convulsions that

are believed to cause most spinal injuries in PDC (and

possibly AC). Injuries might still occur during transition

between these states and when fish are removed from the

field. If some spinal injuries do occur during tetany, as

has long been suspected but unproven, the sustained

muscular tension would have to be sufficiently strong to

permanently compress one or more portions of the spinal

column, burst blood vessels, and possibly fracture vertebrae. Aside from this possibility, and unlike severe stress,

fatigue, and mortality, measures to specifically reduce

the intense zone of tetany around an electrode might not

have much impact on the frequency of spinal injuries.

Orientation of fish when first exposed to the effective portion of the field is probably as significant a factor

in electrofishing injuries as in other responses and mortality. However, based on limited evidence, greatest effect appears to occur when fish are perpendicular to rather

than parallel to the lines of current (minimum rather than

maximum head-to-tail voltage differential). If so, experiments to assess the injurious effects of electric currents

on fish might be confounded or biased to minimum effects if fish are held parallel to the direction of current.

Pulse frequency appears to be a primary factor affecting the incidence of spinal injuries in PDC and may be

a significant secondary factor in electrofishing mortalities. As expected if spinal injuries are caused primarily by

sudden changes in electrical potential, the incidence of

injuries is generally lowest for low-frequency currents

and increases with pulse frequency. With regard to incidences of spinal injuries, the CPS pulse train with a primary frequency of 15 Hz appears comparable to simple

low-frequency currents (and DC). It is unknown whether

other pulse trains or complex variations of PDC also result in as few injuries as low-frequency PDCs.

The effects of pulse shape or waveform, pulse width

or duty cycle, and voltage spikes on mortality and spinal

injuries have been inadequately investigated and data

that are available are difficult to compare and sometimes

contradictory. Although exponential and half-sine PDCs

have been implicated as particularly lethal and half-sine,

quarter-sine, and square PDCs as particularly injurious,

the effects of PDC waveforms on electrofishing mortality

and injury remain inconclusive. Likewise for AC waveforms, despite one comparison of sine-wave and triangular-wave AC which revealed no significant differences in

incidence of externally obvious injuries but notable differences in the nature and perhaps severity of those injuries. The little data that exists with regard to pulse duration

or duty cycle suggests no effect on mortality and a tendency for fewer spinal injuries using currents with longer

pulses or greater duty cycles. A limited-scope investigation suggested that voltage spikes have little or no impact on electrofishing injuries or mortality.

Evidence to date strongly indicates that Salmoninae

(trout, char, and salmon) are more susceptible to spinal

injuries, associated hemorrhages, and probably mortality

during electrofishing than most other fishes. Among other

species, burbot and sculpins (Cottidae) were reported to

be particularly susceptible to electrofishing mortality, at

least under some environmental and electrical-field conditions, whereas goldeye, some suckers (Catostomidae),

channel catfish, largemouth bass, walleye, and possibly

paddlefish were reported to be more susceptible to

electrofishing-induced spinal injuries and associated

hemorrhages. Electrofished mountain whitefish have been

reported to be particularly susceptible to bleeding of the

gills.

Because voltage differential across fish or specific

tissues increases with size, larger fish have been expected

to be more susceptible to electrofishing mortality and

injury than smaller fish. However, laboratory and field

data suggest that increases in electrofishing mortality

with size might only occur with increases in exposure

time and some researchers have reported greater

electrofishing mortality among smaller fish. Some data

support an increased frequency of spinal injuries as fish

size increases, but other data do not, and so the importance of size remains questionable.

The physical condition of fish can affect their susceptibility to electrofishing injury and mortality, but assessment of this factor is based mostly on suppositions

and casual observations rather than specific experiments

and data. Fish in poor health may respond less strongly

to electric fields, thereby reducing chances for spinal injury, but they also may be less able to withstand the

stresses of tetany and apnea during narcosis, thereby

increasing probability of death. On the other hand, weakened skeletal systems probably make fish especially susceptible to spinal injuries. Temperate fishes electrofished

during late fall through early spring may be less likely to
84 INFORMATION AND TECHNOLOGY REPORT--2003-0002

suffer either spinal injuries or mortality due to lower water temperatures that substantially reduce metabolism and

slow responses.

If there are significant harmful impacts on fish resulting from single electrofishing events, the effects of multiple events should be cumulative. In at least some cases,

the stress of repeated handling has greater impact on

delayed mortality than repeated exposures to electric

fields. The incidence of total injuries among captured

fishes inhabiting repeatedly sampled waters increases

cumulatively, not only during multiple-pass sessions, but

in successive seasons or years of sampling. Some newly

captured fish may have been injured during prior treatments or sampling but at that time either escaped the

effective portion of the electric field or were missed by

netters.

Effects on Long-Term Survival

Most investigations suggest that long-term survival

(that beyond a week or two) is seldom significantly affected by electrofishing. Apparently, most electrofishing

mortality occurs immediately or shortly after capture as a

result of asphyxiation or severe physiological stress. Also,

if not too severe, most fish survive spinal injuries caused

by electrofishing (see discussion under “Relation between

Mortality and Injury”). Still, there is some evidence that

injury or severe stress in fish as a result of exposure to

electric fields can result in long-delayed as well as shortterm or intermediate-term mortality.

Maxfield et al. (1971) conducted one of the earliest

investigations of the effects of electric fields on longterm survival. They exposed YOY and yearling rainbow

trout to low-frequency PDC and concluded that there

was no consistent long-term effect. Several lots of finclipped YOY were exposed for 30 s in a homogeneous

field of 8-Hz, 40-ms pulses at 1 Vp/cm (water 11–13° C,

143–172 μS/cm; 32% duty cycle). Fin-clipped yearlings

were similarly exposed but in a field of 5-Hz, 60-ms pulses

at 0.75 Vp/cm (water 9–11° C, 114–132 μS/cm; 30% duty

cycle). During exposure, none of the YOY but 4 to 84% of

the yearlings were narcotized. All narcotized fish revived

immediately and all fish were alive 2 days after treatment.

The fish were held with untreated controls of the same

age group until maturity. Cumulative mortalities were 9.9%

after 3 years for trout exposed as YOY versus 16% for

controls and 7.1% after 2 years for those exposed as yearlings versus 10.4% for controls.

Ellis (1974) narcotized age-2 channel catfish with 60

s of DC, 60-Hz AC, or 15, 20, or 25-Hz, exponential-wave

PDC at 1.5V/cm and found no significant effect on survival 133 days later. The fish were confined to cages in

ponds. Mortality ranged from 0 to 25% among treatment

fish and 0 to 23% for controls and was attributed to vegetation-limited exchange of water, predation by snakes,

and escape through holes torn in the cage netting by

turtles.

Barrett and Grossman (1988) reported no significant

differences in delayed mortality for mottled sculpin (3–9

cm SL) collected in late winter by DC electrofishing (600

V, 200 W continuous) and kick seine (0–11% and 0–15%

mortality, respectively). Although sample sizes were too

small for statistical analyses, they also reported little or

no mortality for largescale stoneroller, rosyside dace,

warpaint shiner, Tennessee shiner, longnose dace, creek

chub, and northern hog sucker. The fish were collected

from a North Carolina stream (5–8° C, 10–15 μS/cm) and

monitored for 1 month.

Taube (1992) conducted a series of controlled experiments in the slowly flowing water of a hatchery raceway

(11° C, 103 μS/cm) with heterogeneous fields of DC, CPS,

and four other variations of square-wave PDC, but after

monitoring treatment fish, large rainbow trout ( = 38–42

cm FL), in a raceway for 128 days, he reported no statistically significant differences in survival despite mortalities of 0% for DC and 8 to 25% for the PDCs (probably

due to small sample sizes of 12 fish per treatment). Furthermore, comparing these data to the observation of 10%

mortality for control fish not subjected to electric current

and maintained in a raceway for another long-term survival experiment (203 days–discussed below), Taube

could not attribute any mortality in this experiment to the

treatments. For this experiment, trout were individually

placed at the distal end of the exposure area and scared

or chased towards the electrodes into the effective portion of the field where they were shocked (stunned) for

5 s. Output voltage was 200 V for all treatments except

CPS for which voltage had to be doubled to stun fish.

Incidence of spinal injury was assessed by X-ray and

ranged from 8 to 67%. Most, if not all, mortalities occurred within 21 days.

Schneider (1992) stated that although AC

electrofishing was an important technique in fishery management and research, he had not found prior quantitative information about its effects on survival and growth

of fish under typical field conditions. Accordingly, he

analyzed tag data for largemouth bass and walleye initially captured by 3-phase AC electrofishing, trap netting, or angling in Michigan lakes and ponds during

mark-recapture investigations and reported no long-term

differences in survival among these capture methods. As

noted earlier under “Type of Current,” he also found that

3-phase AC electrofishing did not measurably increase

the shorter-term mortality (1–33 days) of several species

of warmwater and coolwater fishes.

After monitoring adult northern pike (38–77 cm FL)

for a month, Roach (1992) detected no significant

0
 SNYDER 85

differences in mortality among shocked fish with spinal

injuries (5%), shocked fish without spinal injuries (9%),

and controls (9%). The fish, which were initially collected

from South Dakota and Colorado reservoirs by gill net

and seine, were exposed to 5 s of homogeneous 120-Hz

PDC (50% duty cycle, 4.2 ms pulse width) at 0.14 to 2.21

Vm/cm ( = 0.93 V/cm). Treatment fish and controls were

tagged, measured, X-rayed, and placed in ponds to monitor

survival and growth during the next month. Spinal injuries,

mostly misalignments (class 2), were induced in 28% of

the treatment fish.

Zeigenfuss (1995) indirectly assessed survival by

comparing catch rates among shocked-injured, shocked

uninjured, and control rainbow trout stocked in a highly

controlled lake fishery in Colorado. Despite observing a

lower catch rate for shocked fish in the first of two study

years, he reported that differences in survival were not

significant for either year. As summarized earlier under

“Field Intensity,” treatment fish (15–35 cm TL) were

shocked for 2 s in homogeneous fields of 60-Hz, square

PDC with sufficient intensity to induce spinal injuries, Xrayed to document injuries, measured, and tagged before

release in April of two consecutive years. The lake was

open to anglers from May through September each year

and all fish caught were processed at an exit check station. Captures of control fish were greater than shocked

fish in the first year (22% and 17%, respectively) but similar in the second year (15% and 16%, respectively). Most

of the difference in capture rates for the first year was due

to a lower catch rate of shocked-injured fish. For this

reason and because the incidence of spinal injury and

mortality within 24 h of treatment was notably greater for

shocked fish in the first year than in the second,

Zeigenfuss (1995) suggested that although not statistically significant, the observed difference in capture rates

that first year was probably due to differential survival.

Dwyer and White (1995) exposed hatchery rainbow

trout ( = 33 cm TL) to single 10-s exposures of 250-Hz,

half-sine PDC and reported 8% mortality within 24 h but

no delayed mortality during the next 35 days of the experiment. The fish were individually exposed in a homogeneous field at 3.5 to 3.9 Vp/cm (0.9–1.0 Vm/cm; 8° C,

270–340 μS/cm) and presumably stunned (not specifically stated by the authors). Based on X-rays (and follow-up necropsies when possible injuries were detected),

no spinal injuries were detected among fish preserved

immediately after treatment, those that subsequently died,

or a 40% sample of surviving treatment and control fish

35 days after exposure (hemorrhages and non-spinal injuries were not reported).

Tipping and Gihuly (1996) found that tag returns for

adult steelhead (rainbow trout) subjected to electrical

anesthesia (29–53%) were consistently 6 to 18% less ( =

8%) than for those subjected to carbon-dioxide anesthesia

(35–60%), but the differences were significant only for an

intermediate electrical treatment (8-s exposures at 1 Vp/

cm with returns 18% less than for corresponding fish

anesthetized with carbon dioxide). Fish were anesthetized

with homogeneous fields of CPS from 0.2 Vp/cm for 100 s

to 1.7 V/cm for 3.6 s or carbon dioxide bubbled through a

tank at 10 l/min (fish left in the tank until narcotized, usually

several minutes). They were then processed, tagged, held

for 1 to 2 days, released downstream in the river, and later

caught by anglers or at upstream hatcheries. No mortalities

were observed prior to release. Although requiring more

exposure time to induce sufficient narcosis, electrical

treatments using the lowest field intensities (0.2 and 0.3

Vp/cm) produced a less violent response and resulted in

the least differences in returns after release (only 6–7%

less than fish anesthetized with carbon dioxide). Tipping

and Gihuly (1996) concluded that electroanesthesia might

be detrimental, possibly due to spinal or related injuries

(in preliminary experiments at 1.7 Vp/cm, spinal

compression fractures occurred in 8% of exposed fish),

but that if it is used as an alternative to chemical

anesthesia, injuries and mortalities caused at the lower

field intensities might be acceptable.

Although they lacked controls for comparison,

Dalbey et al. (1996) implied that 40 to 46% mortality recorded for wild rainbow trout within 335 days after capture by DC, 60-Hz PDC, or a hybrid of the two currents

(Fig. 5J) was comparable to expected annual mortality of

age-2 and older rainbow trout in Montana rivers and therefore probably not caused by electrofishing. Captured fish

were X-rayed to document spinal injuries then maintained

for observation in an irrigation pond. Dalbey et al. (1996)

found no differences in mortality among currents of capture or relative to presence and severity of spinal injury

upon capture.

Kocovsky et al. (1997) investigated the long-term

effects of annual electrofishing on stream fish and detected no adverse population effects for brook trout,

brown trout, or rainbow trout (populations remained

stable or increased), but a notable effect on longnose

sucker (population declined significantly in the third year).

They conducted three-pass electrofishing with 100-Hz

PDC in three streams for a period of 8 years. Incidences

of externally detected spinal injuries (mostly cumulative

from prior years of electrofishing) were assessed during

three of those years and despite lack of negative effect

on population size, were found to increase cumulatively.

Based on X-rays of over 100 specimens, Kocovsky reported, like many other researchers, that actual incidences

of spinal injury were much higher than could be detected

externally.

Ruppert (1996) and Ruppert and Muth (1997) reported

no significant effect on survival for juveniles of the

endangered bonytail 98 days after exposure to

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electrofishing currents. They exposed 720 fish (5–8 cm

TL) to CPS or 30-, 60-, or 80-Hz PDC at intensities sufficient

to induce taxis, narcosis, or tetany. No externally obvious

injuries were observed and no vertebral damage was

found among the half of each treatment and control group

examined for internal injuries, but 13% of the exposed

fish suffered spinal hemorrhages.

Unlike most investigations, one study reported by

Taube (1992) resulted in significantly greater mortality

among fish exposed to an electric field than among controls monitored for 203 days. In addition to an experiment

discussed earlier in this section (for which differences in

mortality were not significant), Taube (1992) exposed 102

rainbow trout (32–54 cm FL) to 5 s of homogeneous 60-

Hz, 50%-duty-cycle PDC at 2.3 Vm/cm (10–12° C, 95–104

μS/cm). Along with 50 control fish, the exposed fish were

X-rayed, weighed, measured, and maintained in a raceway to assess long-term survival and growth. Taube

(1992) reported 52% mortality for exposed fish that suffered spinal injuries (mostly class 2 injuries,

misalignments), 29% for exposed but uninjured (with respect to detectable spinal injuries), and 10% for the controls; 83% of the deaths occurred within the first 30 days.

Based on mark-recapture and radio-tag investigations

in the Colorado River Basin, long-term survival of initially electrofished endangered species does not appear

to be a serious problem. Many Colorado pikeminnow and

smaller numbers of humpback chub and razorback sucker

have been electrofished, radiotagged, and subsequently

monitored for extended periods (Tyus and McAda, 1984;

Wick et al., 1985, 1986; Tyus et al., 1987; Osmundson and

Kaeding, 1989; Valdez and Masslich, 1989; Tyus and Karp,

1990). Most survived electrofishing and radio-tag implant

surgery and were assumed to behave (move about) normally between and during subsequent contacts. A far

greater number of endangered and other fish initially collected by electrofishing were tagged with dangler, anchor, coded-wire, or PIT (passive integrated transponder)

tags. Some of these fish have been recaptured one or

more times by electrofishing or other means, sometimes

several years later (Hawkins, personal communication).

The fate of fish that were not recaptured is unknown, but

if recaptured fish had incurred electrofishing injuries, the

injuries were not externally obvious or not documented.

Effects on Growth and Condition

Even if exposure of fish to electric fields typically

has little, if any, affect on long-term survival of fish (most

electrofishing mortality is immediate or occurs within a

few days), it might impact subsequent growth and

condition of at least some species. Results of many, but

not all, investigations discussed below suggest that such

effects may be significant, especially in the case of multiple

exposures over short periods of time and among fish

known to have suffered spinal injuries. Accordingly,

impacts on growth and condition could be a serious

concern for fishery managers and others seeking to

safeguard, recover, or enhance aquatic ecosystems.

Most investigations discussed in this section also

included a survival component. Refer to corresponding

accounts in the preceding section, “Effects on Long-Term

Survival,” for additional information on nature of the studies and survival results.

Contrary to the results of most investigations discussed below, most early and some more-recent investigations of growth subsequent to electrofishing events

or controlled electrical exposure failed to reveal significant adverse effects. According to Halsband (1967), even

long treatments with different types of current did not

affect the general condition or growth of common carp.

In a very long-term study, Maxfield et al. (1971) exposed

YOY and yearling rainbow trout to low-frequency PDC,

monitored them through maturation, and concluded that

there were no consistent effects on subsequent growth.

Likewise Ellis (1974) narcotized 2-year-old channel catfish with 60 s of 60-Hz AC, 15- to 25-Hz, exponential-wave

PDC, or DC at about 1.5V/cm and found no significant

effect on growth 133 days later. Kynard and Lonsdale

(1975) held yearling rainbow trout (~12 cm) under DC

narcosis for 1, 2, 4, and 6 h (0.25 V/cm, 13–21° C, 450 μS/

cm) but reported no mortalities for exposures up to 4 h

and no effect on growth 25 days later, even for survivors

of 6-h trials (7% short-term mortality). Based on 1- to 2-

year mark-recapture tag data, Schneider (1992) reported

no significant differences in growth for largemouth bass

and walleye initially captured by AC electrofishing, trap

netting, or angling in five Michigan lakes and ponds.

In a laboratory experiment, Ruppert (1996) and

Ruppert and Muth (1997) reported that long-term (98-d)

growth in juvenile bonytail (an endangered species) was

not significantly affected by exposure to any of several

electrofishing currents or levels of intensity. After 49

days, mean weights increased 24 to 39% for treatment

fish and 26 to 27% for controls; after 98 days, mean weights

had increased by 42 to 54% and 42 to 44%, respectively.

Based on necropsy of half of the treatment and control

fish, they found that 13% of the shocked fish (range 3–

27% among treatments) had spinal hemorrhages but no

obvious damage to the vertebrae. They cautioned readers that significant negative effects on growth due to

these injuries might be more likely to occur in a dynamic

riverine ecosystem than under laboratory culture.

Among investigations reporting significant effects

of electrical exposure on growth, Dwyer and White (1995)

reported significantly less growth for shocked adult

rainbow trout (9% less in TL, 34% in g) and yearling Arctic

grayling (15% in TL, 23% in g) during 35 and 28-day
 SNYDER 87

monitoring periods, respectively, after exposure than for

controls. In contrast, both shocked and control yearling

cutthroat trout experienced very little increase in length

during the 28-day monitoring period, with no significant

difference between them, and actually lost weight, with

shocked fish losing significantly more than controls. The

rainbow trout (initially = 33 cm TL) were subjected to

10 s of 250-Hz PDC (8° C, 270–340 μS/cm) and the Arctic

grayling and cutthroat trout (12–18 cm TL) to 10 s of 500-

Hz PDC (9.6° C). X-rays of 44 exposed rainbow trout

revealed no spinal trauma (20 frozen immediately after

treatment, 4 upon death within 24 h, and 20 of 50 monitored

for 35 days); the yearling fish were not X-rayed for spinal

injuries.

In a field investigation, Thompson (1995) and

Thompson et al. (1997b) assessed the effects of

electrofishing on growth and body condition of brown

trout and rainbow trout (>18 cm TL) 1 year after initial

capture and concluded that annual electrofishing had

some adverse effects on fish growth or condition, but

not consistently. Fish in three Colorado rivers were

captured using 60-Hz, half-sine PDC with a mobile

(throwable) anode and marked with visible implant tags.

Recaptured fish were compared to control fish not

captured the year before and assumed to not have been

shocked in that previous year (authors recognized that

some fish might have been exposed but either escaped

the electric field or capture). Growth was based on backcalculations for the last-annual increment of scales and

validated for tagged fish by comparison with differences

in length since initial capture. Unreadable scales or failure

to validate back calculations reduced sample size such

that only four species-river-age comparisons were

statistically valid. In one river, age 4 and 5 unshocked

brown trout grew significantly more than shocked brown

trout. Likewise for age-5 rainbow trout in another river.

Although the mean growth increment for previously

shocked fish also was less than for controls in the

remaining two valid comparisons, the differences were

not statistically significant. In one river-year comparison,

average condition factors were significantly higher for

unshocked brown trout (22–36 cm TL) and rainbow trout

(32–41 cm TL) than for previously shocked fish–average

weights were 11% and 9% greater, respectively. In eight

other species-river-year comparisons, condition factors

were not significantly different. Based on other

investigations, Thompson et al. (1997b) suggested that

differences they observed in growth and condition may

have been caused by electrofishing injuries.

Acknowledging earlier reports of no significant

impact on growth after single electrical exposures, Gatz

et al. (1986), noted that several short-term physiological

effects had been identified by Horak and Klein (1967),

Schreck et al. (1976), Bouck et al. (1978), and Burns and

Lantz (1978), and hypothesized that repetition of these

physiological effects through repeated electrofishing, as

in multiple-capture studies, might measurably affect

subsequent growth. In a field study carried out for 1 year

in very low-conductivity streams in Tennessee and North

Carolina (5–10 μS/cm, salt blocks were necessary to

increase conductivity), Gatz et al. (1986) monitored the

individual growth of rainbow trout and brown trout of

various ages that were electrofished with 600-V, 120-Hz

PDC twice within a 1 to 3-day period repeatedly at

intervals of 1.5 to 7 months. They reported that significant

numbers of fish lost weight, in both the short term (1–3

days, 81% lost an average of 5% of their body weight)

and long term (48% of the fish electrofished within 3-

month intervals). The percentage of fish with

instantaneous growth rates less than average was

significantly greater for fish that were electrofished four

or more times during the year, at intervals of less than 3

months, or at a young age (ages 1 and 2). Gatz et al. (1986)

concluded that “studies should be designed to avoid

repeated electroshocking, especially at intervals of less

than 3 months.” They also suggested that “growth studies

in which more than a small fraction (e.g., >20%) of the

total population is repeatedly electroshocked at short (<3

month) intervals are likely to underestimate growth rates.”

Although no external signs of injury were noted, Gatz

et al. (1986) mentioned tissue damage which might require

up to 3 months for complete recovery as a possible

explanation. Fish were not examined by X-rays or necropsy

to confirm this suspicion.

Based on a laboratory experiment, Gatz and Adams

(1987) also concluded that time intervals between repeated

electrofishing should be maximized to limit impacts on

growth. They exposed hybrid bluegill x green sunfish to

400-V, 120-Hz PDC once a week for 3 months and found

that growth was about 37% less than for controls and

29% less than for fish exposed only once or at 2 or 4 week

intervals (differences between the latter two groups were

not significant).

Dwyer and White (1997) followed their singleexposure PDC short-term growth experiments on juvenile

Arctic grayling and cutthroat trout (Dwyer and White,

1995, discussed above) with a longer-term, multipleexposure investigation and reported significant effects

varying with species and electrical current, intensity, and

exposure. Using juvenile Arctic grayling (15–25 cm TL,

29–121 g) and Yellowstone cutthroat trout (12–17 cm TL,

14–41 g) and the same homogeneous field exposure tank

as in the earlier experiments (390 μS/cm, 7° C), they

compared the long-term effects on growth of two 5- or

10-s exposures, 10 to 14 days apart, in smooth DC or 60-

Hz, square-wave PDC at 0.75 Vp/cm (lower intensity; 25%

duty cycle for PDC) and 1.5 Vp/cm (higher intensity field;

33% duty cycle for PDC). For Arctic grayling 100 days

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88 INFORMATION AND TECHNOLOGY REPORT--2003-0002

after initial treatment relative to controls, they reported:

(1) no significant difference in growth for fish exposed to

5 or 10 s of low-intensity DC; (2) significantly less growth

in length (22–29% less) but not weight for fish exposed

to 5 or 10 s of high-intensity DC; and (3) significantly less

growth in length (56–71% less) and weight (63–76% less)

for fish exposed to 5 or 10 s of low or high-intensity PDC.

For the cutthroat trout relative to controls, they reported:

(1) no significant difference in growth for fish exposed to

5 or 10 s of high-intensity DC or 5 s of high-intensity

PDC, (2) significantly less growth in length but not weight

for fish exposed to 5 s of low-intensity PDC (19% less) or

10 s of low-intensity DC (15% less), and (3) significantly

less growth in length and weight for fish exposed to 5 s of

low-intensity DC (12% less in TL, 10% in g) or 10 s of low

or high-intensity PDC (25–27% less in TL and g). Some of

the significant differences for cutthroat trout appear

counter-intuitive and all are much lower in magnitude than

for Arctic grayling. Based on results for both species, the

authors concluded that smooth DC was less harmful.

Although not as strongly supported by their results, they

also recommended lower voltages and shorter exposures

to minimize potential long-term effects on growth.

Although initially much smaller in size, control and

treatment juvenile cutthroat trout in the above experiments (Dwyer and White, 1997) grew nearly 50% more in

length and 25% more in weight in 100 days than Arctic

grayling. In contrast, growth after 28 days for juvenile

cutthroat trout in their single-exposure experiment (Dwyer

and White, 1995; discussed above) was almost nil, but

PDC frequency and peak field intensity were much higher.

Because of problems in objectively assessing the

degree and impact of electrofishing injuries on a fishery,

Holmes et al. (1990) recommended assessing effects at

the population level by testing for differential survival

and growth over time between fish with electrically induced spinal injuries and control groups. Three such investigations have been conducted with mixed results.

Unlike the significant mortality reported by Taube

(1992) for injured rainbow trout ( = 39 cm FL; mostly

spinal misalignments) during a 203-day monitoring period after exposure to 5 s of homogeneous 60-Hz PDC at

2.3 V/cm (most mortality within the first 30 days; see discussion above under “Effects on Long-term Survival”),

he found no significant differences in growth among exposed-injured, exposed-uninjured, and control trout.

Mean increases in length were 29, 42, and 37 mm, respectively, and corresponding mean increases in weight were

320, 381, and 355 g. However, results were compromised

by small sample sizes due to lost tags, especially for injured fish. Taube suggested that differences might be

significant in a wild population where injury could affect

ability to capture prey.

In the first, but not second, of two annual trials,

Zeigenfuss (1995) found that average daily growth rate

among captured rainbow trout (15–35 cm TL) stocked in

a highly controlled Colorado lake was significantly lower

for shocked fish with spinal injuries than for either controls or shocked-uninjured fish (no significant difference

between the latter two groups). Most of that overall difference occurred among fish measuring 29 to 32 cm TL.

No significant differences in growth rates were detected

for any size groups during the second-year trial. Differences in results between years might be explained, at

least in part, by the greater incidence of spinal injury

observed among electrically exposed trout during the first

year.

Dalbey et al. (1996, also Dalbey, 1994) compared

changes in length, weight, and condition factor for wild

rainbow trout 335 days after capture by DC, 60-Hz PDC,

or a hybrid of the two currents (Fig. 5J) and reported

significant differences in growth relative to both

electrofishing current and presence and severity of

electrofishing injury upon capture. Captured fish were Xrayed to detect spinal injuries, then maintained for longterm observation in an irrigation pond. Among trout

captured by the different currents, growth did not differ

significantly 100 days later but it was significantly greater

for the hybrid current than for DC or 60-Hz PDC at 335

days (respectively, 112% and 51% greater for mean increase in length and 79% and 58% greater for mean increase in weight). There were no significant differences

in mean condition factors relative to electrofishing current. Combining data for all currents, uninjured fish grew

significantly more through the first 100 days than fish

with spinal injuries (~1.5 times more in mean length and

16 times more in mean weight). By 335 days, uninjured

fish and those suffering only vertebral compression fractures (class 1) grew significantly more than fish with more

severe (class 2 and 3) spinal injuries (>3 times more in

both mean length and mean weight). Fish with the most

severe spinal injuries (class 3) actually lost mean weight.

Condition factor declined for all groups by 100 days after

exposure, but the decline was significantly greater for

injured fish. By 335 days, condition factors increased

beyond that at capture for all groups except the most

severely injured fish and was significantly greater for

uninjured than for injured trout. Dalbey et al. (1996) concluded that the negative effects of electrofishing injury

on growth and condition are likely to persist for at least a

year after injury and speculated that in a dynamic stream

environment, spinal injuries could have even greater negative effects.

Among recaptured endangered and other native

cypriniform fishes that were initially captured by

electrofishing, tagged, and subsequently recaptured one

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 SNYDER 89

or more times in the Upper Colorado River Basin (see last

paragraph under “Effects on Long-term Survival”),

Bestgen et al. (1987) and Hawkins (personal communication) found some fish that had grown very little in length,

not at all, or even lost length between captures, even a

year or more after the initial or prior capture. Spinal injuries, including compressed vertebrae, or long-term physiological stress might account for at least some of these

poor or no growth observations.

Effects on Reproduction and Gametes

Spawning or near-ripe fish often aggregate in accessible localities and are sometimes considered more vulnerable to electrofishing than at other times of the year

(Stewart, 1967, as cited by Lamarque, 1990; Kolz and

Reynolds, 1990b). For these reasons, some fish are targeted for collection by electrofishing during the spawning season. Also, broodstock for experimental or hatchery

culture are often collected by electrofishing or subjected

to electronarcosis prior to hormone injection or extraction of gametes for artificial fertilization of eggs. If fish in

spawning condition are particularly susceptible to

electrofishing injury or there are significant adverse effects of these practices on spawning behavior or gametes,

the impacts might in turn affect hatchery operations or

natural reproduction, a matter of particular concern for

small isolated populations or endangered species.

Most knowledge of the effects of electric fields on

fish reproduction and gametes is based on collection of

broodstock, hatchery operations, and survival of artificially fertilized eggs. Unfortunately, conclusions drawn

from these observations and experiments are mixed but

sufficient to warrant caution, ongoing scrutiny, and perhaps reevaluation of the practices.

