Biological Services Program

and

Division of Ecological Services

LI!:o; i;:OI .. ' , ,. " ,." . f 1 ' : .<; ;'..." ;'.: v. ;1 :1 ? es<::J.rch Center

L !.· ~.'l ;-" :,;.rl~~l f;3 Service

' ";- ,.":i:j-: ; .~ r; E:;e

L:...••,}c t~ L;l. 70506

FWS/O8S-82/1 0.17

JULY 1982

HABITAT SUITABILITY INDEX MODELS:

NORTHERN PIKE

r:'i<:)h and Wildlife Service

S. Department of the Interior SK

36 1

. U54

n o. 82 -

10.17
The Biological Services Program was established within the U.S,. Fish

and Wildlife Service to supply scientific information and methodologies on

key environmental issues that impact fish and wildlife resources and their

supporting ecosystems. The mission of the program is as follows:

• To strengthen the Fish and Wildlife Service in its role as

a primary source of information on national fish and wildlift resources, particularly in respect to environmental

impact assessment.

• To gather, antlyze, and present information that will aid

decisioJl11lkers in the identification and resolution of

problems associated with major changes in land and water

use.

• To provide better ecological information and evaluation

for Department of the Interior development programs, such

as those relating to energy development.

Information developed by the Biological Services Program is intended

for use in the planning and decisionmaking process to prevent or minimize

the impact of development on fish and wildlife. Research activities and

technical assistance services are based on an analysis of the issues, a

determination of the decisionmakers involved and their information needs,

and an evaluation of the state of the art to identify information gaps and to determine priorities. This is a strategy that will ensure that

the products produced and disseminated are timely and useful.

Projects have been initiated in the following areas: coal extraction

and conversion; power plants; geothermal, mineral and 011 shale development; water resource analysis, including stream alterations and western

water allocation; coastal ecosystems and Outer Continental Shelf development; and systems inventory, including National Wetland Inventory, habitat classification and analysis, and information transfer.

The Biological Services Program consists of the Office of Biological Services in Washington, D.C., which is responsible for overall planning and

management; National Teams, which provide the Program's central scientific

and technical expertise and arrange for contracting biolo~ical services

studies with states, universities, consulting firms, and others; Regional Staffs, who provide a link to problems at the operating level; and staffs at

certain Fish and Wildlife Service research facilities, who conduct in-house

research studies.

This model is designed to be used by the Division of Ecological Services in conjunction with the Habitat Evaluation Procedures.
FWS/OBS-82/10.17

July 1982

HABITAT SUITABILITY INDEX MODELS: NORTHERN PIKE

by

Peter D. Inskip

Habitat Evaluation Procedures Group

Western Energy and Land Use Team

U.S. Fish and Wildlife Service

Drake Creekside Building One

2625 Redwing Road

Ft. Collins, CO 80526

Western Energy and Land Use Team

Office of Biological Services

Fish and Wildlife Service

U.S. Department of the Interior

Washington