Several investigators have reported evidence of no

harmful effects of electrofishing or electroanesthetizing

broodstock on the viability of artificially fertilized eggs

and recently hatched offspring. Halsband (1967) reported

that gonads were not harmed by electrofishing, and

Halsband and Halsband (1975, 1984) stated that “Harmful

genetic effects–or harmful effects to the progeny–are also

not produced.” According to Vibert (1967b), “McGrath

reported that . . . no ill effects have been recorded in

hatcheries on the offspring of wild trout caught by

electricity.” Maxfield et al. (1971), who subjected YOY

and yearling rainbow trout to 8-Hz and 5-Hz PDC,

respectively, and documented the lack of effects on longterm survival and growth (see discussions in

corresponding sections above), also reported that

subsequent fecundity of those fish and mortality of their

offspring through eyed-egg, hatching, and initial feeding

stages were not consistently different from those of

unexposed fish. Khakimullin and Parfenova (1981)

reported no ill effects of pulsed 6-Hz, 40-ms AC (probably

PDC from rectified AC) on Siberian sturgeon spawners or

subsequent (pituitary-induced) gamete maturation and

development of eggs and larvae. Similarly Valdez (personal

communication) and Pfeifer (personal communication)

reported no adverse effects of PDC electrofishing on ripe

lake trout and walleye, respectively, or on the survival of

their artificially fertilized eggs. Even when broodstock

are injured by exposure to electricity, eggs may not be

adversely affected. Tipping and Gihuly (1996) reported

that in one instance, ripe eggs were successfully extracted

from a few coho salmon that had apparently suffered spinal

injury and swam upside down for more than 2 weeks.

Walker et al. (1994) found that survival of fertilized

northern pike eggs through an eyed stage was nearly the

same whether broodstock was electrically (10 s of 50-Hz,

7.6-ms, PDC at 0.6 V/cm) or chemically (tricaine

methanesulfonate = MS-222) anesthetized (55% vs. 56%).

Ripe broodstock (45–97 cm SL) for this comparison were

collected from the Mississippi River by frame nets, anesthetized, and stripped of eggs and milt for hatchery propagation. All electrically anesthetized fish were swimming

upright within 3 min and none of the fish died or showed

external signs of injury within 24 h of electrical exposure.

Tipping and Gihuly (1996) reported that mortality of

chinook salmon eggs and larvae through swim-up was

significantly greater for those reared from unshocked

broodstock (mean 12%, range 6–19%) than from electrically anesthetized fish (means 6–7%, range 4–8%). The

latter were subjected to 18 s of CPS at 0.4 Vp/cm or 4 s of

CPS at an estimated 1.4 Vp/cm immediately prior to being

killed for collection of eggs and milt. There was no significant difference in egg and larval mortality relative to

electro-anesthetic protocols. For all treatment and control groups, most mortality occurred by the eyed-egg

stage.

In contrast to discussion above, Marriott (1973) and

other investigators have documented significantly greater

mortality for the progeny of captive or wild broodstock

that were subjected to electric fields prior to spawning or

extraction of gametes. Marriott (1973) compared mortality of artificially fertilized pink salmon eggs from

unshocked and electrocuted (110-V, 60-Hz AC) males and

females. He found mortality through a late-eyed stage to

be 12% higher for eggs from electrocuted females. Two

electrocuted females had severely ruptured internal organs, and most of their eggs were loose and bathed in

body fluids that might have accounted for at least some

subsequent egg mortality. Additional exposure of a batch

of fertilized eggs from electrocuted adults to an electric

field resulted in an additional 15% mortality, 27% greater
90 INFORMATION AND TECHNOLOGY REPORT--2003-0002

mortality than for eggs never exposed to an electric field.

Marriott (1973) recommended that electrofishing not be

used to capture ripe females.

Newman and Stone (unpublished manuscript, 1992)

subjected ripe walleye to 120-Hz, quarter-sine PDC (400

V), stripped and artificially fertilized the eggs, and documented significantly greater mortality for these embryos

(63–65%) than for embryos from controls ( = 37%).

Broodstock were exposed in a net enclosure as an

electrofishing boat made two slow passes about 0.7 m

from the net.

Newman and Stone (unpublished manuscript, 1992)

also reported that the manager of the Lac du Flambeau

Tribal Hatchery had severe viability problems with eggs

from electrofished brown trout. He and other hatchery

managers observed broken eggs when stripping

electrofished brown trout and suspected that albumen

from the eggs might have clogged the micropyles in many

unfertilized eggs. According to Newman and Stone (unpublished manuscript, 1992), some biologists also suspect that electrofishing might cause a loss of sperm

motility in ripe males, but experimental confirmation of

such has not been published.

Roach (1996) monitored the fertilized eggs from wild

broodstock of four salmonids captured by electrofishing

and reported significantly greater mortality than for eggs

from controls. Using 60-Hz, square-wave, PDC (50% duty

cycle), he electrofished chinook salmon (63–98 cm FL;

145 μS/cm; 11° C), least cisco (29–41 cm FL; 138 μS/cm;

7° C), and humpback whitefish (37–48 cm FL; 138 μS/cm;

7° C) and exposed weir-trapped Arctic grayling (25–38 cm

FL) in a net pen (340 μS/cm; 5° C). Arctic grayling and

chinook salmon were exposed to mean field intensities

up to 1.4 and 1.2 V/cm, respectively, at 2.5 cm from the

anode. All fish were netted while in a state of narcosis.

Eggs from these fish and corresponding controls caught

by other means were stripped, fertilized, incubated, and

monitored up to the eyed stage. Eggs were collected on

the day of exposure for all but chinook salmon which

were stripped 3 days after exposure. Mortality up to the

eyed-stage for embryos from shocked versus unshocked

parents was 4% vs. 2% for Arctic grayling, 20% vs. 1%

for chinook salmon, and 57% vs. 52% for least cisco;

mortality for humpback whitefish was 41% but controls

were not available for comparison). The difference in

mortality figures was greatest for chinook salmon (19%),

perhaps because its eggs were collected 3 days after exposure rather than on the same day.

Muth and Ruppert (1996) subjected ripe broodstock

of endangered razorback sucker to electrofishing fields

and reported injuries to the adults, premature expulsion

of gametes, and significantly greater mortality of progeny through hatching than for controls. Captive ripe males

(50–55 cm TL) and near-ripe females (55–60 cm TL) were

transported from a national hatchery and the females injected with hormone to induce ovulation. Treatment fish

(2 replicates, each with 2 males and usually 2 females)

were exposed to 10 s of either CPS or 60-Hz, 4-ms, squarewave PDC, each at a homogeneous intensity of 1 Vp/cm

(610 μS/cm; 20° C). Tetany was induced in all fish but was

incomplete for those exposed to CPS (fish continued to

quiver). All fish expelled gametes during treatment, at

least several hundred eggs per female. No external hemorrhages were observed but subsequent necropsy and

X-ray analysis revealed spinal injuries or associated internal hemorrhages in 50% of the fish exposed to 60-Hz

PDC and 14% of those exposed to CPS (see earlier discussion under “Pulse Trains”). No spinal injuries or hemorrhages were detected in control fish and no damage to

internal organs was observed for either treatment or control fish. Fertilized eggs were divided into lots of 500, five

for each treatment replicate and ten for controls, incubated at 18° C, and checked twice daily for removal of

dead eggs until hatching. About 8 to 12% of samples of

treatment and control eggs preserved prior to fertilization

had ruptured chorions. Mortality through hatching for

controls, 65 to 79% ( = 74%), was quite high but within

the range reported by hatcheries for razorback sucker

(45–77%). Mortalities for 60-Hz PDC and CPS treatments

were significantly higher at 83 to 96% ( = 89%) and 90 to

98% ( = 95%), respectively, but the difference between

the two treatments was not significant. Muth and Ruppert

(1996) recommended that the practice of electrofishing

spawning aggregations of endangered razorback sucker

be carefully reevaluated.

In the only investigation of effects of electrofishing

ripe or near-ripe fish on natural reproductive behavior,

Sorensen (1994) concluded no long-term consequences.

He subjected spawning goldfish (32 females and 24 males;

spawning induced by injection of prostagladin F2 in females) to 15 s of 100-Hz, square-wave PDC in an aquarium.

Recovery times were greater than 10 min, but no shortterm mortality or brands were reported. Twenty-four hours

after being stunned, females were again injected and the

fish spawned normally. In a field investigation, Sorensen

exposed naturally spawning brook trout (11 males and 9

females) to 30 s of rippled DC in a Minnesota stream.

After recovery, all fish were released back into the stream

in good condition and nearly half (5 males and 3 females)

were observed to spawn.

Effects on Early Life Stages

Electric fields are of no value in the collection of

already spawned fish eggs, and few biologists have applied electrofishing technology to the collection of fish

larvae and early juveniles (Snyder, 1983; Copp, 1989; Kelso

and Rutherford, 1996). Accordingly, most concern about

0

0

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 SNYDER 91

harmful effects on fish eggs and larvae pertains to their

incidental exposure during electrofishing operations for

larger fish. Although based on very few investigations

and limited primarily to salmonids, Lamarque (1990) noted

that evidence to that date suggested that embryos are

particularly sensitive between fertilization and eyed-egg

stages and recommended that electrofishing over active

spawning grounds should be avoided.

Godfrey (1957) determined that mortality in brook

trout and Atlantic salmon embryos was low when exposed

to electric fields during the first few hours (water hardening; precleavage stages), high when exposed at some

point thereafter until the eyed-egg stage, then low again

when exposed anytime during the remainder of embryonic development. As discussed below, more recent investigations on other species have generally

substantiated Godfrey’s (1957) observations, and one,

as surmised by Kolz and Reynolds (1990b), suggested

that this pattern of greater sensitivity to electricity before

the eyed-egg stage is similar to that for mechanical shock.

Dwyer et al. (1993; also Dwyer and Fredenberg, 1991)

found that for rainbow trout embryos reared at 10° C,

cumulative mortality to day 26 or 27 (just a day or two

before hatching) followed a nearly normal distribution

relative to age at exposure (2 to 26 days) for both electrical and mechanical shock with peaks on day 8 (10 days

before eye up and 20 days before hatching). Mortalities

for embryos treated at this most sensitive time (day 8)

averaged 99% for eggs dropped 15 cm from one container

to another, 58% for eggs exposed to 10 s of homogeneous 250-Hz PDC at about 3.4–3.8 Vp/cm (0.9–1.0 Vm/cm,

270–340 μS/cm, 8° C), 30% for those handled but not

shocked, and about 20% for unhandled controls.

Noting that walleye have a much shorter incubation

period than rainbow trout and that peak sensitivity to

mechanical shock for walleye embryos occurs at about

24 h after fertilization, Newman and Stone (unpublished

manuscript, 1992) found that walleye embryos also were

more sensitive to 120-Hz, quarter-sine PDC (400-V, 3-A

output) when exposed at 24 h than 48 h of age. For their

experiment, eggs were placed in nylon mesh bags and

laid on a lake bottom over typical walleye spawning substrate then exposed to a single pass of current from an

electrofishing boat. The difference in average mortality

between embryos exposed at 24 h and controls was 19%

(64% vs. 45%), whereas the difference between those

exposed at 48 h and their respective controls averaged

only 3% (56% vs. 53%).

Embryos also may be detrimentally irritated by electric fields near the end of the embryonic period. Luczynski

and Kolman (1987) used AC to induce premature hatching in powan embryos.

The impact of electrical factors such as duration of

exposure, field intensity, and type and waveform of current

on embryos has been investigated by several researchers.

As early as the beginning of the 1920’s, Scheminzky (1922,

according to Lamarque, 1990), subjected trout eggs to

long exposures in a DC field and reported movements of

embryos and one incident of high mortality. Although

exposures in Scheminzky’s (1922) experiments were far

longer than those likely in normal electrofishing operations

(Lamarque, 1990), perhaps they were not so different from

what drifting eggs or larvae (e.g., freshwater drum, emerald

shiner, striped bass) might experience near electric screens

or barriers.

In addition to determining the most sensitive stages

during embryonic development, as discussed above, both

Godfrey (1957) and Dwyer et al. (1993; also Dwyer and

Fredenberg, 1991) found that mortality during these stages

increased with both field intensity and exposure time. In

Dwyer et al.’s (1993) investigation, 8-day-old cutthroat

trout embryos were exposed to 5, 10, or 20 s of homogeneous CPS at 2.4, 3.8, 5.3, and 6.7 Vp/cm and assessed for

mortality 10 days later (about eyed-egg stage). Combining exposure times, mortality increased with field intensity from less than 15% for controls and treatments at 2.4

Vp/cm to 20 to 45% at about 3.8 Vp/cm, 85 to 100% at

about 5.3 V/cm, and approximately 100% at 6.7 Vp/cm. For

embryos subjected to 3.8 Vp/cm, they reported significant increases in mortality with exposure duration from

approximately 20% for 5-s to 30% for 10-s and 42% for 20-

s exposures.

Furthermore, Dwyer and Erdahl (1992, 1995) found

that field intensity had greater impact on mortality of cutthroat trout embryos than either current type or PDC pulse

frequency. They exposed separate batches of 2- to 18-

day-old embryos every second day to 10 s of homogeneous DC at 1.4 and 2.2 Vp/cm, 30- or 60-Hz, square-wave

PDC (50% duty cycle) at 1.4 and 2.2 V/cm (probably peak–

if mean, corresponding peak voltage gradients would be

2.8 and 4.4 V/cm), or CPS at 3.4 and 4.3 Vp/cm; 10-day-old

embryos were exposed also to CPS at 1.4 and 2.4 Vp/cm

(7.8° C, 388 μS/cm; voltage gradients calculated). Mortalities for all treatments were assessed on day 18 (eyed

embryos). Mean mortalities for controls ranged from 3 to

11% and was greatest for 12-day-old embryos. Mean

mortalities were similar to controls for all treatments at 1.4

V/cm except DC on days 8, 10 and 12 (19, 22 and 29%,

respectively), all treatments on day 2 except DC at 2.2 V/

cm (47%) and CPS at 3.4 V/cm (52%, no data for 4.3 V/cm),

and, as in investigations by Dwyer et al. (1993) and Roach

(1996, discussed below), for all treatments beyond day 14

regardless of field intensity. The greatest mortalities were

observed at the highest intensity levels for DC (2.2 Vp/

cm; 84–99% for exposures on days 8–14; no data for days

4 and 6) and CPS (4.3 Vp/cm; 81–99% for days 4–12).

Mean mortalities for 30- and 60-Hz PDC treatments at 2.2

V/cm and CPS at 3.4 V/cm on days 6 through 12 ranged
92 INFORMATION AND TECHNOLOGY REPORT--2003-0002

from 22 to 76%. Unlike comparable experiments by Dwyer

et al. (1993) and Roach (1996, discussed below) using

PDC, mortalities for 30- and 60-Hz PDC at 2.2 V/cm peaked

on days 6 and 12 (61–76%) with notably lower mortality

between on days 8 and 10 (22–32%). Among current types

tested at comparable field intensities, cutthroat trout embryos were at times more sensitive to DC than PDC or

CPS. There were no consistent differences in mean mortality between 30- and 60-Hz PDC at comparable field intensities.

Roach (1996), who conducted similar experiments with

Arctic grayling and chinook salmon, also reported significantly greater mortality for embryos exposed to electric fields through the eyed-egg stage and that mortality

during the more sensitive stages generally increased with

increasing field intensity, but he also found a strong species effect and that mortality was usually greater for embryos from recently shocked parents than unshocked

parents. Roach (1996) exposed embryos from shocked or

unshocked broodstock to 5 s of homogeneous 60-Hz PDC

(50% duty cycle) at high (1.3–1.5 Vm/cm), medium (0.8–1.0

V/cm), low (0.3–0.5 V/cm; Arctic grayling only), or no

field intensity. Batches were treated once at difference

ages, every second or third day from fertilization to or

beyond acquisition of dark eye pigment. Mortality rates

for exposed embryos were much lower for Arctic grayling

(<12%) than for chinook salmon ( 100%) and peak sensitivity occurred notably later in development (day 10 vs.

day 6 for chinook salmon).

For exposed Arctic grayling, Roach (1996) reported

that mean mortality rates increased with age at exposure

from a low for 2-day-old morulas to a peak for 10-day

embryos (~2 days after optic vesicles appeared and 2

days before the eyes became faintly pigmented), then

decreased for embryos exposed at later stages. Mortalities for high-intensity treatments of embryos from shocked

parents ranged from 7% for day 2 exposures to 12% for

day 10 exposures versus 2 to 4%, respectively for controls from unshocked parents. Significant differences in

mean mortality included: (1) shocked embryos from

shocked parents (6%) versus shocked embryos from

unshocked parents (4%); (2) all combinations of embryos

shocked by level of field intensity (7% for high, 6% for

medium, 4% for low, and 3% for none); (3) embryos

shocked at 2 days (4%) versus all other stages (5–7%);

and (4) embryos shocked at 10 days versus all other

stages (mean percentages not reported).

For chinook salmon, Roach (1996) reported that

greatest mean mortality rates were about 90 to 100% for

embryos exposed to high field intensity at any stage

between morula and early epiboly (3–12 days), regardless

of whether parents had been shocked or not. This was

comparable to the high mortality Dwyer and Erdahl (1995)

reported for 4 to 12-day-old cutthroat trout exposed to

high-intensity CPS. For chinook salmon embryos treated

at high intensity on day 14 (optic vesicles visible and

brain lobes differentiated) or later, mortality dropped

sharply to 32 to 42% for those from shocked parents and

2 to 8% for those from unshocked parents. At medium

field intensity, response was more like previously

discussed experiments with sensitivity increasing

progressively to an early peak of 75 to 78% mortality for

treatments on day 6 (gastrula stage), again regardless of

whether parents were shocked or not. For later-stage

treatments at medium field intensity, mortality dropped

regressively by the day 14 treatment to 22 to 32% for

embryos from shocked parents and 1 to 2% for those

from unshocked parents. Mean mortality for

corresponding controls was consistently 16 to 24% and

1 to 3%, respectively.

In experiments similar to those by Dwyer and Erdahl

(1995) on cutthroat trout embryos (discussed above),

Muth and Ruppert (1997; also Ruppert and Muth, 1995,

Ruppert, 1996) investigated the harmful effects of electricfield intensity and PDC pulse frequency and pattern on

embryos of the endangered razorback sucker and

determined that moderately early embryos were most

sensitive, mortality following exposure at this early stage

was significantly greater for all fields tested, and the

highest intensity and highest frequency simple PDC fields

tested were most harmful. Embryos at 33 h (early epiboly),

78 h (early tail bud), or 122 h (finfold) postfertilization

were exposed for 10 s in one of six homogeneous fields

(19° C, 650 μS/cm): CPS or 30-Hz (4-ms, 12% duty cycle),

60-Hz (4-ms, 24% duty cycle), or 80-Hz (5-ms, 40% duty

cycle), square-wave PDC at 1.2 Vp/cm; or 60-Hz PDC at 5

or 10 Vp/cm. Mortalities from all treatments through

hatching (between 128 and 140 h after fertilization), except

for 78-h and 122-h embryos exposed to CPS and 30-Hz

PDC, ranged from 22 to 97% and were significantly greater

than corresponding controls (5–12%). As for most

investigations discussed above, sensitivity to all electric

fields was greatest for embryos in a moderately early stage

of development (33-h) and decreased significantly with

age at exposure except between 78-h and 122-h for CPS

and 30-Hz PDC treatments. At 1.2 Vp/cm, differences in

mean mortality between treatment and control lots ranged

from 38 to 85% for embryos treated at 33 h, 27 to 49% at

78 h (excluding insignificant differences of 2% for CPS

and 17% for 30-Hz PDC), and 17 to 25% at 122 h (excluding

insignificant differences of 3% for CPS and 5% for 30-Hz

PDC). For 78-h and 122-h embryos treated to 1.2-Vp/cm of

30-, 60-, or 80-Hz PDC, mortality increased with increasing

pulse frequency but only differences between the 30-Hz

and 80-Hz treatments were significant. Mean mortalities

for all 33-h treatments were similar at 50 to 60% except for

the most intense (10 Vp/cm) 60-Hz PDC treatment which

suffered 97% mortality. Among all 60-Hz PDC treatments,
 SNYDER 93

mortality increased with increasing field intensity but the

differences were only significant for the most intense

treatments with 33-h and 78-h embryos. No notable

external morphological anomalies were observed for any

larvae that hatched from these treatments. Muth and

Ruppert (1997) concluded that electrofishing adult

razorback sucker over active spawning grounds could

significantly reduce survival of embryos present in or on

the substrate.

Muth and Ruppert (1997) also subjected recently

hatched, 36-h-old razorback sucker larvae (9.4–9.7 mm

SL; ~96 h before swimup) to the same treatments discussed above for embryos and reported a significant reduction in growth during the next 4 weeks, regardless of

electrical treatment, but no significant effect on survival.

Larvae were immediately tetanized upon exposure to 60-

Hz PDC at the higher field intensities (5 and 10 Vp/cm),

but during all 1.2 Vp/cm treatments they severely twitched

and rapidly swam in random directions. Larvae subjected

to 30-Hz PDC and CPS recovered almost immediately after being shocked whereas those subjected to other treatments remained on the bottom of nylon-mesh treatment

baskets for several seconds after current was switched

off. No fish died within 48 h of treatment and no behavioral anomalies were observed during the 4-week monitoring period after exposure during which survival and

growth were assessed at 7-day intervals. Mortality

through day 28 averaged 8% for controls and 5% (30-Hz

PDC, 1.2 V/cm) to 15% (60-Hz PDC, 1.2 V/cm) for exposed

larvae. Growth for all treatment larvae 4 weeks after exposure averaged 0.07 to 0.09 mm/d and was significantly

less (31–46%) than the average growth rate of 0.13 mm/d

for controls. However, in most cases, significant differences in growth were not detected until 21 days after

treatment. There were no significant differences in growth

among treatments. The authors discussed the probability that reduced growth rates could significantly affect

already limited first-year survival by prolonging vulnerability to starvation and predation.

Based on a limited experiment with precleavage Atlantic salmon eggs buried under about 20 cm of gravel

and exposed to DC for about 2 min, Godfrey (1957) concluded that eggs in gravel redds received some protection from shock (mortality 10% vs. 81% for unburied eggs),

but such protection was not substantiated in other investigations. In a similar experiment, Dwyer et al. (1993)

subjected 8-day-old cutthroat trout eggs in Vibert boxes

buried 15 cm deep in several artificial redds to 10 s of 250-

or 500-Hz PDC or CPS at voltage gradients of 0.9 to 1.0

Vm/cm. Voltage gradient appeared to be negatively affected by gravel depth when using 250-Hz PDC but not

500-Hz PDC (not mentioned for CPS). Still, cumulative

mortalities 10 days after exposure were 95% for CPS, 68%

for 250- or 500-Hz PDC, and 56% for controls. The high

mortality for the controls and a portion of each shocked

group was attributed to sedimentation in redds. Based

on these results, other experiments noted above, and

measurement of voltage gradients of about 1 Vm/cm at 20-

cm depths in artificial redds, Dwyer et al. (1993) concluded

that electrofishing in streams where trout have recently

spawned can adversely affect egg survival. Roach (1996)

also determined that burial in gravel offered little protection to eggs other than as a physical barrier keeping electrodes dragged over the surface at least burial depth away

from the eggs. He reported that with an electrode at the

surface of a gravel substrate, the drop in voltage gradient

with depth in gravel was nearly the same as in water alone

(70–72% drop over 40 cm in his experiments). However,

he also reported that drop in field strength within the first

10 cm of substrate depth was greater for larger size gravel

and cobble than for silt or gravel and silt.

Among the few biologists who used electric fields to

capture larvae, Braem and Ebel (1961) used electrified dip

nets and McLain and Dahl (1968) an electrified net sled

(beam trawl) to capture lamprey ammocoetes, Maty et al.

(1986) used electric fields to capture Atlantic salmon larvae after emergence from redds, and Noble (1970) used

an electric grid in front of a Miller high-speed sampler to

improve the catch of larger larvae and juveniles. Noble

(1970) found that the latter electrified sampler had little

effect on the catch rate of small larvae. However, because

small fish larvae are much less likely to evade the sampler

than larger larvae or early juveniles, catch rates of the

former might not be expected to differ even if the field

was effective.

Perhaps the greatest proponent for use of an

electrofishing technique for capture of larvae and small

juveniles is G. H. Copp of France. Copp and associates

(Copp and Penaz, 1988; Copp, 1989, 1990; Persat and

Copp, 1990) used the same portable PDC electrofishing

gear that was used locally to capture larger fish but reduced the size of the anode to a 10-cm ring and increased

the size of the cathode. This intensified the field within 30

cm or less of the anode sufficiently to induce taxis or

narcosis (possibly tetany) in most fish as small as 5 mm

SL. The anode, mounted on a 2.5-m handle, was dipped

into the water as the deadman switch on the handle was

closed for a second or two, then a fine mesh dip net was

immediately thrust under the anode to collect the fish.

The advantage of this technique over simply using dip

nets or hand seines to collect larval and early juvenile

fish was samples with a relatively unbiased size range.

Usually for larger juvenile and adult fish, both electrodes

should be as large as practical to reduce the zone of tetany

and maximize the effective size of the field. However, Copp

and his associates (Copp and Penaz, 1988; Copp, 1989,

1990; Persat and Copp, 1990) effectively used their very

limited range for smaller fish to advantage by combining
94 INFORMATION AND TECHNOLOGY REPORT--2003-0002

the technique with a sampling strategy that consisted of

numerous, small, randomly distributed, microhabitat

samples (point abundance sampling). The matter of

electrofishing injuries and mortality with this method was

not a serious concern because sample size was usually

small and the collected fish were killed, fixed, and preserved for subsequent processing.

The effects of electrofishing on early life stages appear to vary with species and size. Godfrey (1957) observed that newly hatched Atlantic salmon exhibited

increased swimming movement but not taxis in response

to a DC field. Maxfield et al. (1971) noted that 30-s exposures to homogeneous, low-frequency PDC fields of 1 Vp /

cm failed to induce narcosis in 5-cm YOY rainbow trout,

whereas exposures to similar fields at only 0.75 Vp /cm

were sufficient to induce narcosis in at least some 19-cm

yearlings. Among harmful effects, Lamarque (1990) noted

that mortality was common for larval zander but rare for

trout larvae; also, salmon parr did not suffer unduly in

fields that killed larger smolts. Lamarque (1990) suggested

that electric fields that are dangerous to adult fish are

probably dangerous to juveniles as well, but because

fish larvae and early juveniles are extremely fragile, mortality due to handling and the stress of capture might be

as great as that due to electrofishing fields. The occurrence and significance of physical electrofishing injuries

to fish larvae and early juveniles was not documented in

literature reviewed for this report.

Results — Summary of Results — Summary of

Survey Responses Survey Responses

A questionnaire to assess local observations and

recommendations with respect to electrofishing was distributed directly, or through endangered-species program

leaders, to fishery biologists with electrofishing experience in the Colorado River Basin and to fishery faculty

and graduate students at Colorado State University. Survey requests or questions, excluding parenthetical elaboration, were:

1. Please describe the nature and extent of your

electrofishing experience.

2. What environments and under what environmental conditions have you sampled with

electrofishing gear?

3. What species and size groups have you sampled

or monitored with electrofishing gear?

4. What electrofishing equipment and techniques

have you used?

5. Describe observations of adverse or injurious effects, especially with regard to endangered or

related species.

6. Based on your experience, what recommendations

would you offer for optimal electrofishing efficiency while minimizing injury to fish?

7. Please read the attached material abstracted from

my report, which is still in preparation, and relate

your response, thoughts, oversights, or criticisms

on the content.

Eleven written responses were received–two from

the lower basin, seven from the upper basin, and two

from university graduate students without Colorado River

Basin experience. Pertinent comments from discussions

with two additional upper basin biologists were also considered in the following summary of responses.

Experience

The level of experience represented by survey respondents was extensive, both within and outside of the Colorado River Basin. Most respondents had at least 6 years of

electrofishing experience and had served as crew leaders or

supervisors; one had over 20 years of electrofishing experience. At least four took the U.S. Fish and Wildlife Service

Fisheries Academy course on electrofishing. One taught a

course on electrofishing. At least two had been involved in

the development or modification of electrofishing gear.

Most respondents had electrofished in a variety of

habitats, from large rivers to small streams and from major

reservoirs to small lakes and ponds. Most electrofishing

was done during spring through fall, but a few respondents also had experience electrofishing during winter in

icy conditions. Temperatures during electrofishing were

usually between 10° C and 20° C but were sometimes as

low as 0° C or over 30° C. Most electrofishing, especially

in the Colorado River Basin, took place in water conductivities of 300 to 1,500 μS/cm, but some respondents had

experience with electrofishing in conductivities so low

(down to 10 μS/cm) that salt blocks had to be used to

increase conductivity or so high (2,000–5,000 μS/cm) that

the power supply would shut down. Turbidity ranged

from clear to very turbid, often moderately to highly turbid in the Colorado River Basin. Most respondents had

experience with day and night electrofishing.

According to respondents, boat and raft

electrofishing were typically used in Colorado River Basin

studies and monitoring programs, but most respondents

also had experience with wading systems (backpack, barge,

or bank equipment). At least one had experience with

fixed-position electrical grids and electric seines. Most

systems were commercial (Coffelt, Smith-Root, and

Georator). The Coffelt VVP-15 was mentioned most

frequently. PDCs were the most frequently used currents,

but a few respondents noted that when the situation

allowed (e.g., low to moderate conductivities), they

preferred to use DC. PDC parameters were seldom
 SNYDER 95

reported, but two respondents stated that they used

frequencies of 30, 40, or 60 Hz. One respondent never

bothered with pulse width and frequency controls

because the consensus seemed to be that these factors

were not very important. When reported, voltages and

currents for effective electrofishing, mostly in the

Colorado River Basin, were reported as 200 to 350 V and

4 to 8 A, but some reported use of up to 12 A. One

respondent noted that in very turbid waters, the system

had to be “cranked up” as high as possible to bring the

fish to the surface (a procedure since modified due to

concern for injury). Use of AC was reported by only one

respondent–many years ago in stream-wading situations.

Electrode use was highly variable. Spheres were favored

as anodes for boat and raft electrofishing, but dropper

rings and single or multiple cables were also used. Metal

boats, very long single or multiple cables, and spheres

were typically used as cathodes. Some respondents

changed electrode size or configuration according to the

specific waters being sampled (e.g., smaller spheres for

more conductive waters).

Observations of Harmful Effects on Fish

Some respondents noted that most electrofishing

efforts were inadequately documented. Not only were

notes on specific electrofishing gear, configuration, procedure, waveform, instrument settings, meter readings,

and physical measurements frequently neglected, but

also, records of electrofishing mortalities, injuries, and

other harmful effects. Most respondents had to rely on

their memories for recollections of such adverse effects.

This matter has been rectified in some recent Colorado

River Basin investigations (Valdez, personal communication). But even with comprehensive records, a few

respondents suggested that because of differing environmental conditions, equipment configurations

(especially type, number, and size of electrodes), and control box settings, it would be very difficult to correlate the

incidence of injuries with those factors. Actual measurements of field intensities (voltage gradients) for

determination of intensity distribution and field size

would have taken into account many of these variables,

but were overlooked in most Colorado River Basin investigations. Such data could be invaluable for comparing

electrofishing results and adjusting power output and

electrode size or configuration to maintain comparable

fields within and between sites. In lieu of in-situ measurements, field-intensity distribution can be approximated

by calculation if water conductivity, the size and shape of

the electrodes, and peak output voltage, amperage, or

power are known. Except for one investigation in which

fish were dissected for contaminant assessments

(Krueger, personal communication), no provision was

made for assessment of spinal or other internal injuries

caused by electrofishing. Most observations of injuries

noted below and included in Appendix B were based

solely on visible, external signs of injury (e.g., brands).

Of course, until recently, few researchers suspected the

occurrence of spinal injuries, and often even brands were

not considered serious.

Respondents reported that in the Colorado River

Basin they electrofished most species present in the

areas sampled but that they rarely (in some cases never)

experienced mortalities or injuries directly attributable

to electrofishing, except for occasional brands. Several respondents have electrofished a wide range of

size groups, from less than 4 cm to over 90 cm TL.

Among fishes that were injured or branded, salmonids

were found to be most susceptible (Appendix B). In

one case, where fish were filleted for contaminant

analysis, many captured rainbow trout and brown trout

were found to have broken spinal columns posterior to

the dorsal fin, whereas such damage was not observed

for other species (Krueger, personal communication;

Burdick, personal communication). Nearly all salmonids with damaged vertebrae also had externally obvious brands. Such brands were sometimes observed on

over half of the trout collected. Brands or other injuries and deaths observed by respondents were frequently assumed to be caused by direct contact with

anodes, especially cable anodes. No obvious signs of

injury were reported for channel catfish, but two respondents noted that the species was extremely susceptible to tetany and slow to recover.

Electrofishing injuries or mortalities have been rarely

reported for field-captured Colorado pikeminnow, humpback chub, or razorback sucker. Observations that have

been recorded include brands (probably resulting from

spinal injuries) in all three species, and at least one mortality and one occurrence of bleeding gills in Colorado

pikeminnow (Appendix B). As further evidence that these

endangered species do not appear to be seriously affected by electrofishing, some respondents noted that

many electrofished and tagged specimens have been recaptured, sometimes repeatedly over a period of several

years, and displayed no obvious aftereffects. Also, many

electrofished and radiotagged specimens were successfully tracked for extended periods. Valdez (personal communication) suggested that with regard to long-term

effects, physiological stress and damage to the nervous

system may be the greatest impacts on these fish, but

such effects would be difficult to assess.

One comparison of particular interest to many biologists who are concerned about spinal injuries, is whether

Coffelt’s new pulse-train current, CPS, has an advantage

over typically used constant-frequency PDCs (usually

generated via Coffelt’s VVP-15 and Smith-Root’s GPP
96 INFORMATION AND TECHNOLOGY REPORT--2003-0002

units) in reducing injuries while maintaining electrofishing

efficiency. Contrary to many reports (Meyer and Miller,

1991, unpublished manuscript, 1991; Wyoming Game and

Fish Department, 1991; Fredenberg, 1992, personal communication; Sharber et al., 1994), Trammell (personal communication) observed that there seemed to be

proportionately more brands among rainbow trout and

humpback chub collected with CPS in the Grand Canyon

than with constant-frequency VVP-15-generated PDCs

in the upper basin. However, Valdez (personal communication) noted that such comparisons are questionable

without both units being used similarly in the same waters at about the same time.

With regard to experiences outside the Colorado River

Basin, respondents submitted several notable observations. Gowan (personal communication) noted that among

salmonids, electrofished specimens seldom showed external signs of spinal injury upon initial capture, but spinal injuries were sometimes evidenced a year later in fish

that had stopped growing in the caudal region and became football-shaped. He also noted that the only significant electrofishing mortality he observed was among

sculpins (Cottidae) captured in shallow riffles with outputs of 300 V or greater. The gills of these fish flared

(probably in a state of tetany), and many fish died.

Pfeifer (personal communication) reported high mortalities among paddlefish electrofished with PDC in the

Yellowstone and Missouri Rivers. Upon necropsy, the

notochords of these fish were found to be completely

ruptured. Obviously, spinal injuries are not restricted to

fish with bony endoskeletons and vertebral centra. Pfeifer

noted that the rivers electrofished were very turbid and

suspected that many of the fish had made direct contact

with cable anodes or were exposed to excessively high

field intensities.

In Alaskan streams, Valdez (personal communication)

reported a high incidence of brands among all sizes of

Dolly Varden, pink salmon, and threespine stickleback

electrofished with AC. However, many of these fish were

recaptured a year or two later.

Two respondents used electrofishing to capture ripe

fish for culture. Valdez (personal communication) reported

taking a 9-kg female and several 0.9 to 2.2-kg male lake

trout with no apparent ill effects on the subsequently

released fish or their progeny. Egg survival was high.

Pfeifer (personal communication) reported similar use of

electrofishing to capture ripe walleye. He also observed

no detrimental external effects on the brood fish or the

percentage of eggs that hatched.

Many respondents suggested that handling of fish

during and after netting probably has a greater effect on

mortality and delayed recovery than the electric field.

Overcrowding and stagnant, poorly oxygenated, holding

water was recognized as a serious problem.

Respondents’ Recommendations for Minimizing

Harmful Effects

Approximately half the respondents suggested the

following measures for minimizing harmful effects on fish:

1. Use the lowest power output that still provides

for effective electrofishing (sufficiently large field for taxis

and narcosis). In the Upper Colorado River Basin, Tyus

(personal communication) suggested that amperage

should normally be no more than about 5 or 6 A and if red

shiner are being stunned, the amperage is too high. Gowan

(personal communication) recommended that fish be observed following capture to ensure that they recover equilibrium within 1 to 2 min; if not, power should be reduced.

Kinsolving (personal communication) suggested that the

critical measure with respect to fish injury is voltage gradient, not output voltage or amperage per se. A simple

home-built meter can be constructed (probe with appropriate voltmeter) and used to quantify or monitor field

intensity in different waters and to locate hot spots in the

field. Field intensity should be closely monitored in highly

conductive backwaters and flooded tributaries. Hawkins

(personal communication) noted that in spring, Colorado

pikeminnow often occupy such habitats, wherein they

are especially susceptible to electrofishing.

2. Use the least damaging current available, DC

whenever circumstances allow; do not use AC. However,

the occurrence of brands and extended tetany indicates

that harmful effects are still a problem, even when using

currents designed to be less harmful.

3. Use spherical electrodes and vary the number

and size of spheres according to water conductivity and

desired size and intensity of the field. However, Valdez

(personal communication) noted that while spherical electrodes are theoretically superior to cables, he had not

observed a significant difference in catch rate or the incidence of brands. Also, spherical electrodes limit the depth

from which fish are drawn; Valdez (personal communication) suggested that spherical anodes and cable cathodes appear to be the best combination. Tyus (personal

communication) recommended that anodes be kept high

in the water to draw fish to the surface, where they can be

easily netted.

4. Minimize exposure to the field and specimen handling–rapidly net fish before they get too close to the

anode, and quickly, but gently, place them in oxygenated

holding water. Tyus (personal communication) suggested

that the foot-switch should not be closed continuously

and that it should be released as soon as fish are observed near the anode. He also warned against overworking specific sites to maximize the numbers of fish captured.

Buntjer (personal communication) cautioned that netters

should not allow fish to remain in the net too long or
 SNYDER 97

repeatedly dip fish back into an active electric field. Valdez

(personal communication) noted that underwater lights

improve netting efficiency.

5. Change the holding water frequently to ensure

adequate dissolved oxygen and to avoid excessive temperatures on hot days; process the fish frequently to

reduce crowding.

Some respondents emphasized the need to use

trained personnel to properly operate the equipment under changing conditions and the best netters to quickly

spot and remove fish from the electric field. Tyus (personal communication) emphasized that electrofishing trips

should be scheduled to take advantage of conditions for

the most efficient capture of target species (e.g., spring,

when conductivity is relatively low and endangered species of fish are still in shallow, near-shore habitats).

Electrofishing should not be attempted under turbid or

windy conditions–the fish cannot be seen easily. Valdez

(personal communication) emphasized the need to adequately document electrofishing operations and observations of harmful effects; those that have done so in the

past have a valuable source of information. Analysis and

summarization of such information might be useful in resolving the question of electrofishing injury, at least for

the specific situations documented.

Conclusions Conclusions ConclusionsConclusions

Electrofishing, the use of electric fields in water to

capture or control fish, has been a valuable sampling technique in North America for over half a century, but it

involves a very dynamic, complex, and poorly misunderstood mix of physics, physiology, and behavior. To be

effective, the electric field generated around and between

electrodes in water must be sufficiently strong at appropriate distances from the electrodes to elicit the desired

responses by target fish. The size, shape, and nature of

that field are defined by the distribution of electrical intensity which is determined largely by the peak electrical

potential (voltage differential), type of current, and waveform generated between and around the electrodes; position, size, and shape of those electrodes; conductivity

of the water; conductivity of bounding and surrounded

media; and water-basin size and configuration.

What we know or believe about the responses of

fish to electric fields is the cumulative result of many

years of individual and often piece-meal research. In a

much more concerted effort, many of these responses

were intensively investigated and others revealed in the

1960’s at the Barritz Hydrobiological Station in France

(Blancheteau et al., 1961; Lamarque, 1963, 1967a, 1990;

Vibert, 1963, 1967b; Blancheteau, 1967). However, many

questions remained and the interpretation of some results was either difficult to understand or questionable.

In a more recent attempt to better understand and explain

the interaction between fish and electric fields,

electrofishing has been treated as a power-related phenomena. According to this “power-transfer theory for

electrofishing,” the relationship between electrical power

in water and in fish is a function of the ratio of conductivity of water to the effective conductivity of fish (Kolz and

Reynolds, 1989a; Kolz et al., 1998). Even more recently, it

has been suggested that the observed responses of

fishes to an electric field, including twitches, taxis, narcosis, and tetany, are essentially aspects of the same phases

of epilepsy (automatism, petit mal, and grand mal) that

are observed in humans and other animals subjected to

electroconvulsive therapy (Sharber et al., 1994, 1995;

Sharber and Black, 1999). Most of the currently accepted

or proposed concepts for explaining or better understanding the responses of fish to electric fields, and the mechanisms involved, need to be further explored, validated,

refined, and integrated to advance the science and technology of electrofishing. This might be accomplished best

through a well-coordinated, cooperative program for future electrofishing research.

Stress, injuries, and sometimes mortalities among

captured fish are unavoidable consequences of

electrofishing and most other collection techniques.

Among the more effective gear and techniques available

for collecting fish, biologists usually select those known

to be least harmful, but comparative data on harmful effects are often lacking or inconclusive.

In many cases, especially prior to the late 1980’s,

electrofishing had been considered not only the most

effective but also the least harmful means to capture fish,

particularly moderate to large-size specimens. Despite

occasional reports of substantial harm to fish, the relatively benign nature of electrofishing had been assumed

because generally fish recovered quickly and few, if any,

mortalities or external injuries were observed or reported.

Also, the most frequently noted external effects, brands,

were often dismissed by experienced electrofishers as

harmless, temporary effects rather than as indicators of

potentially serious spinal injuries or hemorrhages. But

since the late 1980’s, many investigators have shown that

assessment of electrofishing injuries based only on externally obvious criteria can be highly inadequate.

Sharber and Carothers (1988) X-rayed and necropsied

many large rainbow trout captured by electrofishing, found

substantial numbers of spinal injuries and associated

hemorrhages, and concluded that without such analysis,

most of these injuries would go undetected unless they

were especially severe. Especially severe spinal injuries

or muscular hemorrhages can be represented externally

by brands (particularly those that are in fact bruises),
98 INFORMATION AND TECHNOLOGY REPORT--2003-0002

bent backs, punctures, or abnormal swimming, but in most

fish even severe injuries are not externally obvious. When

electrofished specimens were similarly examined in

subsequent investigations by other biologists (e.g.,

Holmes et al., 1990; Meyer and Miller, 1991; Fredenberg,

1992; Newman, 1992; McMichael, 1993; Hollender and

Carline, 1994), they too documented in some species,

especially salmonids, significantly, and sometimes

dramatically, greater numbers of electrofishing injuries.

As a result, new research was, and continues to be, funded

to assess the extent of such injuries in specific

applications, longer-term impacts, causes, and

modifications to gear and techniques that might reduce

harmful effects. Based on these studies, some agencies,

institutions, and researchers have been reevaluating their

use of electrofishing and instituting policies or guidelines

to reduce the potential for injury. But we must better

understand the problem, the factors involved, and how

to minimize injuries.

Although verification through targeted research is

still needed, the immediate cause of most spinal injuries

and related hemorrhages appears to be strong myoclonic

jerks (perhaps epileptic seizures referred to as

automatisms) elicited by sudden changes in electrical

potential, as when current beyond some threshold of

intensity is switched on or off, pulsed, or alternated. As

might be expected if this is true, comparative

investigations generally have revealed that DC causes

the fewest spinal injuries and hemorrhages and that lowfrequency PDCs (<30 Hz, the lower the better) and at

least one complex PDC (CPS) with a low inter-pulse-train

frequency (15 Hz) cause substantially fewer spinal injuries

and hemorrhages than higher-frequency PDCs and AC.

Accordingly, these currents are recommended to minimize

potential injuries. However, very low-frequency PDCs

(e.g., 15 Hz) are generally considered less effective for

inducing taxis and capturing fish than higher-frequency

PDCs, perhaps because they, like DC, generally have

higher field-intensity thresholds for the desired

responses. More power might be needed to use them

effectively.

The threshold magnitude of change in field intensity

required to cause spinal injuries is probably the threshold for twitch, which in a heterogeneous field occurs in

the zone of perception (reactive detection), well outside

the zones of taxis, narcosis, and tetany, the effective portions of the electrofishing field. Accordingly, at least as

many fish injured in the zone of perception are likely to

escape the electric field as move into its effective zones

for capture. Limited evidence suggests that field intensities greater than the threshold for sporadic muscular convulsions might not increase the frequency or severity of

spinal injuries. If true, efforts to reduce the size of the

most intense portions of the field, particularly the zone of

tetany, might not have any impact on the incidence or

severity of spinal injuries. But such efforts would still be

beneficial in reducing potential for severe stress and

mortality due to excessive fatigue and asphyxiation. Although as yet untested, increased duration of exposure

under PDC would proportionally increase the number of

pulses to which fish are exposed and thereby likely increase the probability of spinal injury. Regardless of exposure time and also as yet untested, sudden muscular

convulsions, and therefore spinal injuries, are not likely

to occur while fish are in a state of narcosis (petit mal)

and probably not while in a state of full tetany (grand

mal), although they may occur during transition to or

between these states.

Except in very severe cases, electrofishing injuries

in fish heal and seldom result in immediate or delayed

mortality. Instead, most electrofishing mortalities appear

to result from asphyxiation due to extended tetany or

poor handling. However, electrofishing injuries may significantly reduce subsequent growth, at least until they

fully heal. When sufficiently severe, spinal injuries may

affect physical appearance or swimming ability. Still, even

for highly injury-susceptible species, such as the

salmoninae, significant effects at the population level are

unlikely except in the case of very small or very extensively and intensively sampled populations, as is sometimes the case for threatened and endangered species.

Electrofishing can also affect reproduction and early

life stages. In addition to or as a result of injuries, exposure of ripe fish to electrofishing fields can cause significant damage to, or premature expulsion of, gametes and

sometimes reduces viability of subsequently fertilized

eggs. Electrofishing over active spawning grounds can

also significantly affect survival of embryos on or in the

substrate if exposed during their more sensitive stages

(prior to acquisition of eye pigment). Exposure of recently

hatched larvae might not cause significant mortality but

can reduce growth rates for at least a few weeks. Field

intensity and duration of exposure appear to be the most

critical electrical factors affecting embryos and larvae.

In the Colorado River Basin, electrofishing has been

considered one of the most effective and least injurious

techniques available for capturing the larger juveniles

and adults of endangered and other large fishes. As

elsewhere, relatively few fish other than salmonids have

been reported to be killed or injured by electrofishing.

But again, these fish had seldom been X-rayed or

sacrificed for necropsy. Based on the few investigations

in which endangered or native cyprinids were examined

internally after exposure (adult Colorado pikeminnow and

roundtail chub captured in two field studies and large

juvenile Colorado pikeminnow, small juvenile bonytail,

and small juvenile humpback chub exposed in laboratory

experiments), neither spinal injuries nor other harmful
 SNYDER 99

effects (mortality, severe hemorrhages, or for one species,

subsequent short-term growth) appear to be a serious

problem for these species using current electrofishing

gear and techniques with DC or tested PDC waveforms

and frequencies. However, experiments with endangered

razorback sucker and their progeny suggest that at least

ripe adults may be quite susceptible to electrofishing

injuries and hemorrhages and that electrofishing them in

this condition, especially over active spawning grounds,

should be avoided. The survival and physical condition

of endangered and other native cypriniforms (including

razorback sucker) that had been electrofished in recapture

and radiotag investigations also suggest that

electrofishing injuries or mortality are probably not a

serious problem. Even so, the sensitivity of the matter

warrants a heightened awareness of the potential for

electrofishing injuries, a continuing effort to minimize any

harmful impacts by every practical means, and a readiness

to adjust, alter, or abandon electrofishing techniques if

and when potentially serious problems are encountered.

Other sampling gear or techniques may need to be

evaluated and adopted as appropriate.

Electrofishing is a valuable tool for fishery management and research, but when resultant injuries to fish are

a problem and cannot be adequately reduced, we must

abandon or severely limit its use and seek less harmful

alternatives. This is our ethical responsibility to the fish,

the populace we serve, and ourselves.

Responses to Specific Questions

The remaining conclusions of this investigation are

best provided as responses to specific questions, mostly

regarding endangered and other native species of the

Colorado River Basin. Prior to the original version of this

review, M. Yard (Grand Canyon Ecological Studies Aquatic

Coordination Team, Bureau of Reclamation, Flagstaff,

Arizona) assessed information needed by the National

Park Service and assembled a list of questions to be addressed by this and, if need be, subsequent investigations. The questions (edited and reordered as appropriate)

and answers follow:

1. Does electrofishing impact native species of fish

as severely as rainbow trout?

With the probable exception of native salmonids and

possibly ripe razorback sucker, evidence to date suggests

that native species are not as susceptible to electrofishing

injury as rainbow trout. But only one investigation

(Meismer, 1999) directly compared the extent of immediate mortality and injury between rainbow trout and a native species. In that study, adult rainbow trout and

similar-size subadult Colorado pikeminnow were exposed

to several commonly used or recommended currents at

field intensities corresponding to thresholds for typical

responses. Except for one treatment with Colorado

pikeminnow, the frequency of spinal injuries (all minor,

Class 1, except in one Colorado pikeminnow) was insignificantly low for both species. However, rainbow trout

experienced significant mortality (10 and 30%) when exposed to tetanizing intensities of 60-Hz, square-wave PDC

and significant incidences of branding regardless of treatment, whereas Colorado pikeminnow experienced no immediate mortalities, no brands, and very few muscular

hemorrhages regardless of treatment. Incidence of hemorrhages could not be assessed in the trout but, based

on the frequency of brands, were probably common and

at least moderate in severity.

Many Colorado pikeminnow, humpback chub, and

razorback sucker have been captured by boat or raft

electrofishing, some repeatedly in tagging studies. In over

two decades of field research and monitoring studies,

Colorado River Basin biologists reported very few incidents of immediate mortality, brands, or other externally

obvious injuries in these fish due to electrofishing. Still,

most of these fish were not X-rayed or necropsied for

detection of spinal injuries, and except for a higher incidence of brands, the same can be said for trout.

2. Do we know the effects of electrofishing on all

native fish species; if not, what fish would be most

representative of humpback chub anatomically and

physiologically?

No, but as discussed in this report and summarized

in Table 2 and Appendix B, we now have some response

or injury data for: adult roundtail chub, razorback sucker,

cutthroat trout, mountain whitefish, and mottled sculpin;

subadult Colorado pikeminnow (data on field-collected

adults not yet reported); juvenile humpback chub and

bonytail; and embryonic and larval razorback sucker. Electric fields can probably elicit somewhat similar responses

in most, if not all, fishes, but all else being the same, the

field-intensity thresholds and specific nature and degree

of those responses will vary with species, size, and condition of the fish. Likewise for susceptibility to mortality

and injury.

Roundtail chub, Gila chub, and bonytail are very close

relatives of the humpback chub and are similar to it in

morphology and physiology. Of these, roundtail chub is

most common (only species of these not considered threatened or endangered) and has served as a surrogate for

humpback chub in a field investigation of the injurious

effects of electrofishing by Cowdell and Valdez (1994).

For laboratory, raceway, or pond experiments, any of these

species should be suitable surrogates if enough reared

specimens of humpback chub are not available. However,

Ruppert and Muth (1997) reported that response thresholds were notably lower for yearling humpback chub than
100 INFORMATION AND TECHNOLOGY REPORT--2003-0002

similar-size (but younger) bonytail. On the other hand,

they also reported no spinal injuries for either species

and no significant differences between them in the frequency of muscular hemorrhages.

3. What exists in the literature related to physiological

responses and stress due to electrical stimulation?

Well over a hundred publications indexed by Snyder

and Johnson (1991) include information on these matters. See the “Literature Cited” section at the end of this

report and the bibliography by Miskimmin and Paul (1997a)

for more-recent publications. All responses to electric

fields, from reactive detection through tetany, are physiological. Even electrocution and the momentary but powerful convulsions believed to cause spinal and related

injuries are physiological phenomena.

Exposure to tetanizing field intensity results in respiratory failure and synaptic fatigue, but if not exposed too

long, fish usually recover normal breathing activity and

equilibrium within minutes after removal of or from the

field. However, excessive exposure to tetanizing currents

can result in very long recovery periods or death. Fish

may also cease respiratory movements under strong narcotizing field intensities, but remain relaxed and can survive longer exposures than under tetanizing currents.

Stress disrupts normal behavior and osmoregulatory

functions. All capture methods and handling are stressful. Stress caused by electrofishing is similar to stress

caused by hypoxia and intensive muscular activity. Reported changes in blood chemistry include increases in

adrenal hormones, lactic acid, and blood clotting agents,

which indicate overworked muscles and possibly traumatized tissues. Physiological recovery usually requires

6 to 24 h. However, some stresses, such as those related

to physical injury, can persist for weeks or even months.

4. What are the physiological and anatomical effects

of electrofishing on musculature, bone structure, blood,

and reproductive organs?

According to Sharber and Black (1999), exposure of

fish to an electric field of sufficient intensity overstimulates the central nervous system and results in epileptic

responses. The level of central nervous system

overstimulation, subsequent stimulation of muscles

through the motor nerves, or failure of such causes the

various behavioral responses observed when

electrofishing (e.g., taxis, narcosis, tetany). Under certain

conditions, particularly when potential of the field across

the fish changes suddenly with sufficient magnitude (as

when the current is switched on or off, pulsed, or alternated), some body muscles are stimulated to contract in

very powerful convulsions. These presumably petit mal

responses and possibly grand-mal responses of tetany

(a sustained series of very rapid convulsions sometimes

referred to as quivering or pseudo-swimming followed at

a higher field intensity by a continuous convulsion making the body very rigid) can result in trauma to vertebrae,

associated bones, muscle, and blood vessels. Vertebrae

and associated bones can be separated, compressed, fractured, splintered, or misaligned (Figs. 1, 16, 18, and 19).

Muscles can be bruised and torn, and blood vessels can

be ruptured or blocked (Figs. 1 and 17). In extreme cases,

such seizures may damage nerves and visceral organs.

These internal injuries are often not obvious without Xrays or necropsy. When present, external signs include

abnormal swimming behavior, bent backs (Figs. 3 and 4),

brands (Fig. 2), and bleeding at the vent, gills, or base of

the fins. Effects on physiological stress, including blood

chemistry, are discussed in response to the preceding

question.

Except when muscular convulsions are sufficiently

severe to damage gonads or injure and force premature

expulsion of nearly mature (ripe) gametes, general consensus is that there is probably no significant effect of

electrofishing on the development or function of gonads

or developing ova and sperm. However, specific data on

such effects are limited and based mostly on salmonid

broodstocks. Because fish are often targeted for sampling during the spawning season, the matter deserves

specific investigation in wild fish, especially endangered

species. Although there is some evidence to the contrary, electrofishing just prior to spawning might alter or

inhibit subsequent reproductive behavior or physiology.

5. Are there differences in impact related to the age

of the fish?

Yes. Early embryos have undeveloped neural and

muscular systems, early larvae of many fish have incomplete skeletons and sensory systems, and all early life

stages are substantially smaller than later juveniles and

adults. As a result, not only are specific electrogenic structures (nerves and muscles) affected by electric fields either lacking or different than in older fish, but also the

organisms as a whole are subject to much smaller potentials or voltage drops across the body. Taxis and narcosis

are obviously not possible in the earliest embryos, and

vertebral damage is not possible in recently hatched larvae of many species. Other effects, such as disruption of

embryonic development, premature hatching, and even

mortality at particularly sensitive stages, can occur.

Because age is reflected by size in juveniles and

young adults, there may be size-related, and therefore

age-related, differences in their susceptibility to

electrofishing injuries. Some biologists reported that injuries to juvenile and adult fish are more frequent among

larger specimens, whereas others found no consistent

differences between age or length groups.

If poor condition is characteristic of very old fish of

a particular species, these fish may differ from younger

cohorts of the same species in their sensitivity to electric
 SNYDER 101

fields and susceptibility to injury. This matter has not

been addressed in the literature.

6. Are there any differences related to water quality?

Yes. Water chemistry determines its conductivity and

affects physiology of fish, both of which influence the

field-intensity thresholds for various responses by the

fish. Also, very turbid waters make fish difficult to see

and net, thereby reducing electrofishing efficiency and

increasing the amount of time fish are exposed to the field

before being captured. This, in turn, increases the probability of deaths and, in PDC and AC, injuries.

7. Is there an impact from exposure time and

electrical frequencies?

Yes. Exposure time is especially critical in the zone of

tetany, at least with regard to stress, fatigue, and mortality. In full tetany, active breathing ceases and death or

damaging oxygen debt can quickly ensue because of sustained muscular tension. Breathing motions also cease in

full narcosis, but skeletal muscles are relaxed and oxygen

deficit accrues more slowly. Fish must be removed from

zones of high field intensity as soon as possible and

allowed to recover in well-oxygenated water. Based on

very limited data, at least the lethal effects of PDCs appear to be exacerbated by increases in pulse frequency.

With regard to spinal injuries, exposure time does

not appear to be a significant factor, at least for DC and

AC. This is logical for DC if, as has been suggested,

spinal injuries occur primarily with sudden changes in

field intensity as when the field is switched on and off or

pulsed. However, in PDC, the frequency of injuries generally increases with pulse frequency and might be expected to also increase with exposure time since both

conditions increase the number of pulses to which the

fish are exposed. The same might be expected for AC but

limited evidence with respect to exposure suggests not.

Pulse frequency also affects the strength of taxis and the

field-intensity thresholds for various responses. Optimal

frequencies for these responses vary with species.

8. What influences the incidence and extent of injury

to fish besides the shape of the electrical pulse, power

density (field intensity), and frequency of pulses; is one

parameter more influential than another?

Susceptibility to electrofishing injury varies with

species and, based on current data, is greatest for the

Salmoninae (trout, char, and salmon). Other biological

factors such as size and condition may also influence

susceptibility.

Among physical factors, fish position and orientation in a heterogeneous electric field determine the field

intensity to which the fish is subjected and, along with

fish size, the voltage drop across the fish’s body, and

thereby its response to the field. Accordingly, position

and orientation also determine whether a sudden change

in field intensity is powerful enough to elicit a seizure and

possibly whether the nature of the seizure is likely to

cause spinal injuries. Although subject to minimum voltage across the body, fish perpendicular to the lines of

current turn convulsively towards the anode and, based

on limited data, are more likely to incur spinal injuries

than fish parallel to the current.

Among electrical parameters other than field intensity, waveform, and frequency, the type of current has a

strong influence on electrofishing injuries. DC generally

causes less harm than AC or PDC. The possibility of detrimental voltage spikes when electric fields are switched

on and off has been ignored, in part because some researchers believe the duration of such spikes is too short

to have an effect, but this matter deserves specific investigation.

Field intensity is probably the most important electrical parameter affecting mortality, but above a certain

relatively low threshold, it does not appear to be important with respect to spinal injuries. The only exception is

that when it is high enough to induce a state of full narcosis or tetany, fish are probably not susceptible to the

sudden convulsions resulting in spinal injuries. In PDC,

pulse frequency seems to have the greatest effect on

spinal injuries. Absence of pulses, as in DC, usually results in the least number of injuries, but they still can

occur (up to 30% in some rare cases), probably when the

current is switched on or off or the fish are removed (netted) from the field. The role of waveform or pulse shape

remains unclear with contradictory results from various

investigations.

9. What is the threshold level of injury for each fish

species and can it be identified?

Defining injury broadly to cover most harmful effects

of an electrofishing field, injury is caused primarily by

two distinct conditions–tetany (grand mal) and the

convulsive seizures of petit mal. Excessive exposure to

currents at or above the threshold for tetany can result in

severe stress, fatigue, and cessation of respiratory

activity, possibly leading to death. Exposure to any

current intensity at or above the threshold for twitch can

elicit sudden and very powerful convulsions of the body

musculature. These seizures can result in injuries such as

compressed, broken, or misaligned spines; fractured or

broken vertebrae, bones, or joints; ruptured blood vessels;

and possibly a host of other traumatized tissues and

organs. Such physical injuries have long been attributed

to only the sustained contractions of tetany, but now it is

uncertain whether they even occur during tetany, and if

so, whether they differ in nature, frequency, or severity

from those generated in less intense portions of the field.

Outside the zone of tetany, some stress also occurs in the

zone of reactive detection, stress and fatigue in the zone

of taxis (especially in AC and PDC), and apnea in deep
102 INFORMATION AND TECHNOLOGY REPORT--2003-0002

narcosis, but aside from the stress and fatigue of physical

injuries, these effects are generally mild and recovery is

rapid. The threshold for spinal and related physical injuries

can be identified as that for twitch and the threshold for

severe stress, fatigue, and apnea as that for tetany.

However, these thresholds vary with species, size of fish,

water conductivity, type of current, and other factors.

10. Is power density the main parameter associated

with electrotaxis, narcosis, and injury, or are these

physiological responses independent of each other?

 Field intensity, whether defined in terms of voltage

gradient, current density (voltage gradient x water conductivity), or power density (voltage gradient x current

density) is the primary electrical factor eliciting taxis, narcosis, and tetany in fish. Field-intensity thresholds for

these responses tend to be lower in PDC and AC than

DC, and in PDC appear to be inversely related to pulse

frequency. Unlike taxis in DC, once the field-intensity

threshold is achieved for taxis towards the anode in PDC,

or oscillotaxis around the anode in AC, body flexures

(swimming motions) appear to be, at least in part, a function of pulse or cyclic frequency. Once in a state of tetany,

duration of exposure becomes the critical factor resulting

in injurious fatigue or death due to asphyxiation; long

exposure in a state of deep narcosis can also result in

asphyxiation.

As noted above, sudden changes in voltage differential (field intensity) that occur when switching current

or pulses on and off, if of sufficient magnitude relative to

orientation of the fish, appear to be the cause of myoclonic jerks resulting in spinal and related injuries. At or

above the relatively low field-intensity threshold for

twitch, these sudden, potentially injurious, convulsions

appear to be random and independent of other responses

except when fish are actually in a state of narcosis or

tetany. The incidence of these seizures and sometimes

resulting injuries, may or may not increase with the magnitude of change in field intensity, but in PDC, and probably AC, they do increase with pulse or cyclic frequency

and probably exposure time.

11. Does injury result from power densities that exceed those required for electrotaxis or that cause tetany?

Yes. Severe stress, fatigue, and hypoxia are caused

by excessive exposure to tetanizing (or deeply narcotizing) currents and can result in death or possibly longterm physiological injury. Also, fish may be electrocuted

and perhaps burned by extremely high field intensities or

contact with an electrode. However, high field intensities

are not prerequisite for spinal and related injuries caused

by convulsive seizures. These physical injuries, and associated stresses, can occur anywhere in the effective

field at or above the threshold for twitch in the zone of

reactive detection.

12. What is the relation between narcosis and

compression fractures?

There does not appear to be a specific relation between the two effects. Since fish in narcosis are fully

relaxed it is logical to assume that they are no longer

subject to the sudden convulsions believed to cause spinal or related injuries, including compression fractures.

This also applies to fish in a state of full tetany unless the

sustained muscular contractions characteristic of this

state are sufficiently strong to compress and fracture

vertebrae or burst blood vessels. The latter, historically

assumed possibility has yet to be experimentally tested.

It is likely that fish are still susceptible to convulsive

seizures during transition to and from either narcosis or

tetany.

13. Is there a relation between injury and type of

equipment used?

Yes. Adverse effects and mortality resulting from tetanizing currents can be reduced by minimizing the effective zone of tetany. This can be accomplished by enlarging

the electrodes, reducing power to the electrodes, or using DC with its higher threshold for tetany. Injuries resulting from momentary convulsions can be minimized

by using DC rather than PDC or AC, reducing pulse frequencies in PDC to no more than 30 Hz (preferably less),

or using Coffelt’s CPS (or similar pulse trains if proven no

more harmful).

If the rapidity with which pulses reach their peak

voltage is a factor, use of waveforms with gradual rather

than sharp rising pulses might reduce the incidence of

injury. However, data from waveform comparisons are inconsistent and half-sine waveforms appear to be just as

injurious as square, quarter-sine, and exponential waveforms. Voltage spikes often occur when current rises or

falls very sharply (e.g., when DC is switched on and off

and with each pulse in square-waveform and exponential

PDCs). If such voltage spikes are ever shown to be a

factor, they might be eliminated or minimized with electronic filters. But voltage spikes are not reported to be

characteristic of half-sine waveforms, and these waveforms appear no less injurious than others.

14. Is there an impact on eggs and developing

alevins?

Yes. Some investigators, particularly European

authorities, have concluded that exposure to electric fields

has no significant effect on developing eggs or larvae,

but most, especially recent, investigations suggest

otherwise. Most specific studies reviewed herein,

including a recent study of razorback sucker, documented

increased mortality among embryos exposed to electric

fields and that this additional mortality increases with

exposure time and field intensity. The effects appear similar

to the effects of mechanical shock with embryos being
 SNYDER 103

most susceptible to mortality prior to eyed-egg stages.

Exposure late in the embryonic period might induce

premature hatching. Nonfatal developmental effects, aside

from premature hatching, have not been adequately

investigated. There is little information on the harmful

effects of electric fields on fish larvae and early juveniles,

but published observations indicate that some species

are more sensitive than others (e.g., mortality more likely

for zander larvae than for trout larvae). Recently hatched

razorback sucker larvae exposed to the same electric fields

found to increase mortality among embryos suffered no

significant effect on survival or behavior but did

experience reduced growth rates during the subsequent

4 weeks. Obviously, it would be prudent to avoid

electrofishing over active spawning grounds, especially

for endangered species. Because larvae and early

juveniles may be more susceptible to predation while

recovering from exposure to electric fields, it might be

wise to also limit use of electrofishing in discrete nursery

habitats (e.g., backwaters and floodplains) heavily used

by endangered species.

Some investigators reported significantly reduced

production from ripe broodstock captured by

electrofishing, but others reported no effect. The only

investigation of such for an endangered species of the

Colorado River Basin, razorback sucker, resulted in significantly reduced survival for embryos from parents exposed to electrofishing fields.

15. Can experiments be designed to quantifiably

determine whether changes in an electrical system will

reduce or eliminate spinal injury?

Yes. Several such experiments have been reviewed

in this report, but much more can and should be done to

confirm or establish specific cause-and-effect relations.

The results of such investigations could lead to development of electrofishing gear and techniques that would

further minimize adverse effects.

16. Are there means in use, or documented in literature that would reduce or eliminate injury to fish?

Yes, reduce but not eliminate. Where practical and

when the power source is sufficient, use of well-filtered

or straight DC is the best way to minimize spinal injuries

and perhaps tetany-related effects. Researchers switching from PDC to DC may have to substantially increase

field intensity and otherwise modify their electrofishing

operation to maintain effectiveness. For example, some

biologists working from boats use mobile or throwable

anodes (Fredenberg, 1992) to take advantage of DC taxis.

However, special safety concerns arise with the use of

such techniques.

If DC is not practical and somewhat higher incidences

of injury are acceptable, spinal injuries can be reduced in

PDC by using the lowest effective pulse frequencies (preferably no more than 30 Hz) or Coffelt’s CPS (other complex PDCs might be just as effective but have not been

adequately tested). Unfortunately, very low pulse frequencies (e.g., 15 Hz) may produce insufficient taxis for effective electrofishing. Evidence regarding the relative

harmfulness of different PDC waveforms (e.g., square vs.

quarter-sine or exponential) is too limited and inconsistent for recommendations at this time.

There is some evidence that AC, especially 3-phase

AC, might not be as bad as its reputation and that it is

perhaps no worse than higher-frequency PDCs with regard to spinal injuries. However, until proven otherwise,

AC should be avoided, especially in work with endangered fishes. Because comparative information on the

effects of AC and PDC on spinal injuries is very limited,

AC should be included in future research to evaluate

harmful effects. When taxis to the electrode is not critical

and if its harmful effects can be minimized (or accepted as

when collected specimens are to be immediately killed or

preserved), AC might still be a useful current.

With any type of current, tetany-related stress, fatigue, injuries, and mortalities can be minimized by reducing the zone of tetany immediately around the electrodes,

and all harmful effects can be minimized by limiting the

range of the zone of perception and removing fish from

the effective portion of field as soon as possible. This

can be accomplished by: (1) prudent selection of electrode size, shape, and configuration for the waters being

sampled; (2) using the minimum power to those electrodes

needed for effective electrofishing; (3) optimizing technique for capture and subsequent handling; and (4) only

electrofishing when and where it can be done safely and

effectively.

Within limits imposed by water conductivity and

generator capacity, use of electrodes (or electrode arrays)

with the largest practical surface area for each situation

will minimize electrode resistance and the high-intensity

zones of tetany around them. Generally, small-diameter

cables should be avoided. Local hot spots of very high

field intensity around the electrodes can be eliminated or

minimized by selecting electrodes without sharp corners

or edges.

With any electrode configuration, reductions in power

output will reduce the zone of tetany, but it will also reduce

overall field intensity and thereby the size of other

response zones and possibly the effective range of the

field. Electrode configuration and power output should

be balanced such that the zone of tetany is minimized, the

zone of narcosis does not extend beyond the reach of

netters, and the zone of taxis is sufficiently large for

effective electrofishing. The larger the electric field

potentially perceived by fish, the greater will be the number
104 INFORMATION AND TECHNOLOGY REPORT--2003-0002

of fish likely to encounter it and be injured by it. Power

output beyond that needed for effective electrofishing

must be avoided. Methods for determining minimum

effective power are described by Kolz et al. (1998).

Sampling techniques should minimize potential for

exposure to tetanizing intensities and contact with the

electrodes and facilitate rapid removal of fish from the

electric field. Restricting use of electrofishing to near

optimal conditions (e.g., relatively clear and calm or

smooth flowing waters) will enhance the ability of netters

to quickly spot and remove fish from the water. Holding

facilities should be optimized to speed recovery and minimize further stress (e.g., frequently replaced water, oxygenation, avoidance of excessive crowding). Processing

techniques should minimize handling and return recovered fish to the water as soon as possible.

17. What types of research identify the lower limits

(thresholds) of field strength and pulse frequency for

efficient electrofishing (good catch per unit effort)?

Controlled pond or field experiments. However, laboratory studies using homogeneous fields can simplify

experimental design for these experiments by first identifying field-intensity thresholds for target species and size

groups over a range of temperatures and conductivities

for the currents to be tested (including PDCs covering a

range of pulse frequencies). With this data and knowledge of conductivity and temperature conditions in the

waters to be sampled, a range of potentially good

electrofishing fields can be calculated and tested. Of

course, the calculated electrofishing fields should be verified by actually mapping or spot checking field intensities before proceeding with experiments.

18. Are there threshold levels related to injury, and

do these vary with species, sex, size, length, mass, and so

forth?

Yes and probably. If muscular convulsions are the

cause (or principal cause) of spinal and related injuries

and the twitches observed well below field-intensity

thresholds for taxis are such seizures, then the relatively

low thresholds for twitches are also the thresholds for

electrofishing (or electrical-field) injuries. Indeed, spinal

injuries have been reported for Colorado pikeminnow and

rainbow trout exposed to various currents at field-intensity thresholds for twitch. Similarly, field-intensity thresholds for tetany are the thresholds for the adverse effects

of tetany, including death if sustained long enough.

Thresholds for tetany under specific environmental and

electrical conditions have been determined for many species, and lethal exposure times at tetanizing intensities

have been approximated for a few species within certain

size ranges. These response thresholds appear to vary at

least somewhat with species, size (length or mass), and

condition of the fish.

19. How comparable are previous studies when most

researchers do not have the ability to use an oscilloscope

to accurately determine field strength?

Not very. Without an adequate set of in-water electric-field measurements (either with an oscilloscope or

peak-voltage meter), comparisons between studies, trips,

or even sites within a trip can only be made on faith that

the electrofishing controls and meters remained accurately

calibrated and equipment was operating properly. Even

when equipment is known to function properly, few researchers, especially in field investigations, record sufficient information to approximate field size and intensity.

Without a reasonable approximation of field intensity and

size, and knowledge of the specific waveform, frequency,

and duty cycle utilized, results can neither be related to

field and system (circuit) parameters nor properly compared with results from other studies or even different

habitats within the same study. Failure to report whether

output or field intensities are peak or mean (rms in AC)

values and to recognize the difference between the two

has confounded the results of many investigations. Electric fields with similar mean intensities can have substantially different peak intensities, and it is the peak field

intensity that is believed to be biologically significant.

20. What studies have been conducted to assess delayed mortality resulting from electrofishing injury; how

long have most fish been observed after exposure to an

electric field?

Several studies held electrofished specimens for

specified periods to assess delayed mortality (see above

section on “Long-term Survival and Growth”). Monitoring periods for most of these studies ranged from a day

to several weeks, but some have spanned several months

to a year. Except when fish were seriously injured or fatigued, most of these studies reported little long-term

delayed mortality attributable to electrical-field injuries.

Some fish and game agencies routinely obtain broodstock

by electrofishing and sometimes use electric fields in other

hatchery operations (e.g., to anesthetize fish). Delayed

mortality has not been reported to be a significant problem in these situations.

21. What species of fish have been used in

electrofishing experiments; have any cyprinids been

used other than grass carp and goldfish?

Many species, including marine fishes and many

cyprinids, have been used in electrofishing experiments

or field studies to assess responses and adverse impacts

(see Appendix B and index to bibliography by Snyder

and Johnson, 1991). However, in most cases, the

objectives, methodologies, and conditions of these

investigations differed such that the results of these

studies are seldom directly comparable. Trout, particularly

rainbow and brown trout, have been used most frequently.
 SNYDER 105

Recent experiments have targeted response thresholds

of and adverse effects on endangered species of the

Colorado River Basin, specifically certain life stages of

reared Colorado pikeminnow (Meismer, 1999), bonytail

and humpback chub (Ruppert, 1996; Ruppert and Muth,

1997), and razorback sucker (Ruppert and Muth, 1995;

Ruppert, 1996; Muth and Ruppert, 1996, 1997). Also wild

specimens of another endemic native, the roundtail chub,

were studied to assess spinal and related injuries (Cowdell

and Valdez, 1994); likewise for wild Colorado pikeminnow

that were field X-rayed and returned to the water in

another investigation (Hawkins, personal

communication). Other species represented in the

Colorado River Basin that have been used in experiments

or observations on the effects of electrofishing include:

cutthroat trout, mountain whitefish, northern pike,

common carp, goldfish, white sucker, channel catfish,

flathead catfish, bluegill, green sunfish, largemouth bass,

yellow perch, walleye, and mottled sculpin.

22. Does injury occur at the onset of electronarcosis or tetany, relative to body position in the field, or as

the fish enters the electric field?

Spinal and related injuries resulting from convulsive

seizures can occur anywhere in the field at or above the

relatively low field-intensity threshold for twitch in the

zone of reactive detection. If fish do not actually detect

and respond in other ways to the field at still lower field

intensities, then electrofishing injuries may occur even

as fish enter the outermost reaches of the perceivable

field. The convulsive seizures sometimes resulting in

injuries are believed to occur primarily when fish are

subjected to sudden changes in potential at or above

that threshold, as when current or individual pulses are

switched on or off or fish are quickly removed from or

placed in the field. Whether these myoclonic jerks and

potentially resulting injuries also randomly occur under

conditions of constant current (DC) is not known, but if

so, their frequency is normally much less than in PDC.

Fish are probably susceptible to these sudden muscular

convulsions and injuries during transition to and from

narcosis and full tetany but probably not while they are

in those states. Once fish are in a state of tetany, they are

subject to severe stress, fatigue, and hypoxia depending

on the magnitude of field intensity above the threshold

for tetany and exposure time. Whether the sustained

muscular tension of tetany can be strong enough to also

cause spinal compressions, fractured vertebrae, or burst

blood vessels has not been documented. The magnitude

of voltage differential actually experienced by fish (across

their bodies) would certainly vary with their position and

orientation in the field. There is some evidence that upon

a sudden change in voltage differential, fish oriented

perpendicular to the lines of current suddenly turn much

more sharply toward the anode and are more likely to

suffer spinal injuries than fish oriented more parallel to

the lines of current.

23. Is injury a relation of size, mass, length, and

cross-sectional width, or is it species specific?

Based on field experiments in Alaska and Montana

and controlled experiments elsewhere, there appear to be

substantial differences in susceptibility of various species to spinal injuries caused by electrofishing. Size (at

least length and width) affects the voltage differential a

fish actually experiences (i.e., head-to-tail or across-body

voltage) at any particular point and orientation in an electrical field, and therefore, the thresholds for various responses. Beyond the threshold level for electrically

induced injuries (presumably that for twitch), the effect

of size is uncertain. However, recent field studies on

electrofishing injury indicate no significant size-related

difference in injury frequency or severity among

electrofished rainbow trout and brown trout between 20

and 58 cm TL (injuries were also observed among fish

less than 20 cm TL, but data were insufficient for analysis

of size-related differences). The relation between size and

mortality is even less clear, but in at least one controlled

experiment, size was not found to be a critical factor.

Future Research

Since the late 1980’s, research on electrofishing and

its injurious effects has expanded dramatically. Biologists

have repeatedly confirmed the potential for electrofishing

induced spinal injuries and hemorrhages and begun to

explore the specific nature and causes thereof, as well as

the relative susceptibility of different species. Fishery

managers and biologists have begun to recognize such

injuries as a potential problem and address it in their

policy and practices. Manufacturers have developed new

complex PDCs specifically to reduce the risk of such injuries. And new hypotheses have been advanced regarding “power transfer” to fish and the epileptic nature of

their responses to electric fields. But much remains to be

done.

Major technological advances to assure that the

potentially harmful effects of electrofishing are minimized,

while maintaining or improving its efficiency, will probably

depend on a better understanding of the mechanisms

involved, especially those resulting in injury. Before

proceeding with intensive experimentation to this end, a

thorough review of what is already known regarding the

effects of electric currents on humans and other animals

might be enlightening with respect to effects on fish and

help focus future research. Sharber and Black (1999)

suggested that the principal responses of fish to

electricity are phases of epilepsy (Bozeman paradigm)
106 INFORMATION AND TECHNOLOGY REPORT--2003-0002

and essentially the same for all vertebrates. However, even

the mechanisms involved in producing epileptic

responses in humans and other animals are not fully

understood. Also, discrepancies between the Bozeman

paradigm and the previously accepted Biarritz paradigm

must be explored and resolved. Perhaps it is time for a

concerted, well-funded, national or international effort to

better understand the responses of fishes to electric fields,

document the specific mechanisms resulting in injury, and

develop innovative gear, currents, and techniques to make

effective electrofishing safer for man, fish, and other

aquatic organisms.

In the meantime, work must continue on building a

database of experimentally derived response thresholds

and associated susceptibilities to injury for various species and size groups of fishes. Two major projects to this

end are currently underway by Reynolds (personal communication; Standardized evaluation of electrofishing

injury among North American freshwater sport fishes)

and Miranda and others (Miranda, personal communication; Effects of electrofishing configuration on catch efficiency and injury rates of warmwater fishes). Such

experiments should cover a variety of currents including

DC, AC, and a range of typically used, currently recommended, and newly developed PDCs.

To supplement data from controlled experiments,

standard practice in electrofishing, and especially research

on electrofishing techniques and effects, should include

observation and documentation of at least obvious injuries, abnormal behavior, and mortalities. When possible

and consistent with research goals, biologists are encouraged to examine fish for internal injuries and monitor

them for delayed mortality as well as for long-term behavioral and physiological effects. To better facilitate comparison and interpretation of results, biologists also are

encouraged to more fully describe their electrode systems and document the physical and electrical parameters of their operations and experiments. Data should

include water conductivity and temperature, output or,

preferably, in-water measurements of field intensity, and

if possible for PDCs, verification of waveform shape and

pulse width and frequency. Output and field intensity

values must be specified as either peak or mean values.

Recommendations Recommendations

Interim Policy to Minimize

Electrofishing Injury

The superintendent of Grand Canyon National Park,

J.H. Davis, suggested in a 12 July 1990 memorandum to

the GCES project manager that electrofishing for

humpback chub be kept to a minimum and conducted in

such a way as to minimize possible stress and injury. In

the earlier version of this review (Snyder, 1992a), I

suggested that this policy was warranted and should be

extended on an interim basis to all endangered and native

species of the Colorado River Basin until the harmful

effects on those species are adequately documented and

understood to justify changes. Since that review, the

results of a few new experiments have provided limited

evidence that endangered Colorado pikeminnow, native

roundtail chub, and by inference, endangered humpback

chub and bonytail are less susceptible to electrofishing

injuries and associated hemorrhages than the Salmoninae

(trout, salmon, and char). Although additional experiments

and observations are needed to substantiate this

conclusion, especially with respect to humpback chub

and bonytail, evidence now appears to be sufficient (at

least for the currents and PDC frequencies tested–DC,

CPS, and 15- and 60-Hz PDC for Colorado pikeminnow

and 40-Hz PDC for roundtail chub) to cautiously relax the

minimal use policy and allow careful use of electrofishing

for most monitoring and research efforts likely to

contribute to recovery of these species.

In contrast to the situation for endangered cyprinids

of the Colorado River Basin, policy minimizing use of

electrofishing for the endangered razorback sucker remains warranted pending results of further experimentation. Results of the only experiment to date on adult

razorback sucker suggest that at least reproductively ripe

specimens may be quite susceptible to electrofishing injury, especially when using 60-Hz PDC.

Policy minimizing use of electrofishing is also

warranted for rare, threatened, or endangered salmonids

of the Colorado River Basin. The generally greater

susceptibility of Salmoninae to electrofishing injury is

well documented.

Based on this updated review, the following measures

are recommended to minimize significant harmful effects

of electrofishing:

1.

2.

1. Unless or until there is adequate evidence to the

contrary, assume that available electrofishing techniques

can cause enough injury to targeted or incidental species

to be a potentially significant concern.

2. For species in which electrofishing injury is or 2.

might be a serious concern, especially if the fish are threatened, endangered, or otherwise of special concern,

minimize use of electrofishing. When practical and effective, consider and use means for obtaining needed data

without physical collection of fish (e.g., direct observation, cameras, scuba, sonic techniques) or by collecting

fish with alternative gear and techniques likely to be

less harmful.
 SNYDER 107

2.1.

2.2.

2.2.

2.3.

2.3.

2.4.

2.4.

3.

3.1.

3.1.

. Exceptions to this policy would include cases .

in which use of an alternative to electrofishing would

jeopardize critical comparisons with past data, or when,

during trial of or transition to an alternative, simultaneous

use of both collection techniques is necessary to determine

an acceptable correlation or data-conversion factor.

2.2. A review of literature comparing the

effectiveness and harmfulness of electrofishing with other

collection techniques would be useful. However, most

published comparisons with alternative collection gear

and techniques cover only sampling efficiency and

seldom mention harmful effects. The bibliography by

Snyder and Johnson (1991) lists about 80 such references.

More-recent references are included in the updated

bibliography by Miskimmin and Paul (1997a); a few are

included in the “Literature Cited” section of this review.

2.3. Judgments regarding the injurious effects of

some alternative gear and techniques may have to rely

on the experiences of project biologists and outside

contacts. Unlike many electrofishing injuries, injuries

caused by most alternative gear for physical collection

are more likely to be external and therefore more readily

observed. However, this is an assumption and should be

tested via X-ray and necropsy as it has been to assess

internal injuries in electrofished specimens. Some

alternative gear and techniques (e.g., entanglement nets)

might be more stressful and cause greater mortality than

electrofishing.

2.4. Used carelessly or improperly, any collection

gear and technique can be harmful to fish, other aquatic

organisms, or their habitat. Alternatives to electrofishing

must also be used in such a way as to minimize significant harmful effects.

3. Regardless of target species, if electrofishing is 3.

the least harmful of practical and effective techniques for

obtaining needed data or specimens, it should always be

conducted in such a way and with such currents as to

minimize potential for stress and injury as much as

possible while maintaining sufficient effectiveness. In

most cases, biologists will have to sacrifice use of currents

and field intensities providing the greatest catch rates

per unit time (e.g., high-field intensity using PDC at 60 Hz

or greater). Even when susceptibility of the target species

to electrofishing injuries is low, such may not be the case

for other fishes that will also be subjected to the electric

fields.

3.1. Exceptions to this policy include necessary

investigations to assess susceptibility of a species to

electrofishing injury or test gear, currents, or procedures

to minimize adverse effects, and cases in which

electrofishing can be used as a humane technique for

reducing or eliminating populations of undesirable fish

without significant harm to non-target species.

3.2.

3.2. Based on latest information, update

electrofishing equipment and procedures, including specimen handling, to ensure the least harm to captured fish.

3.2.1. Use the least harmful current available

for effective capture of target fish.

3.2.1.1. Where practical, use DC.

3.2.1.1.1. Until strongly rippled DC is comparatively evaluated for harmful effects, DC produced

from an AC source should be well filtered to make it relatively smooth.

3.2.1.1.2. Because of significantly higher fieldintensity thresholds for desired responses, use of DC

may require either a more powerful generator or acceptance of a smaller effective field.

3.2.1.1.2.1. Some of this limitation might be

overcome by altering the electrofishing technique to take

advantage of DC’s good anodic taxis.

3.2.1.1.2.2. Experimental mobile or throwable

anode techniques take advantage of anodic taxis and are

reported to be effective (Nehring, 1991; Fredenberg, 1992),

but they cannot be recommended unless specific safety

procedures are followed (U.S. Fish and Wildlife Service

policy is to not use a movable anode with a metal-hulled

boat–Temple, personal communication).

3.2.1.2. If DC is not practical, use a lowfrequency PDC (preferably 30 Hz or less, the lower the

better), CPS, or other complex PDC proven as effective in

minimizing spinal injuries. Kolz et al. (1998) provide a

simple step-wise field procedure for determining the

lowest combination of PDC frequency, pulse duration

(width), and field intensity that will effectively catch fish.

However, the impact of pulse duration on injury rates is

uncertain with limited evidence that shorter pulse

durations may be more harmful; the test procedure should

be modified accordingly.

3.2.1.3. Whether warranted or not, AC is recognized by many authorities as the most harmful type of

current used in electrofishing. Until proven otherwise, it

should be avoided for most purposes.

3.2.2. Operate electrofishing systems at the

lowest effective power setting with the largest practical

electrodes to minimize or eliminate the zone of tetany

around the electrodes.

3.2.2.1. Spherical, circular, or dropper array

anodes are generally recommended rather than cables

(especially single or paired, small-diameter cables).

3.2.2.2. Equipment for measuring

conductivity and field intensity (voltage gradients) in

the water should be available on each electrofishing trip

to monitor equipment operation and adjust settings and

electrodes for the desired size and intensity of the field.

However, if the available electrode systems have been

mapped for a specific output voltage and water

conductivity, adjustments for differences in water
108 INFORMATION AND TECHNOLOGY REPORT--2003-0002

conductivity and the desired size and intensity of the

anodic field can be calculated or graphed and a fieldintensity meter may not be necessary except for

recommended verification.

3.2.2.2.1. For in-water measures of field intensity, portable, field-durable oscilloscopes are preferred

because they also can be used to monitor output waveforms and pulse duration, but commercial field-strength

meters or similar home-built units based on voltmeters

should be adequate if they accommodate the specific

waveforms used.

3.2.2.2.2. Field-intensity measurements

should be based on peak voltages. If the meters used can

only measure average voltages, then pulse frequency,

width, and shape can be used to calculate peak voltages.

3.2.2.3. Electrical output (voltage, amperage,

or power) to the electrodes and electrode selection should

be based on (standardized to) predefined field sizes and

intensities that will maximize the range for taxis for the

target species and size group, while minimizing the zone

of tetany and limiting narcosis to a zone within easy reach

of netters. Kolz and et al. (1998) provide procedures for

standardizing applied power over a range of water conductivities.

3.2.3. Adjust the electrofishing technique in

such a way as to net and remove fish from the electric

field as soon as possible.

3.2.3.1. Select and position anodes such that

fish are brought as near to the surface and as close to the

netters as possible before narcosis. Maneuver the boat

with the current in such a way as to improve netter access to the fish.

3.2.3.2. Position netters, and lighting at night,

such that fish are more easily observed and captured;

use polarizing glasses and other aids to minimize glare

and reflections when electrofishing during daylight.

3.2.3.3. Avoid electrofishing when and where

waters are rough, too fast for effective netting, or excessively turbid.

3.2.4. Optimize fish handling and holding facilities for fast recovery and least possible stress.

3.2.4.1. Fresh, well-oxygenated water must

be provided, with a temperature not significantly warmer

than that from which the fish were removed. Consider

installation of a wire-mesh, Faraday-shield live tank

through the bottom of electrofishing rafts or boats not

used as cathodes (Sharber and Carothers, 1987). Isaak

and Hubert (1997) described a live bucket to minimize

holding injury and mortality when electrofishing small

streams.

3.2.4.2. Avoid overcrowding captured

specimens.

3.2.4.3. Some researchers suggest use of an

anesthetic, such as MS-222, to keep fish calm while they

recover or are processed (including X-rays). However,

care must be taken to ensure that the anesthetic does not

interfere with recovery. It might be wise to use anesthetic

only in a second container for fish that have recovered

equilibrium and normal behavior. Some anesthetics, including MS-222, can only be used in accord with U.S.

Food and Drug Administration regulations.

3.2.4.4. Handle fish as gently and as little as

possible. However, if a fish is very slow to recover breathing motions, it may be necessary to force fresh water

over the gills with a hose or tube inserted in the mouth or

gill cavity or by manually moving the fish back and forth

in water.

3.3.

3.3. Ensure that electrofishing equipment is well

maintained and in prime operating condition and that

personnel are adequately trained in its use and emergency

procedures. Properly used equipment and attention to

safety should minimize injury to fish and crew.

3.3.1. Governmental or other comprehensive

and up-to-date guidelines for safe and proper use of

electrofishing gear and techniques should be adopted

and followed closely (e.g., guidelines by Goodchild, 1986,

1991; U.S. Fish and Wildlife Service, 1992; reviews by

Hickley and Millwood, 1990; Goodchild, 1990; Lazauski

and Malvestuto, 1990).

3.3.2. Because electrofishing systems are

subject to extreme conditions, an oscilloscope (or other

appropriate diagnostic equipment) should be used to

check components for proper operation and calibration

at least before, if not periodically during, each

electrofishing trip.

3.3.3. Team leaders should be properly trained

and certified in electrofishing theory and practice. An

appropriate course and certification program is available

through the U.S. Fish and Wildlife Service National Conservation Training Center (Branch of Aquatic Resources

Training) in Shepherdstown, West Virginia (Kolz et al.,

1998). The course is offered in a classroom setting at

various times and locations throughout the country and

as a correspondence course (internet web site—https://

otis.fws.gov). Similar courses or related text material may

be offered by other governmental agencies (e.g., Meyer

and Miller, 1995), universities, or manufacturers.

3.3.4. Other electrofishing team members

should be trained, if not certified, in the proper use of

electrofishing gear and techniques for the specific sampling program (perhaps by the team leader with a refresher

each season).

3.3.5. At least two, if not all, team members

should be prepared to handle medical emergencies
 SNYDER 109

through advanced planning for each trip (procedures and

means to get help or reach medical facilities) and certified

training in first aid and CPR (cardio-pulmonary resuscitation).

3.3.6. Periodic re-certification for

electrofishing, first aid, and CPR should be required, perhaps every 5 years, to refresh and update knowledge

with latest information.

3.4.

3.4. Institute standardized procedures for

documenting each electrofishing event and observations

of adverse effects. This information is necessary to

compare results between sites and studies, evaluate the

conditions under which harmful effects continue to occur,

and refine techniques to further reduce harmful effects.

3.4.1. Record output parameters (voltage and

amperage, current type and waveform characteristics),

description and placement of electrodes, water conductivity and temperature, and field intensity (voltage gradient at specified distances from the anode). It is especially

important to note whether output and field intensity values are peak or mean (rms in AC) and whether water conductivity is ambient or standardized to 25° C.

3.4.2. Look for and detail all occurrences of

injuries or abnormal behavior among individual specimens and associate with other recorded specimen-specific data or tag number; include at least species and length

if not otherwise recorded. If distances of captures from

the anode can be estimated, note that information as well.

Dead fish should be frozen or otherwise preserved for

subsequent examination.

Further Research on Electrofishing

Injuries and Responses

The following are revisions of research recommendations originally outlined in Snyder (1992a) for continuation of Phase II of the Bureau of Reclamation’s

three-phase plan for addressing concern over the potential for electrofishing injuries among endangered and other

native fishes in the Colorado River Basin. Phase II consists of controlled laboratory and field experiments to

answer questions unresolved by past research. As listed

in the Preface to this report, several investigations have

been conducted since 1992 under Phase II, but much more

remains to be learned and understood. If the potential for

electrofishing injuries is significant (as it may be for razorback sucker and for the other endangered species

when using PDC frequencies greater than already tested)

and changes in current equipment and techniques are

recommended to significantly reduce that potential, Phase

III would test those recommendations in practical field

operations. These research recommendations target fish

and concerns of biologists and managers in the Colorado

River Basin but can be modified to address similar concerns elsewhere.

Some of the suggested research parallels studies previously conducted for other species, particularly rainbow trout and brown trout. Accordingly, when possible

for comparative purposes, rainbow trout should be treated

as one of the test species.

The first question is whether the species of concern

are likely to be significantly injured by currently used

electrofishing gear and techniques under all environmental conditions likely to be sampled. If not, the matter of

electrofishing injury to endangered fishes in the Colorado River Basin becomes a non-problem for those gear,

techniques, and environments, and further research either becomes unnecessary for recovery concerns or can

be redirected toward other basin fishes that may be significantly affected. Because endangered species recovery requires consideration of the entire ecosystem,

adverse impacts of electrofishing on other native species

are also a concern. Even if electrofishing injuries are likely

to be significant, that incidence of injury might be acceptable if there are no better alternatives for obtaining

information critical to recovery efforts. Unless already

known to be less or no more harmful than currently used

electrofishing gear and techniques, or sampling conditions, each prospective change in electrofishing gear, technique, or environment sampled should be tested for

harmfulness before the change is adopted.

Recommendations for continuing Phase-II research are:

1. 

1.1.

1.1.

1. Field and pond studies. Continue determining

whether and to what extent electrofishing gear and procedures used in the Colorado River Basin cause physical

injury to endangered or other native fishes. The three

suggested approaches to this end (1.1, 1.2, 1.3) are intended to provide complementary information, but if necessary they could be treated as alternatives.

1.1. Appended investigations. As part of

specimen processing for endangered and other selected

species in ongoing field investigations or monitoring

programs utilizing electrofishing as a capture technique,

include thorough external examination for signs of injury,

X-ray analysis for vertebral and other skeletal damage

when possible, and necropsy of any mortalities or

statistically useful subsets of non-endangered species

sacrificed for assessment of internal injuries. This has

been done for Colorado pikeminnow, but the results have

not yet been reported (Hawkins, personal

communication). This approach assumes that the X-ray

exposures will cause no significant harm to wild fish or

their offspring (an assumption that still needs to be
110 INFORMATION AND TECHNOLOGY REPORT--2003-0002

experimentally verified). Regardless of use of this

approach for assessment of electrofishing injuries,

thorough external examination of all specimens and

documentation of observed signs of injury should

become standard procedure for all investigations using

electrofishing (or other) gear and techniques.

1.1.1. For thorough external examination,

document and describe in detail all external signs of physical injury including brands, bent backs, punctures, and

bleeding at the vent, gills, fin bases, or elsewhere.

1.1.2. For X-ray analysis:

1.1.2.1. Supplement the usual field crews with a

separate team of properly trained and equipped biologists

to expedite field radiography of fish using a portable X-ray

machine and ensure the safety of all personnel.

1.1.2.2. Label each X-rayed fish such that

radiographs can be associated with recorded field and

analysis data for that fish.

1.1.2.3. Document the presence and severity

of spinal injuries using to Reynolds’ (1996) criteria

(Table 3). Be aware that minor vertebral fractures may be

difficult to assess in X-rays of small fish.

1.1.3. For necropsy of electrofishing mortalities (including specimens that are not likely to survive),

specimens sacrificed for other purposes (e.g., contaminants analysis), and statistically useful subsamples of

selected non-endangered species sacrificed for this purpose:

1.1.3.1. Document the presence and severity

of spinal damage and associated hemorrhages using

Reynolds’ (1996) criteria (Table 3).

1.1.3.2. Document other external and internal

damage or anomalies following Goede and Barton’s (1990)

necropsy-based procedures and criteria for fish health

and condition profiles (HCP; blood tests can be omitted).

1.1.3.3. Fish can be iced or frozen and processed at a later time and more convenient location.

1.1.4. If possible, record the behavior and

condition of fish as they are netted–for example, active

(automatism, taxis; note whether swimming motions are

normal or abnormally rapid), unconscious and limp (petit

mal, narcosis; note whether breathing motions have

ceased), or unconscious and stiff (grand mal, tetany) or

quivering (partial tetany, transition from narcosis to full

tetany).

1.1.5. Observe and record any difficulties in

recovery from narcosis or tetany (note nature and duration of problem and special measures taken to aid recovery), mortality if the fish fail to recover, and any abnormal

behavior once breathing motions and equilibrium are reestablished.

1.1.6. Also record:

1.1.6.1. Date, time, water temperature,

conductivity, turbidity, depth, habitat type and other

environmental conditions at each site.

1.1.6.2. Electrofishing electrode configuration (type, size and placement), output voltage (note

whether peak or mean values), type of current and waveform (shape), pulse or cyclic frequency (if PDC or AC),

and pulse width or duty cycle (if PDC).

1.1.6.3. In-water measurements of field intensity at standardized locations relative to the electrodes

and vessel (again note whether peak or mean values).

1.1.6.4. Sampling strategy, including average

and maximum exposure time per “switch-on” event and

for each effort as a whole.

1.1.6.5. When electrofishing parameters and

techniques are standardized for specific environmental

conditions (temperature, conductivity, depth, habitat type)

they need be measured only once under each set of conditions during a trip, except for continual monitoring of

output and periodic checks of field intensity.

1.1.6.6. Such thorough documentation

should be standard practice for all electrofishing operations, regardless of purpose.

1.1.7. If species of trout or other Salmoninae

(which are especially susceptible to spinal injuries) are

captured in statistically useful numbers, they should also

be examined and analyzed for comparative purposes.

1.1.8. If sufficient numbers of the same species and size classes are collected in existing programs

by other gear for statistically useful comparisons, they

should be similarly examined and documented.

1.2.

1.2. Special field studies. Conduct special field

studies, independent of ongoing investigations, to specifically document the incidence and severity of

electrofishing injuries in selected fishes that can be sacrificed, especially native species chosen as surrogates

for the endangered species (e.g., roundtail chub and

flannelmouth sucker). One investigation of this type has

been conducted by Cowdell and Valdez (1994) using adult

roundtail chub as a surrogate for humpback chub. The

investigations of Sharber and Carothers (1988, 1990) on

adult rainbow trout, which initiated recent concern over

electrofishing injuries, were also of this type.

1.2.1. Electrofishing gear and techniques

should be comparable to those normally used and prospectively used in the basin.

1.2.2. Examine all target specimens as per item

1.1.1. above, then euthanize and ice or freeze for

subsequent laboratory analyses by X-ray radiography

and necropsy as per items 1.1.2.2., 1.1.2.3., 1.1.3.1., and

1.1.3.2. above. X-rays could be taken by or at a cooperating

educational or medical facility. If possible, include

statistically useful subsamples by size class.

1.2.3. Observe and document condition upon

capture, difficulties in recovery, and abnormal behavior

upon recovery as outlined above under items 1.1.4. and

1.1.5., and record environmental and electrical parameters

as per item 1.1.6.
 SNYDER 111

1.2.4. If species of trout or other Salmoninae

(which are especially susceptible to spinal injuries) are

captured in statistically useful numbers, they should also

be examined and analyzed for comparative purposes.

1.3.

1.3.

2.

2.1.

2.1.

 Controlled pond experiments. Conduct controlled electrofishing experiments in large ponds to document incidence and severity of electrofishing injuries on

hatchery-reared endangered fishes, preferably fish not

previously subjected to electric fields.

1.3.1. Fish should be tagged for individual

identification and examined for pre-existing injuries or

anomalies based on pretrial X-rays and detailed external

examination.

1.3.2. Follow procedures as suggested for

approach 1.2. above.

1.3.3. Fish remaining in the ponds after

electrofishing should be collected by other means (e.g.,

seining) as soon as possible and processed similarly for

comparison. Assuming the ponds were intensively

sampled by electrofishing, most of these specimens would

likely represent fish that were subjected to electrofishing

fields but escaped capture.

2. Laboratory experiments on juvenile and adult fish. 2.

Conduct laboratory experiments to document and compare the injurious effects and induced responses of currently used and potentially less harmful electrofishing

waveforms on endangered fishes (or surrogates) and other

species of concern in the Colorado River Basin. Identify

waveforms, electric-field characteristics, and conditions

(from among those tested) that will minimize injurious

effects, but still elicit sufficient taxis and narcosis for effective electrofishing. Conduct experiments initially in

homogeneous electric fields and follow up, if appropriate, with comparable experiments in heterogeneous fields.

As of this review, limited versions of such experiments

have been conducted only with small juvenile humpback

chub and bonytail (Ruppert, 1996; Ruppert and Muth,

1997) and subadult Colorado pikeminnow, and for comparison, similar-size adult rainbow trout (Meismer, 1999).

2.1. Currents and waveforms to be tested.

2.1.1. Currents and waveforms typically used

in the Colorado River Basin or believed to reduce the

incidence of injury.

2.1.1.1. Quarter-sine, 120-Hz PDC with 7-ms

pulses (80% duty cycle).

2.1.1.2. Square, 80-Hz PDC with 5-ms pulses

(40% duty cycle).

2.1.1.3. Square and quarter-sine, 60-Hz PDC

with 4-ms and 13-ms pulses (25% and 80% duty cycle,

respectively).

2.1.1.4. Square, 30-Hz PDC with 4-ms pulses

(12% duty cycle).

2.1.1.5. Coffelt’s CPS, a pulse train of three 240-

Hz, 2.6-ms pulses every 15th of a second (12% duty cycle).

2.1.1.6. DC (filtered and conditioned from rectified AC).

2.1.2. Single-phase, 60-Hz AC and threephase, 180-Hz AC should ultimately be tested for comparative purposes.

2.1.3. Others of interest (possibly including

other specially developed complex PDCs such as SmithRoot’s sweeping waveforms).

2.1.4. Waveforms for the above need to be

clean and well-defined. If voltage spikes are found to be

a significant component of a typically used current or

waveform, both it and the clean version of the waveform

should be tested separately and compared.

2.2.

2.2. Variables to be considered in experiments with

each waveform. Except for field intensity, initial experiments should be conducted with these variables held at a

fixed level that approximates typical conditions when

electrofishing in the Colorado River. Subsequent experiments should be conducted over a range of values for

each variable (at least two more levels), one variable at a

time, to assess the effects of those variables on injuries

and responses.

2.2.1. Field intensity (voltage gradient) and

exposure time.

2.2.1.1. To simulate heterogeneous electric

fields gradually moving over fish (or fish in taxis moving

towards anodes), experiments should be run with voltage gradient continuously increased at a steady rate from

a near zero value (e.g., Meismer, 1999). A variable-speed

motorized control of voltage is recommended to ensure a

constant rate of increase in field intensity. Individual tests

should be concluded immediately after the response being tested is achieved either by switching off the current

or gradually reducing field intensity back to zero. The

latter eliminates the possibility of injury caused by the

sudden change in voltage as the field is switched off

(perhaps especially important when testing DC). Tested

responses should include reactive detection (twitch),

taxis, narcosis, and tetany; a level of field intensity well

above the threshold for tetany is also recommended for

comparison. At least in initial or preliminary experiments,

different rates of voltage gradient increase should be

tested, thereby increasing or decreasing exposure time to

the progressively increasing field intensity.

2.2.1.2. To simulate stationary electric fields

positioned over fish when the current is switched on, the

above experiments should be repeated at fixed voltagegradient levels over a range of exposure times (e.g.,

Ruppert and Muth, 1997 except their experiments were

conducted with a single fixed exposure time). The current

should be switched on and off at the preset intensity

level after the fish are appropriately positioned in the

water. The selected voltage-gradient levels should be

slightly greater than or bracket response thresholds
112 INFORMATION AND TECHNOLOGY REPORT--2003-0002

suggested above. Use of levels less than the response

thresholds when tightly bracketed, would document

whether threshold levels are in part dependent on

exposure time. One level well below the threshold for

reactive detection and one well above full tetany should

be included for comparative purposes.

2.2.2. Water conductivity between 10 and

2,000 μS/cm; 500 μS/cm would be a good choice for initial

experiments because it is median to the range of

conductivities typically experienced in Colorado River

Basin endangered-fish investigations. Ultimately, tests

to determine field-intensity response thresholds should

be conducted in at least three or four widely ranging

freshwater conductivities (at least 10, 100, 500, and 1000

or 2,000 μS/cm) to provide threshold data over a range of

conductivities. In addition to documenting response

thresholds relative to conductivity, the resulting data

could be used to determine for these fish the powertransfer relations postulated by Kolz (1989a).

2.2.3. Water temperature between 5 and 25° C;

15 or 20° C would be a good choice for initial experiments.

2.2.4. Fish size groups between 2 and 50 cm.

Because fish under 10 cm are usually easier to handle and

obtain in quantity, they might be a good choice for initial

experiments (Ruppert and Muth, 1997). However, they

may be more difficult to necropsy, and their X-rays may

not be as easy to analyze as those of larger fish.

2.2.5. Fish orientation. Experiments should

be conducted initially with fish oriented (or maintained)

in various directions (e.g., toward the anode, toward the

cathode, transverse to both, and positions between these).

Position or changes in position during an exposure could

significantly affect experimental results. Sharber (personal

communication) reported a much greater incidence of

spinal injuries among trout held perpendicular to the lines

of current than parallel to the current.

2.3.

2.3. Fish species.

2.3.1. Initial experiments should be conducted on surrogates for the endangered species and

rainbow trout, the latter for comparison and verification

of previous observations on that species.

2.3.2. Once the critical ranges for experimental variables are narrowed as a result of the initial experiments, more focused experiments can be conducted on

other species, including endangered species if expendable hatchery-reared specimens are available. If the effects, responses, and response thresholds for the

endangered species are very similar to those for the surrogate species, only enough endangered-species experiments need to be conducted to support that conclusion,

and remaining surrogate-species results can be extrapolated to the corresponding endangered species.

2.3.3. If differences in response thresholds

and susceptibility to injury are found between hatcheryreared and wild stocks of the same species, these differences should be documented and considered in the interpretation of results. Such differences have been

observed for trout (Sharber, personal communication;

Fredenberg, personal communication).

2.4. 

2.5.

2.5.

2.6.

2.6.

Responses and injuries.

2.4.1. Each experimental specimen should be

marked or tagged for individual identification, measured,

examined for external anomalies, X-rayed (to document

any existing spinal anomalies), and acclimated to the water temperature and conductivity in which it will be tested.

2.4.2. Responses for each tested fish should

be recorded on video tape with specimen data, a time

index, and the level of variables tested. Such documentation will allow repeated review of questionable observations and independent analyses.

2.4.3. After each trial, fish should be observed

carefully for rate of recovery and abnormal behavior, examined for external signs of injury, then X-rayed for spinal injuries and dissected for related internal injuries

according to procedures outlined above for mortalities

and sacrificed fish in field experiments (item 1.2.2.).

2.5. Homogenous versus heterogenous fields.

Experiments should be conducted initially in homogeneous fields (rectangular tank with full cross-sectional

electrodes) to minimize confounding factors and allow

comparison with published data. When waveforms or

other experimental factors significantly affect either injuries or typical responses, experiments should be repeated

in heterogeneous fields (e.g., quarter-circular tank with

one full-depth electrode at the apex and the other fulldepth electrode lining the outer wall, or perhaps better

yet in an open-water setting such as a pond with standard electrofishing electrodes).

2.5.1. In heterogeneous fields, fish should

be tested in anodic and cathodic fields. Effects in the

cathodic field have been largely overlooked in past research, especially with regard to injury, and they may

differ from effects in the anodic field.

2.5.2. These experiments will establish the

relation between responses and thresholds observed in

homogeneous and heterogeneous fields.

2.5.2.1. If the relation is well defined and consistent, future experiments with other species may only

need to be conducted in homogeneous fields.

2.5.2.2. If the relation is inconsistent or

difficult to define because of continuously varying

voltage gradients, future experiments for field-applicable

thresholds may be limited to heterogeneous fields.

2.6. Application of results. If these experiments

demonstrate that some electrofishing currents,

waveforms, and conditions are less harmful than others

to endangered or surrogate species, while still eliciting
 SNYDER 113

sufficient taxis and narcosis for effective electrofishing,

then electrofishing gear and techniques used in recovery

and fishery investigations should be modified

accordingly. Based on response threshold data, it might

also be possible to define optimal field sizes and intensities

for consistent electrofishing over a wide range of water

conductivities and temperatures while minimizing potential

for harm. A simplified set of experiments could be designed

for similarly determining optimal electrofishing fields for

other species.

3.

3.1.

3.1.

3.2.

3.2.

3.3.

3.3.

4.

4.1.

4.1.

4.2.

4.2.

3. Laboratory experiments on spawning adults and

early-life stages. Conduct a series of experiments on the

effects of presently used and potentially less harmful

electrofishing fields on the spawning and early life stages

of endangered species (or surrogates), other native species of concern, and rainbow trout (for comparison with

existing observations).

3.1. Effects on spawning adults. Determine the

adverse effects of electrofishing fields on the reproductive capability and behavior of fish exposed while in or

approaching a state of spawning readiness (e.g., in part

for razorback sucker, Muth and Ruppert, 1996). Fish are

sometimes targeted for electrofishing as they aggregate

for or begin spawning.

3.2. Effects on eggs and larvae. Determine the

effects of these electrofishing fields on developing eggs

and larvae in and out of simulated substrate (e.g., for

razorback sucker out of substrate, Ruppert and Muth,

1995; Ruppert, 1996; Muth and Ruppert, 1997). Adults

are sometimes electrofished over spawning grounds.

3.3. Responses and thresholds for larvae and early

juveniles. Document responses and response thresholds

of protolarvae, mesolarvae, metalarvae, and early juveniles, and determine the sizes at which these fish begin to

respond like older fish and incur spinal injuries. Larval

and YOY fish are likely to be present in many habitats

that are electrofished for larger juveniles and adults.

4. Long-term effects of injuries. If incidence of 4.

electrofishing-induced spinal (or related) injuries is significant for endangered species, and changes in technology (e.g., current and waveform) and technique are

unacceptable or do not sufficiently reduce the incidence

of injuries, then conduct a series of 1 year or longer pond

investigations to assess subsequent effects of the injuries on survival, growth, condition, and, if possible, reproductive viability.

4.1. Ponds. Each pond should include approximately equal numbers of treatment and control fish.

4.2. Treatments. Treatment fish would be

intentionally injured (vertebral fractures or misalignments)

by electric fields and X-rayed for verification and

subsequent comparison. Also consider a second set of

treatment fish, those subjected to the electric field but

not sustaining a detectable injury.

4.3.

4.3.

4.4.

4.4.

4.5.

4.5.

5.

6.

6.1.

6.1.

6.2.

6.2.

 Monitoring. All fish should be periodically

captured using nonelectrofishing techniques and monitored for injury healing, survival, growth, condition, and

reproductive state.

4.4. Results. If most electrofishing-induced injuries do heal and subsequently have no detectable effect

on survival, growth, condition, or reproductive behavior,

then the matter of electrofishing injuries is not critical

with respect to population management and recovery of

the species.

4.5. Assessment of past damage. It might be

desirable to conduct these experiments even if changes

in standard electrofishing technique do reduce the harmful

effects; the results might help assess the extent of damage

done to endangered populations by past electrofishing

activities.

5. Investigations into causes and mechanisms with 5.

integration of knowledge for other vertebrates. Employing or in consultation with electro-physiologists, compare and contrast known or hypothesized effects of electricity on fish with those on other vertebrates, then design

and conduct laboratory experiments, beyond the preceding, to confirm or determine the specific causes and mechanisms involved in electrofishing responses and injuries.

6. Improvements to electrofishing gear and tech- 6.

niques. Based on results of the above suggested research,

refine, develop, and test new electrofishing gear and techniques to help minimize harmful effects while maintaining

the size and intensity of electric fields needed for consistently effective electrofishing. Possibilities might include:

6.1. Improved control box meters and displays.

Provide accurate voltage meters and ammeters as standard components of all electrofishing systems for monitoring peak (as well as mean) electrical output. If the

meters are digital, a special circuit could be incorporated

to calculate and also display output power. For PDC, displays of accurately measured pulse frequencies and duty

cycles or pulse widths should also be provided.

6.2. Special meter and probe for monitoring electric

fields. Develop a special electrofishing field meter with

omni-directional probe for mapping electric fields and

monitoring peak voltage gradients, ambient water

conductivity, and water temperature at standardized

locations. (Kolz, 1993, suggested that a voltage-gradient

probe could be designed for simultaneous measurement

in both horizontal and vertical planes and used with a

meter that would add these components and display the

resultant magnitude of voltage gradient. However, such

a probe would still have to be rotated to determine direction

of current flow and maximum voltage gradient.)

6.2.1. The probe, perhaps a spherical

apparatus suspended by gimbals and oriented

magnetically like a compass, would automatically detect
114 INFORMATION AND TECHNOLOGY REPORT--2003-0002

the orientation and direction of maximum current (principal

vector, lines of flux) and sense the peak voltage gradient

along that vector. It would also provide sensors for

conductivity and temperature.

6.2.2. The combination field meter should

accurately measure and display peak voltage-gradient,

direction of the current in both horizontal and vertical

planes, ambient water conductivity, and water temperature. Circuitry could be included to calculate and display

peak power density. Incorporation of a small digital oscilloscope for measuring and displaying peak voltage gradient would have the added benefit of providing specific

waveform information (e.g., shape, including presence of

spikes, frequency, pulse width). The meter should be able

to accommodate or be modified for a variety of

electrofishing currents and PDC and AC waveforms.

6.3.

6.3.

6.4.

6.4.

 Automated control of field intensity for boat

electrofishing systems.

6.3.1. Such a system might be useful when

electrofishing highly variable habitats with different water

conductivities (e.g., when moving from deeper open

waters to very narrow, shallow coves with silt substrate,

shallow riffles with cobble, or across or into the mouths

of tributaries with significantly different conductivity or

temperature). When sampling more uniform habitats, such

a system might be superfluous.

6.3.2. Using a fixed or standard-position sensor for voltage gradient and water conductivity (e.g., the

above mentioned omni-directional probe fixed 30 cm below the surface and 1 m from the center of the anode

towards the boat), a digital peak voltage-gradient meter

or oscilloscope built into the control box, and a controlbox mechanism for automatically controlling output voltage to maintain a preset voltage gradient at the probe.

6.3.3. The preset voltage gradient at the position of the probe might be selected by calculation, table,

graph, or built-in computer program to provide the optimal electrode-specific field-intensity map for the target

species based on its response thresholds (e.g., a distribution of field intensity that would provide for initiation

of taxis and narcosis at some optimal distance from the

anode).

6.4. Improved electrodes. Design electrodes to

minimize tetany-related injury and stress to fish and optimize the effective portion of the electrofishing field.

6.4.1. In addition to using the largest practical electrodes, shield fish from direct contact with, and

the very highest field intensities immediately around,

those electrodes by surrounding or covering the electrodes with a deep, but fine grid of non-conductive material (plastic).

6.4.2. Design boat-electrofishing anodes to

semi-freely float by suspending them from or incorporating

a floatation devise to maintain the effective portion of the

anodes just under the surface of the water and provide

some mechanism to maintain each anode in a fixed position

in front of the boat. As electrofishing boats move through

the water and personnel move about on the boats, they

tend to bob up and down with conventional boomsuspended electrodes either moving deeper into the water

or partially rising above the surface, thereby reducing

the electrodes’ effective surface area. Anodes suspended

from booms also change position relative to the front of

the boat as boats turn or surge forward or backward.

Such changes in the relative position and submergence

of anodes can dramatically affect the effective size of the

electric field and distribution of field intensity therein.

6.4.3. Design hemispherical (or halfsubmerged spherical) anodes or alternative high-surfacearea electrodes with no upward-facing surfaces, and no

sharp corners or edges, to efficiently direct all of the

electric field horizontally and downward. Spherical and

certain other types of electrodes submerged near the

surface direct a significant portion of the field upward to

the surface where it is effectively wasted.

Acknowledgments Acknowledgments

Interlibrary Loan and Science-Reference Departments

of Colorado State University Libraries and S. Johnson

were invaluable in locating and obtaining much of the

pre-1992 literature for this review. U.S. Fish and Wildlife

Service Office of Information Transfer and Fish and Wildlife Reference Service also assisted with searches. G.

Goodchild loaned copies of difficult-to-obtain literature

and provided a computer file of references in the Burridge

et al. (1990) bibliography. Literature was entered in a bibliographic database under Reference Manager, a program

contributed by Research Information Systems, Inc.

S. Johnson helped prepare the database and the indexed

bibliography of pre-1992 literature (Snyder and Johnson,

1991).

In response to published calls for information and a

regional survey (Snyder, 1992a; Appendix II), many researchers provided copies of unpublished manuscripts

and reports and conveyed their experiences, observations, and concerns. Unpublished observations and theories cited herein are used with source permission. M. Yard

provided a preliminary assessment of the problem, a list

of questions to be addressed, and an initial list of references. N. Sharber was especially helpful in interpreting

some of the literature, clarifying pertinent electrical concepts, and conveying his ideas concerning the nature of

fish responses in electric fields. C. Bjork prepared the

original diagram for Fig. 11. Photographs were provided
 SNYDER 115

by M. Quinton for Fig. 3, G. Oliver for Fig. 4,

W. Fredenberg for Figs. 2 and 18, and N. Sharber for

Figs. 16 and 17. Photographs for Figs. 1 and 19 were reproduced with permission from Sharber and Carothers

(1988). Diagrams for Figs. 9 and 10 were copied from

Novotny (1990, with permission) and Sternin et al. (1972,

1976), respectively. The graph for Fig. 6, and graphs for

Figs. 12 and 13 were reproduced with permission from

Kolz (1989a) and Kolz and Reynolds (1989b), respectively.

Table 3 was adapted with permission from Reynolds

(1996).

K. Bestgen, N. Bezzerides, L. Crist, W. Fredenberg,

J. Hawkins, D. Horak, S. Meismer, C. Meyer, R. Muth,

J. Reynolds, S. Seal, N. Sharber, and M. Snyder reviewed

or proofed various drafts or portions of the original report

or this update. J. Zuboy, then fisheries editor for the

National Biological Survey, and referees A. Kolz and

A. Temple provided constructive recommendations for

major revisions of the original report. A. Temple also

reviewed the a near final draft of the revised and updated

manuscript, as did A. Kolz for selected portions thereof.

C. Carlson edited a portion of the near final draft.

R. Wydoski edited the final draft prior to submission for

publication to the U.S. Geological Survey Biological

Resources Division. D. Asherin, J. Carpenter, D. Medellin,

and J. Shoemaker further edited, prepared, and managed

the manuscript through publication.

The original review and synthesis (Snyder, 1992a)

and this updated revision and publication were sponsored by the Upper Colorado Regional Office of the Bureau of Reclamation, U.S. Department of the Interior, Salt

Lake City, Utah. The original report was also sponsored

by the Glen Canyon Ecological Studies Project, Flagstaff,

Arizona. The support, assistance, and patience of

K. Bestgen, T. Chart, R. Muth, D. Wegner, R. Williams, and

especially L. Crist were greatly appreciated. The original

report was Contribution 50 of the Colorado State University Larval Fish Laboratory. This publication is Larval

Fish Laboratory Contribution 114.

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Unpublished Manuscripts

McMichael, G.A., and Olson, J.P., Unpublished manuscript, 1991 draft, Electrofishing injury and short-term

mortality in rainbow trout in a hatchery environment:

Fisheries Management Division, Washington Department of Wildlife, Ellensburg, Washington.

Meyer, C.W., and Miller, D.D., Unpublished manuscript,

1991 draft, Spinal injury in trout electrofished with a

Coffelt VVP-15 or CPS: Wyoming Game and Fish Department, Laramie.

Newman, L.E., Unpublished manuscript, 1991 draft, Spinal injuries of walleye caused by pulsed DC

electrofishing: Office of Fishery Assistance, U.S. Fish

and Wildlife Service, Ashland, Wisconsin.

Newman, L.E., and Stone, F.G., Unpublished manuscript,

1992 draft, Reduced viability in walleye eggs caused

by exposure to pulsed DC electrofishing: Ashland

Fishery Resources Office, U.S. Fish and Wildlife

Service, Ashland, Wisconsin.

Personal Communications

Barton, B.A., Department of Biology, University of South

Dakota, Vermillion.

Bestgen, K.R., Larval Fish Laboratory, Colorado State

University, Fort Collins.

Beyers, D.W., Larval Fish Laboratory, Colorado State

University, Fort Collins.

Buntjer, M., Utah Division of Wildlife Resources, Moab.

Burdick, R.D., Colorado River Fishery Project, U.S. Fish

and Wildlife Service, Grand Junction, Colorado.

Cowx, I.G., Humberside International Fisheries Institute,

University of Hull, Hull, United Kingdom.

Elmblad, W.R., Colorado Division of Wildlife, Grand

Junction.

Fredenberg, W.A., Montana Department of Fish, Wildlife, and Parks, Bozeman (currently with Creston National Fish Hatchery, U.S. Fish and Wildlife Service,

Kalispell, Montana).

Gowan, C., Department of Fishery and Wildlife Biology,

Colorado State University, Fort Collins (currently with

the Department of Biology, Randolph-Macon College,

Ashland, Virginia).

Hawkins, J.A., Larval Fish Laboratory, Colorado State

University, Fort Collins.

Johnson J.R., Smith-Root, Inc., Vancouver, Washington.

Kinsolving, D.A., Research Branch, Arizona Game and

Fish Department, Flagstaff.

Kolz, A.L., Denver Wildlife Research Center, U.S. Department of Agriculture, Colorado (retired, Grand Junction, Colorado).

Krueger, R., Fish and Wildlife Enhancement, U.S. Fish

and Wildlife Service, Grand Junction, Colorado.

McAda, C.W., Colorado River Fishery Project, U.S. Fish

and Wildlife Service, Grand Junction, Colorado.

McMichael, G.A., Washington Department of Fishery and

Wildlife, Ellensburg.

Meismer, S.M., Utah Division of Wildlife Resources, Moab

(currently with the Virgin River Resource Management

and Recovery Program, St. George, Utah).

Meyer, C.W., Wyoming Game and Fish Department,

Laramie.

Miranda, L.E., Cooperative Fish and Wildlife Research

Unit, Mississippi State University, Mississippi State.

Novotny, D.W., Department of Electrical and Computer

Engineering, University of Wisconsin, Madison.

Pfeifer, F.K., Colorado River Fishery Project, U.S. Fish

and Wildlife Service, Grand Junction, Colorado.

Reynolds, J.B., Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska, Fairbanks.

Roach, S.M., Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska, Fairbanks.

Sharber, N.G., Coffelt Manufacturing, Inc., Flagstaff,

Arizona (retired, Flagstaff).

Temple, A.J., U.S. Fish and Wildlife Service National

Conservation Training Center, Shepherdstown, West

Virginia.

Trammell, M.A., Utah Division of Wildlife Resources,

Moab (currently with the National Park Service, Salt

Lake City, Utah).

Tyus, H.M., U.S. Fish and Wildlife Service, Lakewood,

Colorado.

Valdez, R.A., Bio/West, Inc., Logan, Utah (currently with

R.A. Valdez and Associates, Logan, Utah).

Wydoski, R.S., U.S. Fish and Wildlife Service (retired,

Lakewood, Colorado).
 SNYDER 127

Appendix A. Appendix A Appendix AAppendix A Scientific names and families of fishes referenced by common name in this report. Names follow Robins

et al. (1991a,b).

Common name Scientific name Family

American eel Anguilla rostrata Anguillidae

Arctic grayling Thymallus arcticus Salmonidae

Atlantic cod Gadus morhua Gadidae

Atlantic salmon Salmo salar Salmonidae

black crappie Pomoxis nigromaculatus Centrarchidae

black bass Micropterus species Centrarchidae

blue catfish Ictalurus furcatus Ictaluridae

bluehead sucker Catostomus discobolus Catostomidae

bluegill Lepomis macrochirus Centrarchidae

bonytail Gila elegans Cyprinidae

brook trout Salvelinus fontinalis Salmonidae

brown bullhead Ictalurus nebulosus Ictaluridae

brown trout Salmo trutta Salmonidae

bull trout Salvelinus confluentus Salmonidae

bullhead Cottus gobio Cottidae

burbot Lota lota Gadidae

channel catfish Ictalurus punctatus Ictaluridae

chinook salmon Oncorhynchus tshawytscha Salmonidae

chum salmon Oncorhynchus keta Salmonidae

coho salmon Oncorhynchus kisutch Salmonidae

Colorado pikeminnow Ptychocheilus lucius Cyprinidae

common carp Cyprinus carpio Cyprinidae

common shiner Luxilus cornutus Cyprinidae

creek chub Semotilus atromaculatus Cyprinidae

crucian carp Carassius carassius Cyprinidae

cutthroat trout Oncorhynchus clarki Salmonidae

Dolly Varden Salvelinus malma Salmonidae

dragonet Callionymus species Callionymidae

emerald shiner Notropis atherinoides Cyprinidae

European eel Anguilla anguilla Anguillidae

fantail darter Etheostoma flabellare Percidae

fathead minnow Pimephales promelas Cyprinidae

flannelmouth sucker Catostomus latipinnis Catostomidae

flathead catfish Pylodictis olivaris Ictaluridae

flathead chub Platygobio gracilis Cyprinidae

freshwater drum Aplodinotus grunniens Sciaenidae

Gila chub Gila intermedia Cyprinidae

golden shiner Notemigonus crysoleucas Cyprinidae

goldeye Hiodon alosoides Hiodontidae

goldfish Carassius auratus Cyprinidae

grass carp Ctenopharyngodon idella Cyprinidae

green sunfish Lepomis cyanellus Centrarchidae

gudgeon Gobio gobio Cyprinidae

humpback chub Gila cypha Cyprinidae

humpback whitefish Coregonus pidschian Salmonidae

lake chubsucker Erimyzon sucetta Cyprinidae

lake trout Salvelinus namaycush Salmonidae

largemouth bass Micropterus salmoides Centrarchidae

largescale stoneroller Campostoma oligolepis Cyprinidae

least cisco Coregonus sardinella Salmonidae

longnose dace Rhinichthys cataractae Cyprinidae
128 INFORMATION AND TECHNOLOGY REPORT--2003-0002

Appendix A. Appendix A Concluded.

Common name Scientific name Family

longnose sucker Catostomus catostomus Catostomidae

mottled sculpin Cottus bairdi Cottidae

mountain whitefish Prosopium williamsoni Salmonidae

Mozambique tilapia Tilapia mossambica Cichlidae

mummichog Fundulus heteroclitus Cyprinodontidae

northern pike Esox lucius Esocidae

northern hog sucker Hypentelium nigricans Catostomidae

Pacific sardine Sardinops sagax Clupeidae

paddlefish Polyodon spathula Polyodontidae

pink salmon Oncorhynchus gorbuscha Salmonidae

plaice Pleuronectes platessa Pleuronectidae

powan Coregonus lavaretus Salmonidae

pumpkinseed Lepomis gibbosus Centrarchidae

rainbow trout (steelhead) Oncorhynchus mykiss Salmonidae

razorback sucker Xyrauchen texanus Catostomidae

red shiner Cyprinella lutrensis Cyprinidae

river carpsucker Carpiodes carpio Catostomidae

roach Rutilus rutilus Cyprinidae

rosyside dace Clinostomus funduloides Cyprinidae

round whitefish Prosopium cylindraceum Salmonidae

roundtail chub Gila robusta Cyprinidae

roussette (spotted dogfish?) (Scyliorhinus canicula?) Scyliorhinidae

ruffe Gymnocephalus cernuus Percidae

sauger Stizostedion canadense Percidae

seahorse Hippocampus species Syngnathidae

shorthead redhorse Moxostoma macrolepidotum Catostomidae

shovelnose sturgeon Scaphirhynchus platorynchus Acipenseridae

Siberian sturgeon Acipenser baeri Acipenseridae

sicklefin chub Macrhybopsis meeki Cyprinidae

skate (Raja species?) Rajidae

smallmouth bass Micropterus dolomieu Centrarchidae

sockeye salmon Oncorhynchus nerka Salmonidae

sole Solea vulgaris Soleidae

speckled dace Rhinichthys osculus Cyprinidae

striped bass Morone saxatilis Percichthyidae

sturgeon chub Macrhybopsis gelida Cyprinidae

tench Tinca tinca Cyprinidae

Tennessee shiner Notropis leuciodus Cyprinidae

threespine stickleback Gasterosteus aculeatus Gasterosteidae

topsmelt Atherinops affinis Atherinidae

vendace Coregonus albula Salmonidae

walleye Stizostedion vitreum Percidae

warpaint shiner Luxilus coccogenis Cyprinidae

westslope cutthroat trout Oncorhynchus clarki lewisi Salmonidae

white crappie Pomoxis annularis Centrarchidae

white sucker Catostomus commersoni Catostomidae

woundfin Plagopterus argentissimus Cyprinidae

yellow perch Perca flavescens Percidae

Yellowstone cutthroat trout Oncorhynchus clarki bouvieri Salmonidae

zander (pikeperch) Stizostedion lucioperca Percidae
Appendix B. Compendium of reported electrofishing mortalities and injuries by fish species and electrical current. Extracted from published literature, agency docum

and, with permission, unpublished manuscripts.a,b 

Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

Acipenseridae 

Scaphirhynchus platorynchus, shovelnose sturgeon 

#91 TL 5 0% 0% Rivers 600 11 * PDC 60 8? S#77 TL 3 0% 0% Rivers 600 11 * CPS 240/15 2.6/11 S

Polyodontidae 

Polyodon spathula, paddlefish 

highc highc Rivers * PDC 

Hiodontidae 

Hiodon alosoides, goldeye 11-37 TL 19 0% 0% 21% 21% Rivers 351-425 12-15 * DCfy 11-37 TL 23 0% 0% 4% 39% Rivers 351-425 12-15 * PDCfy 40 6.3 S

Cyprinidae 

Campostoma oligolepis, largescale stoneroller 

few ∃0%dl Streams 10-15 5-8 * DC 

Carassius auratus, goldfish 6-9 0d 0d 0d 0d Lab. trough 18-10000 20 = AC 60 S6-9 0d 0d 0d 0d Lab. trough 9.5-10000 20 = DC 6-9 0d 0d 0d 0d Lab. trough 13-10000 20 = PDC 50 10 S6-9 0d 0d 0d 0d Lab. trough 13-9700 20 = PDC 50 2 S12-15 TL 32 0% 0% Aquarium * PDC 100 2.5 SClinostomus funduloides, rosyside dace 

few ∃0%dl Streams 10-15 5-8 * DC 

Cyprinella lutrensis, red shiner 

e 0 0 Rivers etc,CRB <50->5000 >0->30 * PDC 60 

Cyprinus carpio, common carp 

1 1 f f Rivers, CRB * PDC 

0 0 0 Rivers, CRB 500-800 15-25 * PDC 

e 0 0 0? Rivers etc,CRB <50->5000 >0->30 * PDC 60 

0 =37 TL 39 0% 8% Reservoir * PDC 60 S45 TL 1 0 0 Reservoir * PDC 60 SGila cypha, humpback chub 

0 0 Rivers, CRB * 

−0.01% X Rivers, CRB 300-2000 5-25 * PDC 

 Xg Xg Rivers etc,CRB * PDC  Xg Xg Rivers etc,CRB * CPS 240/15 2.6/11 S

5-10 TL 30 0% 0% 0% 0% 20%ft Test tank 940 15 = CPS 240/15 2.6/11 SJuv.-ad. 0 0 Rivers etc,CRB 250-3500 * PDC,DC Gila elegans, bonytail 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

−0.01% 0 Rivers, CRB 300-2000 5-25 * PDC 

5-8 TL 90 0% 0% 0% 0% 3-20%fp Test tank 940 15 = PDC 30 4 S5-8 TL 90 0% 0% 0% 0% 7-27%fq Test tank 940 15 = PDC 60 4 S5-8 TL 90 0% 0% 0% 0% 13-23%fr Test tank 940 15 = PDC 80 5 S5-8 TL 90 0% 0% 0% 0% 7-10%fs Test tank 940 15 = CPS 240/15 2.6/11 SGila robusta, roundtail chub 

 0 Rivers, CRB 600-1000 15-20 * PDC,DC? 

0 0 0 Rivers, CRB 500-800 15-25 * PDC 

22-40 TL 40 0% 0% 0% 0% 0% 5% River, CRB 1,000 20 * PDC 40 5 S

e 0 0 Rivers etc,CRB <50->5000 >0->30 * PDC 60 

Luxilus coccogenis, warpaint shiner 

few ∃0%dl Streams 10-15 5-8 * DC 

Luxilus cornutus, common shiner 5-9 TL >16 Xen Xen Test tank = DC Macrhybopsis gelida, sturgeon chub Adult? 8fz 0% 0% 0% 0% Bucket in River 351-425 12-15 * PDCfz 40 6.3 SMacrhybopsis meeki, sicklefin chub Adults? 9fz 0% 0% 0% 0% Bucket in River 351-425 12-15 * PDCfz 40 6.3 SNotemigonus crysoleucas, golden shiner 5-10 TL 7 0%dx Lakes >66, <520 13-14 * AC 180da Notropis leuciodus, Tennessee shiner 

few ∃0%dl Streams 10-15 5-8 * DC 

Pimephales promelas, fathead minnow 

e 0 0 Rivers etc,CRB <50->5000 >0->30 * PDC 60 

Plagopterus argentissimus, woundfin 

e 0 0 Rivers etc,CRB <50->5000 >0->30 * PDC 60 

Platygobio gracilis, flathead chub 11-24 TL 2 0% 0% 0% 0% Rivers 351-425 12-15 * DCfy 11-24 TL 12 0% 0% 8% 0% Rivers 351-425 12-15 * PDCfy 40 6.3 SPtychocheilus lucius, Colorado pikeminnow 0 = 33 TL 20 0 0 0 15% 5% Test trough 530 18 = DCgi 0 = 34 TL 20 0 0 0 5% 0 Test trough 530 18 = DCgj 0 = 34 TL 20 0 0 0 5% 0 Test trough 530 18 = DCgk 0 = 34 TL 20 0 0 0 0 5% Test trough 530 18 = DCgl 

0 0 Rivers, CRB * 

Juv.-ad. 0 lowk 1k Rivers etc,CRB 250-3500 * PDC&DC 

 0 Rivers, CRB 600-1000 15-20 * PDC,DC? 

Xl Rivers, CRB * PDC? 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

Juv.-ad. 1 1 j j Rivers etc,CRB * PDC S10-90 TL Xh 0 lowh 1h Rivers, CRB 200-2000 0-25 * PDC 

−0.01% X Rivers, CRB 300-2000 5-25 * PDC 

Adults 20 0 5%?i 0 0 Rivers, CRB 300-1500 5-15 * PDC 30,40 20 0 = 34 TL 20 0 0 0 15% 0 Test trough 530 18 = PDCgi 15 4 S0 = 33 TL 20 0 0 0 5% 0 Test trough 530 18 = PDCgj 15 4 S0 = 34 TL 20 0 0 0 10% 5% Test trough 530 18 = PDCgk 15 4 S0 = 34 TL 20 0 0 0 15% 0 Test trough 530 18 = PDCgl 15 4 S0 = 34 TL 20 0 0 0 5% 0 Test trough 530 18 = PDCgi 60 4 S0 = 33 TL 20 0 0 0 0 0 Test trough 530 18 = PDCgj 60 4 S0 = 34 TL 20 0 0 0 0 5% Test trough 530 18 = PDCgk 60 4 S0 = 34 TL 20 Xgn 0 0 0 5% 0 Test trough 530 18 = PDCgl 60 4 S0 = 34 TL 20 0 0 0 0 0 Test trough 530 18 = PDCgm 60 4 S0 = 34 TL 20 0 0 0 0 0 Test trough 530 18 = PDCgm 60 4 1⁄4 0 = 33 TL 20 0 0 0 10% 0 Test trough 530 18 = CPSgi 240/15 2.6/11 S0 = 33 TL 20 0 0 0 5% 0 Test trough 530 18 = CPSgj 240/15 2.6/11 S0 = 34 TL 20 0 0 0 5% 0 Test trough 530 18 = CPSgk 240/15 2.6/11 S0 = 33 TL 20 0 0 0 5% 5% Test trough 530 18 = CPSgl 240/15 2.6/11 SRhinichthys cataractae, longnose dace 

few ∃0%dl Streams 10-15 5-8 * DC 

Rhinichthys osculus, speckled dace 

e 0 0 Rivers etc,CRB <50->5000 >0->30 * PDC 60 

Semotilus atromaculatus, creek chub 

few ∃0%dl Streams 10-15 5-8 * DC 

Catostomidae 

Carpiodes carpio, river carpsucker 

30-58 TL 4 0% 0% 0% 0% Rivers 351-425 12-15 * DCfy 30-58 TL 11 0% 0% 18% 9% Rivers 351-425 12-15 * PDCfy 40 6.3 SCatostomus catostomus, longnose sucker ~43-49TL 5 0% 0% 0% 0% Rivers 351-425 12-15 * PDCfy 40 6.3 SI+& older 229ff 7-13% Stream (c) 34-63 <12-18 * PDC 100 5 SCatostomus catostomus, longnose sucker and C. commersoni, white sucker (combined data) 0 = 41 TL 50 2% 16% River ~450 10 * DC 0 = 40 TL 52 2% 8% River ~450 10 * PDC 15 S0 = 39 TL 50 0% 4% River ~450 10 * CPS 240/15 2.6/11 SCatostomus commersoni, white sucker 

 X Rivers, CRB * PDC? 

0 0 0 Rivers, CRB 500-800 15-25 * PDC 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

e 0 0 0? Rivers etc,CRB <50->5000 >0->30 * PDC 60 

37 TL 1 0 0 Reservoir * PDC 60 SCatostomus discobolus, bluehead sucker 

 0 Rivers, CRB 600-1000 15-20 * PDC,DC?  X Rivers, CRB * PDC? 

−0.01% X Rivers, CRB 300-2000 5-25 * PDC 

0 0 0 Rivers, CRB 500-800 15-25 * PDC 

e 0 0 0? Rivers etc,CRB <50->5000 >0->30 * PDC 60 

Catostomus latipinnis, flannelmouth sucker 

−0.01% X Rivers, CRB 300-2000 5-25 * PDC 

0 0 0 Rivers, CRB 500-800 15-25 * PDC 

e 0 0 0? Rivers etc,CRB <50->5000 >0->30 * PDC 60 

 0 Rivers, CRB 600-1000 15-20 * PDC,DC?  X Rivers, CRB * PDC? 

Erimyzon sucetta, lake chubsucker 5-25 TL 41 0%dx Lakes >66, <520 13-14 * AC 180da Hypentelium nigricans, northern hog sucker 

few ∃0%dl Streams 10-15 5-8 * DC 

Moxostoma macrolepidotum, shorthead redhorse 18-49 TL 4 0% 0% 0% 0% Rivers 351-425 12-15 * DCfy 18-49 TL 20 0% 0% 0% 5% Rivers 351-425 12-15 * PDCfy 40 6.3 SXyrauchen texanus, razorback sucker Juv.-ad. 0 0 Rivers etc, CRB 250-3500 * PDC,DC 

−50 0 0 Rivers, CRB 200-2000 0-25 * PDC 

−0.01% 0 Rivers, CRB 300-2000 5-25 * PDC 

Adults 20 0 10%?m 0 0 Rivers, CRB 300-1500 5-15 * PDC 30,40 20 0.9-1.0SL 60 0% 0% 0%fo Test tank 650 19 = PDC 30 4 S0.9-1.0SL 180 100%fo 0% 0%fo Test tank 650 19 = PDC 60 4 S50-60 TL 8 0% 0% 13% 50% 0% Test tank 610 20 = PDC 60 4 S0.9-1.0SL 60 100%fo 0% 0%fo Test tank 650 19 = PDC 80 5 S0.9-1.0SL 60 100%fo 0% 0%fo Test tank 650 19 = CPS 240/15 2.6/11 S50-60 TL 7 0% 0% 0% 14% 0% Test tank 610 20 = CPS 240/15 2.6/11 S

Ictaluridae 

Ictalurus punctatus, channel catfish Adults? 10 0% ∃60% Hatch. ponds 167 * AC 180da 

 Xn 0 Rivers, CRB 600-1000 15-20 * PDC,DC?  Xo Rivers, CRB * PDC? 

e 0 0 Rivers etc,CRB <50->5000 >0->30 * PDC 60 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

 Esocidae 

Esox lucius, northern pike 13-19 SL 17 24% 0% 24% X ? Aquaria 200-230 12-14 = ACfj 50 S13-19 SL 18 17% 0% ? X 33% Aquaria 200-230 12-14 = ACfj 50 TJuv.-ad. 0 0 Streams 25-200 5-15 * PDC? 52-68 FL 27 11% 0% Test tank 158-188 11-15 = PDC 30 16.7 36-74 FL 60 5% Test tank 109-132 11-16 = PDC100V 30 16.7 36-74 FL 60 10% Test tank 109-132 11-16 = PDC400V 30 16.7 13-19 SL 18 0% 0% 0% 0% 0% Aquaria 200-230 12-14 = PDCfj 50 7.3 45-97 SL 311 0% 0% 0% 0% 0% Test tank 410 11 = PDCfk 50 7.3 36-74 FL 60 8% Test tank 109-132 11-16 = PDC100V 60 8.3 36-74 FL 60 12% Test tank 109-132 11-16 = PDC400V 60 8.3 52-68 FL 27 33%av 15%av Test tank 158-188 11-15 = PDC 60 8.3 40->80FL 32 0.2%as at 16%au 19%au Rivers 210 * PDC 60 8.3 

e 0 0 0? Rivers etc,CRB <50->5000 >0->30 * PDC 60 

38-77 FL 140/172 ar 0%ar 28%ar Test tank 1017-1090 10-13 = PDC 120 4.2 

Salmonidae 

(unspecified species) 

Juv.-ad. 0 mostq Xq Rivers etc,CRB 250-3500 * PDC,DC <46 TL Xp Rivers, streams <100 * PDC 10-24 TL 114fh 44%fh Streams 34-63 <12-18 * PDC 100 5 SCoregonus pidschian, humpback whitefish 33-45 278/60 5%r 3%s 7%s Rivers 80-88 10-11 * PDC 80 5 Coregonus sardinella, least cisco 28-38 106/83 15%t 5%u 30%u Rivers 80-88 10-11 * PDC 80 5 Oncorhynchus clarki, cutthroat trout 

0.5% 10% Streams <100 5-15 DC? * 

Yoy-ad Xv Rivers, CRB <25?-1000 0-19 * PDC 

w <1% Streams 10-100 * DC? 

Oncorhynchus gorbuscha, pink salmon Juv.-ad. high lowx high Streams 5-15 * AC 60 Oncorhynchus kisutch, coho salmon 7 TL 107 78%de Lab shock tank 17 = PDC 8 40 S7 TL 107 0%df Lab shock tank 17 = PDC 8 40 S6-10 SL 197 3%cy Hatch. raceway 67-100 17 * PDC 15 8.3 1⁄2 6-10 SL 207 2%cy Hatch. raceway 200-1000 17 * PDC 15 8.3 1⁄2 6-10 SL 195 9%cy Hatch. raceway 67-100 17 * PDC 15 8.3 S6-10 SL 206 4%cy Hatch. raceway 200-1000 17 * PDC 15 8.3 S6-10 SL 194 7%cy Hatch. raceway 67-100 17 * PDC 30 8.3 1⁄2 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

hemor. 

6-10 SL 207 5%cy Hatch. raceway 200-1000 17 * PDC 30 8.3 1⁄2 6-10 SL 194 18%cy Hatch. raceway 67-100 17 * PDC 30 8.3 S6-10 SL 194 6%cy Hatch. raceway 200-1000 17 * PDC 30 8.3 SOncorhynchus mykiss, rainbow trout >40 FL 22 50% Rivers >50 >7 * >40 FL Xy Rivers,net-pens * −19 TL 48 4%cw Hatch. raceway 306 @25C * AC 5-9.9 TL 45fc 15-23% 0% 0% Stream 14 15 * AC 60 10-23 TL 182fd 2-9% 0-10% 0-6% Stream 14 15 * AC 60 Large 503 X 26% X X X X Xcv Canal 14-21 * AC 60 S11-27 TL 50 0%cu Test tank 475-550 11-13 * AC 60 S4-36 SL 46 4% Test tank 1494? 16-18 = AC 60 0 = 46 FL 12 58% 25% Test tank 100-121 9-13 = AC,100V 60 S0 = 48 FL 12 75% 58% Test tank 100-121 9-13 = AC,400V 60 S16-26 375 3%dh 2%dh di Hatch. raceway 10 6 * AC,350V 300 16-26 375 2%dh 2%dh di Hatch. raceway 10 6 * AC,700V 300 16-26 375 1%dh 1%dh di Hatch. raceway 10 6 * AC,760V 250-300 4-36 SL 91 0% Test tank 1494? 16-18 = DC Large −35%ck <18% Test tank = DC 0 = 46 FL 18 33% 28% Test tank 100-121 9-13 = DC,100V 0 = 47 FL 18 22% 28% Test tank 100-121 9-13 = DC,400V 0 = 40FL 12 0%es 17% Hatch. raceway 103 11 * DC,200V 0 = 32FL 18 11% 47% Stream 30 7 * DC 0 = 34FL 33 3% 0% Stream 30 7 * DC −19 TL 48 2%cx Hatch. raceway 306 @25C * DC −20 TL 16 13%ct 6% Hatchery 6-10 * DC −12 60 0-13%dw Lab shock tank <450dw 13-21 = DC −14-48FL 32/23cl 0%cl 3%cl 4%cl 4%cl Hatch. raceway 80 9-11 * DC,300V −14-48FL 25/29cl −3%cl 8%cl 14%cl 17%cl Hatch. raceway 80 9-11 * DC,400V ~15-39FL 241 5%er 0% 12% Rivers 260 13-16 * DC 30-43 TL 56 13%bk,bl 25%bk,bl Rivers 300-320 6-12 * DC 25-47 TL 21 5%bk 0%bk Rivers 177 10-15 * DC 27-54 TL 28 18%bk,bm 11%bk,bm Rivers 540 18 * DC 0 = 39 TL 50 6% 2% River ~450 10 * DC 0 = 31 TL 20 0 15% 0 0 Test trough 530 18 = DCgi 0 = 31 TL 20 0 20% 0 0 Test trough 530 18 = DCgj 0 = 32 TL 20 0 30% 0 0 Test trough 530 18 = DCgk 0 = 30 TL 20 0 10% 0 10% Test trough 530 18 = DCgl 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

hemor. 

33-43 TL 47 30%bk,bw 72%bk,bw Rivers 300-320 6-12 * Hybridbz 60 <8? 1⁄2 ~15-39FL 309 1%er 1% 40% Rivers 260 13-16 * Hybridbz 60 1⁄2 −20-55TL 152 26%bx 63%bx Rivers 540-900 7-18 * Variousbx Yoy-ad X v Rivers, CRB <25?-1000 0-19 * PDC 20-68 FL 72/32z 14%aa 24% 75%ab 75%ab Rivers 70 6.3 * PDC 4-36 SL 1641 0.3% Test tank 1494? 16-18 = PDC 33%bg 

X?g X?g Rivers etc, CRB * PDC 

0 X 0 Rivers, CRB 500-800 15-25 * PDC 

−0.01% X Rivers, CRB 300-2000 5-25 * PDC 

Juv, 19TL 825 7%?bi Test tank 143-172 11-13 =bi PDC 5 60 SYoy, 5TL 954bj 10%?bj Test tank 114-132 9-11 =bj PDC 8 40 S>30 TL 38 3% Rivers, CRB 600-800 9-11 * PDC 15 4 SAdults 40 3%ae Lab. trough 572 12 = PDC 15 5 S0 = 38 TL 50 20% 22% River ~450 10 * PDC 15 S0 = 31 TL 20 0 0 0 0 Test trough 530 18 = PDCgi 15 4 S0 = 31 TL 20 0 0 0 0 Test trough 530 18 = PDCgj 15 4 S0 = 31 TL 20 0 25% 0 5% Test trough 530 18 = PDCgk 15 4 S0 = 31 TL 20 0 40% 0 0 Test trough 530 18 = PDCgl 15 4 S0 = 41 FL 12 25%es 58% Hatch. raceway 103 11 * PDC200V 20 12.5 S0 = 42 FL 12 17%es 25% Hatch. raceway 103 11 * PDC200V 20 37.5 S0 = 36FL 20 15% 57% Stream 30 7 * PDC 25 30 S>30 TL 38 24% Rivers, CRB 600-800 9-11 * PDC 30 4 S−14-48FL 27/23cl 0%cl 4%cl 22%cl 35%cl Hatch. raceway 80 9-11 * PDC 30 4.2 S0 = 40 FL 12 25%es 33% Hatch. raceway 103 11 * PDC200V 30 16.7 S0 = 41 FL 12 58% 42% Test tank 100-121 9-13 = PDC100V 30 16.7 S0 = 39 FL 12 33% 50% Test tank 100-121 9-13 = PDC400V 30 16.7 S0 = 40 FL 12 42% 58% Test tank 100-121 9-13 = PDC100V 30 25 S0 = 40 FL 12 42% 33% Test tank 100-121 9-13 = PDC400V 30 25 S14-40 TL 34ch 35%ch Rivers 340-350 7-8 * PDC 40 5 S16-31 TL 11ci 0%ci Rivers 340-350 7-8 * PDC 40 5 S14-32 240/4du 1-3%du 0%du River, scap net <250? 11-12 * PDC 45 1.6 S28-61 75 0%dv River, scap net <250? 11-12 * PDC 45 1.6 SLarge −35%ck <20-60% Test tank = PDC 20-60 30-56 TL 99 44% ad Rivers, CRB 600-800 9-11 * PDC 60 4.2 E30-56 TL 55 67% ad Rivers, CRB 600-800 9-11 * PDC 60 4.2 1⁄4 0 = 30 TL 20 0 50% 0 5% Test trough 530 18 = PDCgm 60 4 1⁄4 26-43 TL 50cc #8%cc 60% Rivers 299 * PDCcd 60?cd 7cd 1⁄430-36 TL 9 78%?cf 78% Rivers 600-616 * PDCcf 60 7 1⁄4
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

~15-39FL 316 0.3%er 2% 54% Rivers 260 13-16 * PDC 60 8 1⁄2 20-47 TL 50 18% 28% Rivers 257 6 * PDCeu 60 8 1⁄2 21-48 TL 51 55% 65% Rivers 114 3 * PDCev 60 8 1⁄2 13-51 TL 53ew 64% 43% Rivers 134 6 * PDCew 60 8 1⁄2 14-45 TL 48 6% 13% Rivers 270 7 * PDCex 60 8 1⁄2 22-50 TL 43 40% 49% Rivers 90 2-4 * PDCey 60 8 1⁄2 14-43 TL 52ez 22% 40% Rivers 145 1-5 * PDCez 60 8 1⁄2 Adults?cn #0.5% Streams <100-200 <10-15 * PDCco 60?co 8co 1⁄2 Adults?cn <1-5% <1-5% Streams <100-200 <10-15 * PDCcp 60?cp 8cp 1⁄2 24-45 TL 39bu 59%bk,bu 87%bk,bu Rivers 33-55 4-6 * PDC 60 8 1⁄2 24-48 TL 23 52%bk,bv 61%bk,bv Rivers 150-175 13 * PDC 60 8 1⁄2 31-43 TL 45 42%bk,bt 69%bk,bt Rivers 300-320 6-12 * PDC 60 ~16 1⁄2 23-53 TL 54 50%bk,bn 59%bk,bn Rivers 880-900 7-11 * PDC?bn 60?bn 8?bn S0 = 30 TL 20 0 0 0 0 Test trough 530 18 = PDCgi 60 4 S0 = 30 TL 20 0 45% 0 5% Test trough 530 18 = PDCgj 60 4 S0 = 30 TL 20 0 65% 0 0 Test trough 530 18 = PDCgk 60 4 S0 = 30 TL 20 Xgn 10% 75% 0 0 Test trough 530 18 = PDCgl 60 4 S0 = 31 TL 20 30% 95% 0 15% Test trough 530 18 = PDCgm 60 4 S>30 TL 116 43% Rivers, CRB 600-800 9-11 * PDC 60 4 S>30 TL 60 43% Rivers, CRB 600-800 9-11 * PDC 60 4 S>30 TL 23 65% Rivers, CRB 600-800 9-11 * PDC 60 4 SAdults 33% ae Lab. troughae 572? 12? = PDC 60 4 SAdults 3% af Lab. troughaf 572? 12? = PDC 60 4 SAdults 30 18% Hatch. raceway 60 PDC 4.2 S30-56 TL 55 44% ad Rivers, CRB 600-800 9-11 * PDC 60 4.2 S41-52 TL 30 13%bk 57%bk Streams 195 6 * PDC 60 8? S30-43 TL 42bo 67%bk,bo 91%bk,bo Rivers 33-34 4-6 * PDC 60 8? S26-53 TL 46 68%bk,bp 59%bk,bp Rivers 880-900 7-11 * PDC 60 8? S0 = 48 FL 12 58% 50% Test tank 100-121 9-13 = PDC100V 60 8.3 S0 = 46 FL 12 42% 33% Test tank 100-121 9-13 = PDC400V 60 8.3 S0 = 39 FL 12 8%es 67% Hatch. raceway 103 11 * PDC200V 60 8.3 S0 = 39 FL 102 35%et 25% Test tank 95-104 10-12 * PDC250V 60 8.3 S15-35 TL 628fa 8.5% 70% Test tank 283 10-12 = PDC150V 60 S17-35 TL 184fb 1.7% 41% Test tank 226 8 = PDC300V 60 S(30-120g) 120 0%fv Test tank 240-270 15-20 = PDC200V 60 5 (50-600g) 15 0% Test tank 240-270 7 = PDC200V 60 5 (~100g) 5 0% Test tank 240-270 7 = PDC400V 60 5 (50,100g) 30 0% Test tank 240-270 7 = PDC600V 60 5 (~600g) 15 10-20% Test tank 240-270 7 = PDC600V 60 5 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. hemor. Other injuries subjected to electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

duration 

(ms) 

Where fish were

Pulse 

19-20 TL 102 2%?bh 39%bh Hatch. raceway 242 @ 25C * PDC 60 −35 0 1ac Rivers, CRB * PDC 60 Adults? e 50% 0? 50%? Rivers etc,CRB <50->5000 >0->30 * PDC 60 −14-48FL 26/17cl −0%cl 58%cl 35%cl 53%cl Hatch. raceway 80 9-11 * PDC 90 1.4 SI+& older 1,014ff 2-6% Stream (c) 34-63 <12-18 * PDC 100 5 S13-26 TL 30 0% cu Test tank 500-550 11 * PDC 120 8 1⁄2 28-37 TL 44 5-29%bk Hatchery 238 4 * PDC 250 S~33 TL 50/44ep 8%ep 0%ep 0%ep 0%ep 0%ep Test tank 270-340 8 0 PDCeq 250 4? 1⁄2 >30 TL 37 62% Rivers, CRB 600-800 9-11 * PDC 512 0.2 S>30 TL 43 9% Rivers, CRB 600-800 9-11 * CPS 240/15 2.6/11 S>30 TL 41 7% Rivers, CRB 600-800 9-11 * CPS 240/15 2.6/11 SAdults 30 6% Hatch. raceway CPS 240/15 2.6/11 S0 = 31 TL 20 0 0 0 15% Test trough 530 18 = CPSgi 240/15 2.6/11 S0 = 31 TL 20 0 10% 0 0 Test trough 530 18 = CPS 

gj 240/15 2.6/11 S

0 = 31 TL 20 0 15% 0 CPSgk 0 Test trough 530 18 = 240/15 2.6/11 

S

0 = 30 TL 20 0 60% 5% 0 Test trough 530 18 = CPSgl 240/15 2.6/11 S30-41 TL 4 cg 50% Rivers 600-616 * CPS 240/15 2.6/11 S16-39 TL 51ci 12% Rivers 340-350 7-8 * CPS 240/15 2.6/11 S40-50 TL 30 10%bk 77%bk Streams 195 6 * CPS 240/15 2.6/11 S30-43 TL 12bq 17%bk 25%bk Rivers 35-55 4-6 * CPS 240/15 2.6/11 S20-55 TL 44 43%bk,br 34% 

bk,br Rivers 540 18 * CPS 240/15 2.6/11 S

23-43 TL 26 4%bk,bs 27% 

bk,bs Rivers 158 10-12 * CPS 240/15 2.6/11 S

0 = 38 TL 50 2% 4% River ~450 10 * CPS 240/15 2.6/11 SLarge −35%ck <18% Test tank = CPS 240/15 2.6/11 S0 = 41 FL 12 17% 33% Test tank 100-121 9-13 = CPS 100V 240/15 2.6/11 S0 = 39 FL 12 25% 50% Test tank 100-121 9-13 = CPS 400V 240/15 S

2.6/11 

0 = 38 FL 12 8% CPS 200V es 8% Hatch. raceway 103 11 * 240/15 2.6/11 S0 = 40 FL 8 0% 13% Stream 30 7 * CPS 240/15 2.6/11 SAdults 8% * 

fl Anesthesia tank CPS 240/15 2.6/11 S

Adults 1,015 0% * 

fm Anesthesia tank 4.5 CPS 240/15 2.6/11 S

Adults? X?g X? * 240/15 

g Rivers etc,CRB CPS 2.6/11 S

Oncorhynchus tshawytscha, chinook salmon 0 = 7.9FL ~90 0% Lab test tank 39 = 

ga 12 ACgb 60 20 S

0 = 7.9FL ~60 10% Lab test tank 39ga 12 = ACgc 60 20 S0 = 7.9FL ~30 39% Lab test tank 39 20 ga 12 = ACgd 60 S0 = 7.9FL ~30 57% Lab test tank 39ga 12 = ACge 60 20 S0 = 7.9FL ~90 39ga 0% Lab test tank 12 = AC 

gf 60 20 S

0 = 7.9FL ~60 39ga 62-69% Lab test tank 12 = AC 

gg 60 20 S

0 = 7.9FL ~60 67-79% Lab test tank 39ga 12 = AC 

gh 60 20 S
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. hemor. Other injuries subjected to electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

duration 

(ms) 

Where fish were

Pulse 

Yoy, 7 TL 1843 0-77% −50 

dm Lab shock tank 10-20 = PDC 2 20 S

Yoy, 7 TL 499 2-59%dn Lab shock tank 50 & 85 10-20 = PDC 2 20 S4-6 TL 1441 Xdo 0-57%do Lab shock tank −48 10-20 = PDC 2 20 S6-8 TL 889 X PDC 2 20 do 1-78%do Lab shock tank −48 10-20 = S8-10 TL 201 X Lab shock tank PDC 2 20 do 6-63%do −48 10-20 = S10-12 TL 269 X Lab shock tank 2 do 5-58%do −48 10-20 = PDC 20 S4-10 TL 168 0-1% Lab shock tank dp 43 10-20 = PDC 2 20 S4-12 TL 323 0-7% Lab shock tank dp 55 10-20 = PDC 2 20 S4-12 TL 269 0-15% Lab shock tank dp 83 10-20 = PDC 2 20 S6-10 TL 123 9-10% Lab shock tank dp 133 10-20 = PDC 2 20 S4-12 TL 215 0-13% Lab shock tank dp 180 10-20 = PDC 2 20 S4-12 TL 242 dp 4-58% Lab shock tank 283 10-20 = PDC 2 20 S4-12 TL 52 8-75% Lab shock tank dp 483 10-20 = PDC 2 20 S4-10 TL 93 0% Lab shock tank dp 53 20-25 = PDC 2 20 S4-12 TL 115 0-18% Lab shock tank dp 68 20-25 = PDC 2 20 S4-8 TL 83 0-3% Lab shock tank dp 78 20-25 = PDC 2 20 S4-12 TL 454 0-18% Lab shock tank dp 103 20-25 = PDC 2 20 S4-12 TL 283 4-28% Lab shock tank dp 128 20-25 = PDC 2 20 S4-12 TL 204 10-44% Lab shock tank dp 195 20-25 = PDC 2 20 S4-12 TL 174 15-59% dp Lab shock tank 303 20-25 = PDC 2 20 S4-10 TL 105 49-67% dp Lab shock tank 483 20-25 = PDC 2 20 S4-6 TL 315 0-26%dq Lab shock tank −50? 14 = PDC 2 20 S6-8 TL 449 0-53%dq Lab shock tank −50? 14 = PDC 2 20 S8-10 TL 154 0-50% 14 dq Lab shock tank −50? = PDC 2 20 S4-6 TL 132 0-55% 18 dq Lab shock tank −50? = PDC 2 20 S6-8 TL 678 6-52% 18 dq Lab shock tank −50? = PDC 2 20 S8-10 TL 294 10-80% 18 dq Lab shock tank −50? = PDC 2 20 S9 TL 446 4-58% 12-20 dr Lab shock tank 45-65 = PDC 2 20 S9 TL 412 0-38% 20 dr Lab shock tank 45-65 12-20 = PDC 8 S9 TL 260 0-53%ds Lab shock tank 50 10-16 = PDC 8 20 S9 TL 192 4-38% S

ds Lab shock tank 50 10-16 = PDC 8 80 

4-6 TL 502 0-4% S

dt Lab shock tank 38-68 10-20 = PDC 3-8 20 

6-8 TL 317 3-6%dt Lab shock tank 38-68 10-20 = PDC 3-8 20 S8-10 TL 82 8-50% 20 

dt Lab shock tank 38-68 10-20 = PDC 3-8 S

10-12 TL 46 6-50% 3-8 20 S

dt Lab shock tank 38-68 10-20 = PDC 

4-6 TL 489 9-29% 10-20 10-15 20 

dt Lab shock tank 38-68 = PDC S

6-8 TL 548 0-28% 10-20 10-15 20 

dt Lab shock tank 38-68 = PDC S

8-10 TL 89 48-71% dt Lab shock tank 38-68 10-20 = PDC 10-15 20 S10-12 TL 27 73-75% 20 S

dt Lab shock tank 38-68 10-20 = PDC 10-15 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Shortterm Long- term Brands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp

Current 

type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

0 = 9 FL 71 16-25% Circular tank 73 13 * PDC500V 120 Prosopium cylindraceum, round whitefish Juv.-ad. 0 X Streams 25-200 5-15 * PDC? Prosopium williamsoni, mountain whitefish 

X Rivers eo * all 

0 = 36 TL 51 River * 0% 6% ~450 10 DC 0 = 35 TL 50 * 2% 18% River ~450 10 PDC 15 S0 = 36 TL 50 10 * 0% 2% River ~450 CPS 240/15 2.6/11 SSalmo trutta, brown trout 5-30 −0ag Stream (box) * −19 TL 40 20% Hatch. raceway 306 @25C AC cw * −19 TL 49 4% Hatch. raceway 306 @25C cx * DC 

eh 20 0-6%eh Stream, dip net * DC,500V eh 20 eh 0-17% Stream, dip net * DC,400V ei 

30-51 TL 50 8%by 6%by Rivers 300 6-12 * DC 31-45 TL 50 32%ca 28%ca Rivers 300 6-12 * Hybridbz 60 <8? 1⁄2 Yoy-ad X , CRB -1000 v Rivers <25? 0-19 * PDC 

−0.01% X Rivers, CRB 300-2000 5-25 * PDC 0 X 0 Rivers, CRB 500-800 15-25 * PDC 

eh 20 0-86% Stream, dip net eh * PDC ej Eeh 20 0-93% Stream, dip net eh * PDC ek Eeh 20 0-50% Stream, dip net eh * PDCel 5 66 S

17-38 TL 31ch 26%ch Rivers 340-350 7-8 * PDC 40 5 S17-41 TL 34ci ci 15% Rivers 340-350 7-8 * PDC 40 5 S

e 50% 0? 50%? Rivers etc,CRB <50->5000 >0->30 * PDC 60 

Adults?cn #0.5% Streams <100-200 <10-15 * PDCco 60co 8co 1⁄2 Adults?cn cp <1-5% <1-5% Streams <100-200 <10-15 * PDC 60cp 8cp 1⁄2 17-39 TL 56eu * PDCeu 25% 30% Rivers 257 6 60 8 1⁄2 20-39 TL 33 * PDCev 18% 24% Rivers 114 3 60 8 1⁄2 17-49 TL 64 * PDCew 52% 45% Rivers 134 6 60 8 1⁄2 10-43 TL 56ex 27% 30% Rivers 270 7 * PDCex 60 8 1⁄2 10-40 TL 55ey 31% 13% Rivers 90 2-4 * PDCey 60 8 1⁄2 14-42 TL 53 38% 28% Rivers 145 1-5 * PDCez 60 8 1⁄2 30-56 TL 50 36%cb cb 56% Rivers 300 6-12 * PDC 60 16 1⁄2 17-51 TL 28cc #4%cc Rivers * PDCcd 86% 299 60?cd 7 d 1⁄428-54 TL 17 35%? Rivers cf 82% 600-616 * PDC 60 7 1⁄4

eh 20 27-89% Stream, dip net eh * PDCem 90 −11 1⁄2 

I+& older 1,600ff Stream (c) 3-12% 34-63 <12-18 * PDC 100 5 S
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Short- LongBrands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

term term 

Current 

13-59 TL 8 cg 25% Rivers 600-616 CPS * 240/15 2.6/11 S16-39 TL 59 Rivers CPS ci 14% 340-350 7-8 * 240/15 2.6/11 SSalvelinus confluentus, bull trout 0 = 18 50 0%fu 0%fu Test tank 219 9 = DC,130V Juv.-ad. Xah 0 Streams 25-200 5-15 * PDC? 0 = 18 50 0%fu 0%fu Test tank 219 9 = PDC145V 60 0 = 18 50 100% Test tank 219 9 = PDC293V 60 Salvelinus fontinalis, brook trout ?9-24TLcq 70 13%cr 6%cr Stream 440 11 * AC,125V 250-300 2?9-24TLcq 78 28%cr 19%cr Stream 64 11 * AC,350V 250-300 2?9-24TLcq 65 9% 250-300 cr 8%cr Stream 50 10 * AC,350V 2?9-24TLcq 68 21% 250-300 cr 18%cr Stream 43 8 * AC,400V 29-12 TL 117 16% Streams 43-64, 440 8-11 * AC 250-300 213-17 TL 130 32% Streams 43-64, 440 8-11 * AC 250-300 218-24 TL 34 41% Streams 43-64, 440 8-11 * AC 250-300 2−25 TL 11%cw 47 Hatch. raceway 306 @25C * AC 12- 0.5%dh 24 375 2% 

dh di Hatch. raceway 10 6 * AC,350V 300 

12-24 di 375 0.2%dh 2%dh Hatch. raceway 10 6 * AC,700V 300 12-24 375 0.2% 10 dh 1%dh di Hatch. raceway 6 * AC,760V 250-300 −25 TL 50 0% Hatch. raceway 306 @25C cx * DC Juv.-ad. X 0 Streams 25-200 5- 15 * PDC? Yoy-ad X v Rivers, CRB <25?-1000 0-19 * PDC ?9-24TLcq 68 7%cr 3% 440 11 cr Stream * PDC200V 60 S?9-24TLcq 95 26% Str 11 cr 21%cr eam 64 * PDC400V 60 S?9-24TLcq 72 8% 50 10 cr 10%cr Stream * PDC500V 60 S?9-24TLcq 63 cr 11% 43 8 cr 10% Stream * PDC500V 60 S9-12 TL 128 12% Streams 43-64, 440 8-11 * PDC 60 S13-17 TL 128 26% Streams 43-64, 440 8-11 * PDC 60 S18-24 TL 42 43% Streams 43-64, 440 8-11 * PDC 60 SI+& older 731ff Stream (a) <12- 5-14% 34-63 18 PDC * 100 5 SI+& older 360fg 0-2% Stream (a) 34-63 <12-18 * PDC 100 5 SI+& older 1,304ff 4-23% Stream (b) 34-63 <12-18 PDC * 100 5 SI+& older 275fg 0-3% Stream (b) 34-63 <12-18 PDC * 100 5 SI+& older 2,544ff <12- 2-6% Stream (c) 34-63 18 * PDC 100 5 SI+& older 2,122 0.5% Streams * PDC 100fi 5fi SSalvelinus malma, Dolly Varden Juv.-ad. high lowx high Streams 5- 15 * AC 60 Salvelinus namaycush, lake trout Adultsbf 0 0 0 Lakes 1-5 * PDC 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Short- LongBrands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

term term 

Current 

Thymallus arcticus, Arctic grayling 37-45 TL 25 0% 0% Streams 33 6 * DC 39-43 TL 25 s 0% 4% Stream 33 6 * Hybrid 1⁄2 

bz 60 <8? 

Juv.-ad. X 0 Streams 25-200 5-15 * PDC? 12-37 FL 88 3%ai 16%aj 5% 63% * 40-120 

ak 27%ak Rivers 60-88 9-15 PDC 3.3-10 

23-39 FL 616/60z 1%(+?)al 27%am 62%an 25%an Rivers 39-40 9-11 * PDC 80 5 20-41 FL −0% ao Rivers, lakes >39 8-11 * PDC 80 5 20-32 FL 103/60z 4% 10-11 PDC ap 0% 0-3%aq 5%aq Rivers 80-88 * 80 5 

Gadidae 

Lota lota, burbot 3-30? <25->50% Lakes, littoral 2-18 

go 37-61 * DC

Juv.-ad. 0 0 Streams 25-200 5- 15 * PDC? 

Gasterosteidae 

Gasterosteus aculeatus, threespine stickleback 

Juv.-ad. high low 60 

x high Streams 5-15 * AC

Cottidae 

(unspecified species) 

 oftenax oftenax Riffles <100 * PDC 

Cottus bairdi, mottled sculpin 3-9 SL 90 0-3%dj Streams 5-8 10-15 * DC 4-9 SL −57 0-12% Streams 14-16 dj 10-15 * DC 4-8 SL 50 35-60% dk Exp. channel 12-14 * DC 

Centrarchidae 

Lepomis cyanellus, green sunfish 7-13 TL 51 10% dy Ponds >66, <520 0-4 * AC 180da Lepomis gibbosus, pumpkinseed 6-20 TL 174 0-1%dz 180da 

Lakes >66, <520 13-28 * AC 

67-20 TL 100 ea 180da 

2-12% Lakes >66, <520 16-28 * AC 

7-12 TL 352 9%dy 180da 

Ponds >66, <520 0-4 * AC 

0 = 11 TL 55,

14fe

 0% 

0% 

0% 0% C 

0% 

Lakes 122-789 * PD 30, 60,

120 

E

Lepomis macrochirus, bluegill 8-10 525 Hatch. ponds 167 AC,115V dc 1-19%dd 3-7%dd * 60 8-10 525 dc 1-58%dd 9-16% 167 AC, dd Hatch. ponds * 230V 180da 8-10 1200 0-75% 167 AC, cz Hatch. ponds * 230V 180da 8-10 25 4% AC, db Hatch. ponds 100 * 230V 180da 8-10 25 18% AC, db Hatch. ponds 200 * 230V 180da 8-10 25 24% AC, db Hatch. ponds 250 * 230V 180da 8-10 25 40% AC, db Hatch. ponds 333 * 230V 180da 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or 

develop. 

intervals 

No. 

obs. 

Delayed 

equil., 

recov. 

Short- LongBrands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp type 

Frequency 

(Hz) 

Pulse 

duration 

(ms) 

term term 

Current 

8-10 25 Hatch. ponds 500 * AC,230V 180da 

54% 

db

8-10 25 Hatch. ponds 1000 * AC,230V 180da 

56% 

db

5-18 TL 44 Lakes >66, <520 13-18 * AC 180da 

0-0% 

dx

5-20 TL 102 Lakes >66, <520 22-26 * AC 180da 

2-2% 

eb

7-12 TL 166 Ponds >66, <520 0-4 * AC 180da 

1% 

dy

8-10 dc 1-29%dd 0-3%dd 525 Hatch. ponds 167 * DC,115V 9-17 aw 0-55%aw Test tank 154 10 = PDC 1.6 9-17 aw 0-77%aw Test tank 154 10 = PDC 8.8 9-17 aw 0-94%aw Test tank 154 10 = PDC 16 0 = 13 TL 

142fe

∃5% 0% ∃1fe

0% 

∃5%fe

0% 

 Lakes 122-789 * PDC 30,60, 

120 

568, 5% E

0 = 15 TL 5 0% 0% Reservoir * PDC 60 S15 TL 1 0 1 Reservoir * PDC 80 SMicropterus spp., black basses 

e 0 0 0? Rivers etc, CRB <50->5000 >0->30 * PDC 60 

Micropterus dolomieu, smallmouth bass 0 = 21 TL 145, 

29fe

 1 ∃4% 

14% 

0% 

0%fe

∃4% 14%fe

 122-789 * PDC 30,60, 

120 

 E

Lakes

Micropterus salmoides, largemouth bass 13-25 100/70 −0%dg 0%dd Hatch. ponds * AC,230V 180da 

 167 

 30 −0%ec Lakes * AC 180da 

>66, <520 

0 = 21 TL 454, 

114fe

 1% 1 0% 

1%fe

∃1% Lakes 122-789 * PDC 30, 60, 

120 

 E

2% 

fe

0 = 17 TL 13 0% 8% Reservoir * PDC 60 S0 = 24 TL 3 0% 0% Reservoir * PDC 60 S0 = 32 TL 4 0% 0% Reservoir * PDC 60 S∃20 TL 100's 0% few Reservoirs * PDCfw ~62,73 3.5,8.7 S0 = 34 TL 6 0% 33% Reservoir * PDC 80 SPomoxis annularis, white crappie 15 TL 1 0 Reservoir * PDC 60 S

0

0 = 15 TL 3 0% 0% Reservoir * PDC 60 S19 TL 1 0 Reservoir * PDC 60 S

0

18 TL 1 0 0 Reservoir * PDC 80 SPomoxis nigromaculatus, black crappie 16 TL 0 1 0 Reservoir * PDC 80 S

Percidae 

Etheostoma flabellare, fantail darter 2.5-7.5 aw 0-69%aw Test tank 154 10 = PDC 1.6 2.5-7.5 8.8 aw 0-76%aw Test tank 154 10 = PDC 2.5-7.5 aw 0-95%aw Test tank 154 10 = PDC 16 
Species Adverse Effects Environment Electric Field and Current 

 Mortality Spinal Injuries 

Spine/Vertebrae 

Lengths 

(cm) or

develop.

intervals 

No. 

obs.

Delayed 

equil., 

recov.

Short- LongBrands Extern. Intern. Intern. hemor. Other injuries

Where fish were

subjected to 

electric field 

Conductivity 

(ΦS/cm) 

Temp.

(ΕC) 

Fd

tp type 

Frequency

(Hz) 

Pulse 

duration 

(ms)

term term 

Current

Perca flavescens, yellow perch 5-10 TL 40 0-0%ed Xed 

Lakes >66, <520 10-15 * AC 180da 

8-13 TL 66 X 

0-0%ee ee Lakes >66, <520 6-8 * AC 180da 

10-27 TL 52 

0-9%ef Lakes >66, <520 7-13 * AC 180da 

9-19 TL 16 * AC 

94% ed Lakes >66, <520 19-27 180da 

20 TL 1 0 1 Reservoir * PDC 60 S0 = 25 TL 11 0% 0% Reservoir * PDC 80 SStizostedion vitreum, walleye 

13 −0%ec 180da Lakes >66, <520 * AC 

33-44 TL 5 40% Lakes * PDC? 19-48 TL 12 25% Rivers 17,800? 30 cs 26 * PDC 

e 0 0 0? Rivers etc,CRB <50->5000 >0->30 * PDC 60 

33? TL 1 0 0 Rivers 600 11 * PDC 60 8? S0 = 42 TL 6 0% 17% Reservoir * PDC 60 S0 = 20 TL 24 0% 21% Reservoir * PDC 60 S0 = 21 TL 2 0 0 Reservoir * PDC 60 S40 TL 1 0 0 Reservoir * PDC 80 S18-43 TL 13 31% Lakes 15,300?cs 22 * PDC 120 Stizostedion canadense, sauger∃47 TL 20 0% 0% Rivers −500 7 * PDC 60 8? S32-53 TL 6 0% 0% Rivers 600 11 * CPS 240/15 2.6/11 S

Sciaenidae 

Aplodinotus grunniens, freshwater drum30-43 TL 7 0% 0% 0% 0% Rivers 351-425 12-15 * PDC 6.3 fy 40 S0 = 32 TL 2 0 0 Reservoir * PDC 60 S

General 

(unspecified families or species) 

Xay X Rivers, reserv. * all 

Juv-ad bd 0 be Rivers etc, CRB 250-3500 * PDC,DC 

>0.1% PDC az Rivers, streams <100 * 

bb 0? 0 Rivers, CRB * PDC bc

−0.01% 0 Rivers, CRB 300-2000 5-25 * PDC 

4-96 ba X X Rivers etc, CRB * PDC 4-96 ba X X Rivers etc, CRB * CPS 240/15 2.6/11 S

Footnotes:aAbbreviations in and comments on table headings: Develop. = Developmental; No. Obs. = number of fish observed (i.e., examined, X rayed, or necropsied); D
equilibrium or recovery; Short-term mortality is immediate to several days; Brands are sometimes referred to as burns or bruises and usually indicate spinal damage; 

damage other than brands (e.g., bent back); Intern. = internal observations of damage to the spine or vertebrae based on X-ray analysis or necropsy (regardless of s

internal hemorrhages observed along or in tissues around the spine during necropsy (regardless of severity rating); ΦS/cm = microsiemens per centimeter; Temp. = te

Fd tp = field type (heterogeneous or homogeneous); Hz = hertz; ms = milliseconds; pc = personal communication; um = unpublished manuscript. 
Footnotes (continued) 

d ithin a 6-week period, at least 7-d apart; they were acclimated to test conductivities 2.5-7 d prior to t

exposure.

n to the water surface. ahBrbAbbreviations in body of table: TL = total length; FL = fork length; juv. = juveniles; ad. = adults; yoy = young-of-the-year juveniles; X = one or more observations 

approximately; "<" = less than; ">" = greater than; Lab. = laboratory; CRB = Colorado River Basin; reserv. = reservoirs; Hatch. = hatchery; Exp. = experimental; "gradient diminishes with distance from electrodes); "=" = homogeneous field (voltage gradient constant); AC = alternating current; DC = (continuous) direct curreCPS = trademark for a specific pulse train by Coffelt Manufacturing, Inc. (complex pulse system–trains of three 1.6-ms, 240-Hz pulses at 15 Hz); Sq. = square; Exposine; 1⁄2 Sine = half sine (rectified AC); pc = personal communication; um = unpublished manuscript.

cMortalities due to notochords that were "completely blown apart." 

Specimens were subjected to 2 to 4 5-s electric field exposures w

eFish were captured and filleted for contaminants analyses.fOnly injury was considered a freak accident–specimen was trapped between a plate cathode and the boat.gHemorrhaging (brands?) seemed proportionately greater in Grand Canyon electrofishing using CPS system than in Upper Colorado River Basin using a more typical PDhOnly 1 injury was observed, a brand on a specimen suspected to have touched the cable anode; it and fish knocked out completely and floating on surface recover

release. Specimens that were subsequently recaptured or followed after radiotagging appeared normal.

iTwenty fish were captured by electrofishing, radiotagged, and followed for 4 months (except one that subsequently died of unknown causes).jLarge Colorado pikeminnow which contacted the anode bled at the gills and "everything"; a 112 mm specimen that was captured belly-up with gills still going and n

were removed for study and the remainder of the fish was preserved).

kLarge female with bruise (might be same fish reported by McAda pch); single and multiple recaptures appeared normal.lBrands usually observed in caudal region.

mTwenty fish were captured by electrofishing, radiotagged, and followed for 4 months (except two that were lost).

No visible external injuries or high mortality observed, but fish were typically "knocked out cold" and floated slowly

oMost were completely tetanized, "stiff as boards," and floated to the surface behind the boat.pInjuries from electrofishing in previous years–back half of fish stopped growing and the fish started to look like footballs.qSeveral unspecified injuries among salmonids; necropsied fish same as those reported by Krueger pc.rNo immediate mortality; short-term mortality among seined controls was higher (12%), but most mortality among electrofished specimens occurred on day 1 of the 

seining mortalities (controls) occurred on day 7.

sSpinal injury was higher (9%) and sometimes more severe among seined controls. Hemorrhage was nearly the same for seined controls (5%). None of the injuri

specimens were considered new electrofishing injuries because hemorrhages were not observed in association with specific vertebral damage and injuries appeared 

controls. However, hemorrhages may have cleared up during the 7-d holding period.

tOf the short-term mortalities, 10% were immediate; mortalities among seined controls, immediate and after 7 d, were as great or greater (8% and 42%); mortalities for 

7-d holding period, whereas most mortalities for electrofished specimens occurred during the first day.

uSpinal injuries, internal hemorrhages, and external hemorrhages were as great or greater among seined controls (2%, 19%, and 42%). No spinal injuries were considered 

. 

vBrands were hemorrhages immediately below and posterior to the dorsal fin and were observed on young-of-the-year as well as older fish

wYellowstone cutthroat trout.

xHigh percentage of fish recaptured 1 and 2 years later.

yOncorhynchus mykiss taken within 0.5 m of the electrode seemed likely to be injured.

zNumbers of fish observed for: short-term mortality and bruises/spinal damage and hemorrhages.aaOf mortalities during the 4-d holding period, 4% were immediate.abVertebral damage and hemorrhage data was based on the 10 mortalities and 22 of the survivors > 40 cm FL; of fish over 40 cm in length, 41% of the survivors suffe

suffered either major spinal injury or death.

acSpinal injury was posterior to dorsal fin; based on external examination only.adFish were frozen, X-rayed, and necropsied; those found to have spinal injuries also had associated internal hemorrhages and splintered bones from compression fract

were used as non-electrofished controls.

aeFish were held perpendicular to the lines of current in a plastic cage.afFish were held parallel to the lines of current in a plastic cage.agNegligible, only one mortality in a whole series of tests. Indices of fatigue, 20-240 s (mostly under 60 or 120 s), varied with electrofishing equipment, increased with f

decreased with successive exposures.

ands noted on larger specimens.

aiNo immediate mortality, but all deaths occurred within the first 4 hr.ajBrands disappeared by the end of the 7-d holding period; all branded specimens had associated spinal injuries.akBased on presence of both vertebral damage and hemorrhage in the same location, 17% (21% including mortalities) of the fish determined to have new electrofish

holding period, most hemorrhages may have already dissipated. Fish collected by hook & line (N=11) experienced no mortality, 18% vertebral damage, and no hemor

alAll short-term mortality was immediate; fish were not held beyond normal processing.amSeven fish with spinal injuries had associated brands.
ax

bp

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both.

bv

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both.

Footnotes (continued) anHe

morrhages were observed at the sa

me location as vertebral da

mage in 18% of the fish. Beach seine and hook and line collections yielded 2% im

mediate 

mortality

13% with internal he

morrhages.

aoLong-term

mortality was not significantly different fro

m that for other gear based on year-after recapture rates in a river and a lake. Growth rates for fish 30 cm a

different for fish after one year in the lake; data was insufficient to determine significance of growth differences for s

maller fish and for those from a river.

apShort-term, 7-d mortality was lower than for fish collected by other gear (6%); none was i

m

mediate.

aqSo

me spinal injuries and hemorrhages were greater (12% and 10% respectively) a

mong fish collected by beach seine or hook and line. Regardless of gear, vertebral d

observed in the same locations; however, he

morrhages 

may have begun to clear up prior to specimen analysis.

arMortalities within one 

month holding period a

mong electrofished (n=140) and control (n=70) speci

mens were nearly identical at 8% and 9%, respectively (5% of 

shocked uninjured fish)–no differences (death of all fish in one of five holding ponds was considered an accident and speci

mens were not included in analysis). N=17

exa

mined for spinal injury.

asShort-ter

m

mortality was i

m

mediate (no fish were held); 

mortality for controls was greater (2% for hook and line, 1% for gill nets, and 23% for trap nets) than for electr

atLong-term recapture rates were 9% for controls and 5

% for electrofishing. Long-term growth was not significantly different.

auIncidence of vertebral damage and hemorrhages was 1% and 3%, respectively, for fish taken by hook and line, fyke trap, and gill nets. Based on presence of both vert

the sa

me location, 13% of electrofished speci

mens were considered to have new electrofishing injuries.

avAll fish with he

morrhages also had obvious and severe vertebral da

mage.

aw

Mortality increased progressively with duration of exposure and pulse rate; 

mortality ranged fro

m low or negligible at or below 15 s and even 30 s at very low freque

16 Hz). Recovery time also increased with length of exposure. Fish were held parallel to the lines of current. 

Electrofished sculpins commonly flared their gills and often died. 

aySo

me fish required 5-10 

minutes for recovery of equilibriu

m. 

azMore than 5000 fish held overnight in baskets to assess 

mortality; most were sal

monids under 18 inches.

baNever observed death due to obvious injuries.bbFish in general, except as noted above for Cyprinus carpio and Oncorhynchus mykiss by Kinsolving pc.bcFish other than species listed above by Valdez pc.bdFish other than Upper Colorado River Basin sal

monids and endangered species listed above by Burdick pc.

beNecropsied fish same as those reported by Krueger pc.bfBrood stock: a 20-pound fe

male and 2 to 5-pound 

males. Eggs fertilized well and survival 

was high.

bgDuty cycle.bhDelayed mortality during 35-d holding period 

might have been short-term rather than long-ter

m but no im

mediate 

mortality and delayed 

mortality was less than for con

fishing (5% initial and 3% delayed). The authors specifically noted that brands resulted fro

m internal he

morrhages and possible breakage of the vertebral colu

mn

performance index the day after treatment was significantly lower for electrofished than control or fly-fished speci

mens.

biNo i

m

mediate mortality. Mortality for yearling juveniles over 1.4 yr was si

milar to controls (7% versus 8% for controls). Survival and growth to spawning, fecundity

and larvae) were similar for both exposed and control fish. Voltage gradient was 0.75 V/cm, exposure was 30 s, and 4-84% of the fish were narcotized by the described as having an exponential leading edge and a square trailing edge (square wavefor

m with a leading edge spike?).

bjNo i

m

mediate 

mortality. Data for the first 4 

months were not properly recorded and are unknown (< 10%); the nu

mber of exposed fish were reduced to 856 at that po

yr after exposure,

mortality was less than for controls (10% versus 16% for controls with nu

mbers of exposed fish again reduced to 236 for the last half year of obse

spawning, fecundity, and survival of offspring (eggs and larvae) were similar for both exposed and control fish. Voltage gradient was 1.0 V/cm, exposure was narcotized. 

Waveform pulses were described as having an exponential leading edge and a square trailing edge (square waveform with a leading edge spike?).

bkSi

milar X-rays of 104 gill-net-caught, fish-trap-caught, and hatchery Oncorhynchus mykiss revealed no vertebral da

mage; si

milar necropsy of 16 gill-net-caught and 5

mykiss revealed 13% and 4% with spinal-region he

morrhages, respectively, but all were minor (class 1 by Reynold's u

m 1992 classification).

bl30% injured overall, i.e., speci

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both.

bm18% injured overall, i.e., speci

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both.

bnAccording to Fredenberg's (1992) note on page 8 of his text, current was suspected to be PDC rather than DC as originally recorded and reported in Fredenberg's Appenbo98% injured overall, i.e., specimens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both; sample was a 

mi

and 7 Oncorhynchus clarki. 

78% injured overall, i.e., speci

bqSample was a 

mixture of 9 Oncorhynchus mykiss and 3 Oncorhynchus clarki. 

br55% injured overall, i.e., speci

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both.

bs31% injured overall, i.e., speci

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both.

bt73% injured overall, i.e., speci

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both.

bu90% injured overall, i.e., specimens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both; sample was a 

mi

and 8 Oncorhynchus clarki. 

65% injured overall, i.e., speci
ci

cm dc than 115-V, 60-Hz, single-phase AC).Footnotes (continued) bw64% injured overall, i.e., speci

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both; hybrid current 

(referred to as a half-pulse waveform by Fredenberg 1992).

bxAll branded fish (

N = 39) were X-rayed, necropsied, and found to have spinal injuries, 64% of which were classified as 

moderate to severe; a

mong unbranded fish 

damage or (and) spinal-region hemorrhages, 30% of which were classified as 

moderate to severe; fish were collected from the sa

me river using DC and PDC (60-Hz s

by10% injured overall, i.e., speci

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both.

bzHybrid current with PDC above half-voltage DC (referred to as a half-pulse waveform by Fredenberg 1992 and Dalbey et al. 1996).ca44% injured overall, i.e., speci

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both.

cb62% injured overall, i.e., speci

mens with only vertebral damage revealed by X-rays, only spinal-region hemorrhages revealed by necropsy, and both.

ccInitially, 150 fish were retained for post-electrofishing observation, but only 88 fish were recovered 7 d later due to holes in the in-river pen net.; of these 4 Oncorhy

died. None of the deaths were im

mediate. 50 Oncorhynchus mykiss and 28 Salmo trutta were exa

mined by X ray and necropsy.

cdCharacteristics of the PDC current used were uncertain due to a control box that was found to be "seriously out of calibration."ceAlso reported by Wyoming Ga

me and Fish Department 1990.

cfDeaths for fish taken by 60-Hz PDC occurred in the hatchery truck during transit and were probably due to stresses in addition to electrofishing (any occurrences 

reported); output voltage 280-300V.

cgNo i

m

mediate mortality.

chFish fro

m strea

m seg

ment electrofished with 4 passes. 

Fish fro

m strea

m seg

ment electrofished with only 1 pass. 

cjAlso reported by Meyer and Miller 1991 (abstract) and u

m 1991; output voltage for 40-Hz PDC 370-390V, possibly 1/4 Sine based on Meyer and Miller (1990), for 

CP

ckCo

mbined 

mortality for fish subjected to DC, PDC, and CPS during a 203 d holding period in a hatchery; most deaths occurred in the first 30 d (factors other than or in 

have been responsible).

clNu

mber captured and exa

mined for brands after first exposure (two passes through 5-m wide raceways) / number captured by a second electrofishing effort (again two 

for spinal he

morrhages or ano

malies later; controls showed no signs of spinal he

morrhages or ano

malies. Mortalities were for 7 d period after first exposure, N =−30 f

Brand data from

McMichael and Olson (um 1991), also 

mentioned in McMichael et al. 1991 (abstract); duty cycle 13% for PDCs (McMichael pc) and waveform squar

cnAccount covers and is recorded here under both Oncorhynchus mykiss and Salmo trutta. coWalk electrofishing using Coffelt VVP-2C, assu

med to output 60-Hz, 1⁄2-sine PDC.

cpBoat electrofishing using Coffelt VVP-2C, assu

med to output 60-Hz, 1⁄2-sine PDC.

cqOverall size range for fish collected from all four streams by all 

methods in study; size range for specific stream not reported; however, incidence of injuries overall 

separate entries in this table.

crAmong 46 fish captured by angling from stream with a conductivity of 440 

ΦS/c

m, 12% were deter

mined to have vertebral injuries (but no hemorrhages); h

he

morrhages were found in 39 speci

mens angled fro

m the other study strea

ms which had conductivities ranging from 43 to 64 

ΦS/c

m. Percentages of fish with eith

among all four streams ranged from 14% to 41% using AC and 9 to 41% using PDC.

csReported conductivity or units see

m unlikely–17.8 mS/cm for the river and 15.3 mS/cm for the lake.

ctFish were held for 10 d and 

mortality was delayed; comparable 

mortality for fish captured by seining (

N=16) and angling to exhaustion (

N=16) was 6% and 87% respec

cuBased on X rays, 4% of the fish were found to have vertebral anomalies before exposure to the electric current with no change afterward.cvOther injuries noted included he

morrhaging fro

m the gills or vent, he

morrhaging in the skin around the vent, dilated and clotted blood vessels in the region of the br

vent, and persistent paralysis suggesting injury to the nervous system.

cwMost of these 

mortalities were i

m

mediate (

mostly fish that escaped the scap net and suffered extensive exposure to the current); no 

mortalities occurred a

mong co

current at the beginning,

middle, and end of a 6-wk holding period by holding them in the field with a scap net for 15 s about 30 cm fro

m an electrode (positive electro

cxAll 

mortalities were delayed; no 

mortalities occurred a

mong a si

milar nu

mber of controls. Fish were exposed to the current at the beginning,

middle, and end of a 6-w

in the field with a scap net for 15 s about 30 cm from an electrode (positive electrode in DC).

cyIm

mediate 

mortality; i

m

mediate 

mortality a

mong controls (N=403) was 2%. Co

mparing treat

ments, square rather than half-sine waveforms, higher frequencies (

conductivities (67 

& 100 vs. 200 & 1000 :S/c

m) resulted in higher mortality. Fish were forced through an electrical array of 5 rows of electrodes with a cyclic patte

configurations that changed with every pulse. Fish were held for 30 d; except for frequency, patterns of differences among treatments for delayed mortality (gener

si

milar to those for i

m

mediate 

mortality but not significantly different from controls.

czMortality was highly correlated with exposure times from 1% at 1 s (0% at 4 s) to 75% at 300 s; for exposures ∃60 s, 

most mortalities were immediate, occurring d

holding period; for lesser exposures, 

most

mortalities 

were delayed but occurred within the first 2 h.

da3-phase, 230V AC; 3 sine wavefor

ms phase shifted by a third.

dbExposure ti

me was 60 s; 

most 

mortality occurred in the first 2 h and generally a third to a half was i

m

mediate.

Recovery ti

me for survivors increased with exposure time (and was greater for 230-V, 180-Hz, 3-phase AC

dd

Exposure times varied from 1 to 120 s; 

mortality increased with exposure time but did not correlate well with incidence of injury; incidence of injury was independent 

occurred during the first 2 h of the 24 h holding period.

deFish held parallel to a field of 6 V/cm for 60 s.
dt co

dw

m for 1, 2, 4, or 6 h, (N=15 plus 15 controls each), then held for 25 d; a

narcotized for 6 h; fish narcotized for 1 or 2 h recovered instantaneously whereas those narcotized for 4 or 6 h had difficulty resuming swi

m

ming activity for up to 

growth during the holding period did not differ from that of controls.

en 30 s; recovery delayed in some fish to beyond a 

minute at higher energy densities (125 and 188 

millijoules/c

m3

); longer fish, especially 

required longer recovery ti

mes; fish requiring 

more than 2 

minutes recovery ti

me frequently died.

Footnotes (continued) dfFish held perpendicular to a field of 6 V/c

m for 60 s.

dgN = 100 for 

mortality; N = 70 for spinal injuries. Exposure was varied fro

m 1-300 s and fish were held for 24 h. Only one 

mortality was recorded (60 s, delayed) a

handling rather than the electric current.

dh

Three or four passes; 15 d holding period; im

mediate 

mortality less than 1%; no 

mortality among controls. Data co

mbined for burns and other external signs of spina

on both Oncorhynchus mykiss and Salvelinus fontinalis; no external signs of injury observed among controls.

di2.5% of all 2250 Oncorhynchus mykiss and Salvelinus fontinalis exa

mined are esti

mated to experienced spinal injury (6 of 28 dead, 27 of 36 abnormal, and 23 of 218

from observation of one injury among 96 normal fish examined); no injured vertebrae a

mong controls.

djMortality not significantly different fro

m si

milar numbers of kick-seine "controls" (0-12% for fish collected in late winter at 5-8°C; 0-14% for fish collected in e

exposure at 600V; 30 d holding period.

dk

Mortality not significantly different from si

milar nu

mbers of repeatedly handled kick-seine "controls" (45-50%); probably

most affected by repeated handling; 

multipl

month at 600V; 

monitored between treat

ments.

dlLittle or no 

mortality during 30 d observation period.

dm

Mortality increased with exposure time (0.5-20 

min) and voltage gradient (1, 2, and 4 V/cm).

dn

Mortality increased with exposure time (2-10 

min) and less consistently with water conductivity (and thereby current density); voltage gradient was constant at 2 V/cm

doIm

mediate 

mortality (1 h observation period), 30 s exposure; 

mortality increased with voltage gradient fro

m the low value at 3-4 V/cm to the high value at 15 V/cm

2800 controls was 0.5%.

dpIm

mediate 

mortality (1 h observation period), 30 s exposure at a constant 4 V/c

m.; mortality increased with water conductivity (and therefore current density); at high

to be greater at higher temperatures. No consistent size effect; 

mortality among 2800 controls was 0.5%.

dqVoltage gradient constant at 4 V/cm; mortality increased with exposure time and te

mperature (values at 14°C tend to be less than at 18°C); also tends to increase with s

drVoltage gradient held constant at 2V/cm; mortality increased with exposure time (3-10 

min for trials at 2 Hz and 0.5-3 min for 8 Hz) but occurred at shorter exposu

(cu

mulative current on ti

me 4 ti

mes greater at 8 

Hz).

dsVoltage gradient held constant at 2V/c

m; mortality increased with exposure ti

me (0.5-3.0 

min) but tended to be higher for the shorter pulse duration (20 

ms versus 80 

m

Voltage gradient held constant at 4V/cm, 30 s exposures; 

mortality tended to increase with pulse rate and specimen size, but results could be confounded by variable 

duN = nu

mber exposed/ nu

mber necropsied. 80 fish exposed for 5 s, 80 for 10 s, and 80 for 20 s, then held 13 d for observation; 

mortality similar among 80 

electrofishing. Necropsy of 3 electrofished and 1 control fish that had died during the holding period revealed no signs of spinal injury.

dv25 fish exposed for 5 s, 25 for 10 s, and 25 for 15 s; 

mortality among controls was also 0%. 

Fish were subjected to 1 

min at 0.5 V/c

m to initiate narcosis then held in narcosis at 0.25 V/c

dxHeld for 1 d in cages in the lake.dyHeld for 10 d in pond; some mortalities were probably due to the stress of handling and incomplete recovery of all fish upon draw-down of the pond rather than electrodzHeld for 1-4 d in cages in the lake.eaHeld for 22 d in cages in the lake.ebHeld for 31 d in cages in the lake.ecBased on co

mparable recapture rates 1 to 2 yr after initial capture by electrofishing, trap net, and angling (large

mouth bass only); growth also appeared to be unaffec

angled Micropterus salmoides were notably younger and larger upon capture and had grown most between initial capture and recapture.

edHeld for 3 d in cages in the lake; half of the fish (20) held were selected from among fish with obvious aggregations of blood in the sinus venosus near the base of the 

from those without such an aggregation of blood; The aggregations of blood dissipated within 1 d and the fish seemed other

wise normal.

eeHeld for 10 d in cages in the lake; half of the fish held (33) were selected from a

mong fish with obvious aggregations of blood in the sinus venosus near the base of the 

from those without such an aggregation of blood; The aggregations of blood dissipated within 1 d and the fish seemed other

wise normal.

. 

efHeld for 30-38 d in cages in the lakeegHeld for 40 d in cages in the lake; 

mortality likely due to stress of handling, confine

ment, low food, and high temperatures rather than electrofishing.

ehSpecies assu

med to be Salmo trutta. 10 fish held in net 50 c

m fro

m positive electrode (lower 

mortality value) and 10 held 20 cm fro

m positive electrode (higher 

mor

seconds initially facing positive electrode; all 

mortality assu

med to be im

mediate.

eiRippled DC, generated by s

moothing halfwave, rectified AC.

ej50% duty cycle but frequency not reported.ek33% duty cycle but frequency not reported.el400 V.em400 V; full-wave rectified AC.

62-145 V (up to 180 V?) for 5-
fa

Footnotes (continued) eoHigh incidence of bleeding at the gills observed by Montana biologists regardless of current–“literally dozens of 

mountain whitefish co

me to the electrode under t

water.”

ep20 fish were treated and frozen, for x-ray analysis and if such indicated possible spinal injury, fish were necropsied; likewise for 20 of 50 treated and reared for 35 d

four of the treated and reared fish died within 24 h and were added to the frozen batch, bringing the nu

mber of treated fish examined to 44. There was no indication of 

eqCurrent was generated by a Coffelt BP-6 backpack electrofisher generating a half-rectified waveform (half-sine wave pulses?); output was 

measured at 0.6 A, 365 V 

gradient in the homogeneous field was 0.9-1.0 V/c

m ; fish were exposed for 10 s.

erMortality within first 7 d. For DC, no i

m

mediate mortality and 

most of the 12 

mortalities

may have resulted fro

m increased stress due to recapture following escape fro

storage pond. For hybrid PDC-DC (half pulse) current, two specimens died, one prior to release in a storage pond for 

monitoring subsequent survival and growth. Fo

7 d.

esFish were held in a raceway for 182 d for each of the six treat

ment currents; there was no significant difference a

mong the waveforms despite a

mortality range fro

m 0

Hz PDC, probably because of s

mall sample size. Mortality a

mong controls held 203 d was 10%.

etOf 102 shocked for 5 s at a 

mean of 2.3 V/cm and 50 control fish held for 203 d, 52% of the shocked and injured fish, 29% of the shocked and uninjured fish, a

However, the difference between shocked injured and shocked-uninjured fish was not statistically significant, nor were any length differences among these groups 

class 2. 83% of overall 

mortality occurred within the first 30 d with no significant difference over time between shocked-injured, shocked-uninjured, and controls.

euBoat electrofishing,

mobile (throwable) anode, peak 2.3 V/cm at 15 c

m fro

m anode; sixteen brown trout not evaluated for spinal injuries due to poor x-rays. Data also f

evBoat electrofishing,

mobile (throwable) anode, peak 7.4 V/cm at 15 c

m fro

m anode. Data also from

Thompson (1995).

ewBoat electrofishing,

mobile (throwable) anode, peak 3.4 V/cm at 15 c

m fro

m anode; six rainbow trout not evaluated for spinal injuries due to poor x-rays. Data also fro

exShore-based wade electrofishing, 

multiple 

mobile anodes, peak 0.5 V/c

m at 15 c

m fro

m anode; eight brown trout not evaluated for spinal injuries due to poor x-rays. D

eyShore-based wade electrofishing, 

multiple 

mobile anodes, peak 1.2 V/c

m at 15 c

m from anode; sixteen brown trout not evaluated for spinal injuries due to poor x

(1995).

ezShore-based wade electrofishing, 

multiple 

mobile anodes, peak 1.1 V/c

m at 15 c

m fro

m anode; two rainbow trout not evaluated for spinal injuries due to poor x-rays. D

Percentage 

mortality based on 773 fish, 145 s-rays were unreadable; no injuries observed a

mong a subsa

mple of 20 controls; (peak?) voltage gradient was 1.6 V/c

m in 6

fbPercentage

mortality based on 1134 fish, subsa

mple of 184 fish x-rayed; no injuries observed a

mong a subsa

mple of 20 controls; (peak?) voltage gradient was 3.2 V/cm

fcSubsa

mple of 10 fish x-rayed and necropsied. Short-term

mortalities include 3 fish that died within 24 h and 6 

more that died within 7 d. Data represents handled a

three-pass depletion sampling at three consecutive stations.

fdSubsa

mple of 54 fish x-rayed and necropsied. Short-term

mortalities include 4 fish that died within 24 h and 7 

more that died within 7 d. Data represents handled a

three-pass depletion sampling at three consecutive stations.

feNu

mber of observations for short-term, 3-d, 

mortality; all dead fish had soft tissue da

mage with internal hemorrhaging from da

maged dorsal aorta and other vessels 

and in L. macrochirus from below 

mid-dorsal fin area. Spinal injuries were subluxations (partial separation of vertebrae) or

misalignments.

ffFish taken during three-pass depletion electrofishing in three successive years from a stretch of stream sa

mpled in prior years by electrofishing; externally evident inj

injuries from previous years) progressively (cumulatively) increased during those years for all sal

monids, but greatest in year two and least in year 3 for longnose suck

fgFish taken during single-pass electrofishing in three successive years from a “control” stretch of stream not previously sampled by electrofishing; externally evident inj

injuries from previous years) were absent in first year. 

fhHealed injuries among brook, brown, and rainbow trout found dead (

N=48),that died after electrofishing and handling (

N=62), or were purposely sacrificed to assess d

with externally detected injuries (

mostly healed old injuries) and nu

mber of healed injuries detected by dorsal and lateral x-rays. 153 x-rays were taken, but only 114 

of fish without externally obvious injuries.

fiPDC frequency and duty cycle (50%) assu

med sa

me as for Kocovsky et al. (1997).

fjFor each current (AC-sine, AC-triangular, PDC-36% duty cycle with unreported waveform; all 50-Hz), individual fish were exposed for 10, 30, or 60 s at 15, 30, 45, 6

intensities were calculated at 0.35, 0.70, 1.1, 1.4, 1.7, and 2.1 V/c

m; r

ms for AC, peak for PDC). All injured with sine AC suffered cutaneous hemorrhages along th

triangular AC had cutaneous hemorrhages in their paired and single fins.

fk36% duty cycle, waveform not reported; wild-caught brood stock exposed for 10 s with a peak output of 60V and field intensity of 0.58 V/cm (138 

ΦW/c

m3 if ambient 

.

flFish subjected to peak field intensity of 1.7 V/cmfmFish exposed as groups to peak field intensities of 0.2 to 1.7 V/c

m for 3.6 to 100 s.

fnNo damage was observed externally or to internal organs, however, during 10-s exposures in ho

mogeneous fields with a peak intensity of 1 V/cm, tetany was induced 

all ripe fish, at least several hundred eggs for each fe

male. 

foPreswi

mup larvae tested using 10 s exposures at 1.2 V/cm peak (also 5 and 10 V/cm for 60-Hz PDC); delayed recovery was just several seconds; no external physi

detected, but subsequent growth was reduced over that of controls during a four-week 

monitoring period after treat

ment.

fp10 s exposures, 7% at threshold for taxis (0.5 peakV/cm), 3% at threshold for narcosis (0.7 peakV/cm), and 20% at threshold for tetany (1.1 peak-V/c

m). 

fq10 s exposures, 7% at threshold for taxis (0.5 peakV/cm), 10% at threshold for narcosis (0.6 peakV/c

m), and 27% at threshold for tetany (1.0 peak-V/c

m). 

fr10 s exposures, 23% at threshold for taxis (0.4 peak-V/cm), 13% at threshold for narcosis (0.6 peakV/cm), and 20% at threshold for tetany (1.0 peak-V/c

m). 

. 

fs10 s exposures, 7

% at threshold for taxis (0.8 peakV/cm), 10% at threshold for narcosis (1.0 peakV/c

m), and 7% at threshold for tetany (1.4 peak-V/c

m)

. 

ft10 s exposures at threshold for tetany (0.8 peak-V/c

m)
fuAccording to Barton (pc), shocked fish rolled belly up and were motionless but there was no evidence of tetany, after exposure they quickly recovered within a 

min

external injuries or brands. 

Footnotes (concluded) fvFish were exposed to electric current for 20 s then, to si

mulate additional stress of handling, to 0, 1, 2, or 4 

min of air.

fw

Electrical output originally reported as 80-Hz, 50% duty cycle pulsed AC and pulsed D

C from Coffelt VVP-15 Electrofisher, but later (Van 

Zee et al. 1996) correc

cycle, square-wave PDC and 73-Hz, 8.7-ms, 64%-duty-cycle, square-wave PDC (ignoring trailing positive spike and s

mall, exponential negative pulse betwee

respectively, as determined by digital oscilloscope tracings.

fyOutput voltage about 300 V for DC and 400 V of PDC; Effective field (0.1-1.0 V/cm) mapped at up to 1.6 m from the anode, about 4.5 V/cm at 1 cm from the spheric

between 20 and 40 cm from the anode; voltage gradient at 1 c

m from the boat about 0.11-0.26 V/cm. 

fzFish captured by trawl, placed in plastic bucket with holes, and exposed for 10 s to electrical field in the river about 1 

m from the anode (about 0.18 V/cm); fish succu

m

1 min. Because of size, assess

ment of injuries 

may not be conclusive.

gaConductivity calculated from a reported resistivity of 10,000 ohm per inch cube, which, is assu

med to equate to 10,000 ohm-in or 25,400 oh

m-cm.

gbOneminute exposures at 0.5-0.6 Vm/cm (below to just above mini

mum threshold for stun).

gcOneminute exposures at 0.7 Vm/c

m.

gdOneminute exposure at 0.8 Vm/c

m.

geOneminute exposure at 1.0 Vm/c

m.

gfFiveminute exposures at 0.2-0.5 Vm/c

m

ggFive-minute exposures at 0.6-0.7 Vm/c

m

ghFive-minute exposures at 0.8 Vm/c

m.

giField intensity increased gradually fro

m 0 V/cm to threshold for twitch then maintained for 5 s.

gjField intensity increased gradually fro

m 0 V/cm to threshold for taxis then

maintained for 5 s.

gkField intensity increased gradually fro

m 0 V/cm to threshold for narcosis then 

maintained for 5 s.

glField intensity increased gradually fro

m 0 to 1 Vp/c

m, sufficient to ensure tetany, then 

maintained for 5 s.

gm10 s exposure at 1.5 Vp/c

m, sufficient for tetany and lowest field intensity for experimental setup using Smith Root GPP 5.0.

. 

gnSo

me individuals required 

more than 15 

min to recover equilibriu

m

go

Electrofishing plus handling mortality usually <25%, but >50% during war

m periods; that for all other species collected was al

ways < 11% (taxa unspecified).
 2003

 149

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Standard Form 298 (rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102

NSN 7540-01-280-5500

U.S. Department of the Interior

U.S. Geological Survey

Biological Resources Division

 Snyder, D.E.

 USGS/BRD/ITR--

 2003-0002

Available from the National Technical Information Service, 5285 Port

Royal Road, Springfield, VA 22161 (1-800-553-6847 or 703-487-4650).

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Attn: Help Desk, 8725 Kingman Road, Suite 0944, Fort Belvoir, VA

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Unclassified Unclassified Unclassified UL

Electrofishing and Its Harmful Effects on Fish

Electrofishing, which involves a very dynamic and complex mix of physics, physiology, and behavior, has been

a valuable sampling technique for over half a century, but its potentially harmful effects on fish must be recognized,

monitored, and avoided or minimized, especially with respect to populations of endangered species. Spinal injuries

and associated hemorrhages, although often not externally obvious or fatal, can occur anywhere in the electrofishing

field at or above the intensity threshold for twitch. These injuries are believed to result from powerful convulsions of

body musculature caused mostly by sudden changes in voltage. Significantly fewer spinal injuries are reported when

direct current, low-frequency pulsed direct current (#30 Hz), or specially designed pulse trains are used. Salmoninae

are especially susceptible. Endangered cyprinids of the Colorado River Basin are generally much less susceptible, but

the endangered catostomid Xyrauchen texanus appears sufficiently susceptible to warrant minimal-use policy. Other

harmful effects, including bleeding at gills or vent and excessive physiological stress, are also of concern. Mortality,

usually by asphyxiation, is a common result of excessive exposure to tetanizing intensities near electrodes or poor

handling of captured specimens. Reported effects on reproduction are contradictory, but electrofishing over spawning grounds can harm embryos.

behavior, Colorado River Basin, electric fields, electrofishing, endangered species,

epilepsy, fish, gear, injuries, mortality power transfer, responses, stress

Information and Technology Report (Final)

8327-TJSF3

the U.S. Department of the Interior Bureau of Reclamation, Upper Colorado Regional Office, Salt Lake City, Utah.

This report is also CSU Larval Fish Laboratory Contribution 114.

Prepared in cooperation with the Colorado State University Larval Fish Laboratory and
Fort Collins Science Center Fort Collins Science Center

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Dora E. Medellin

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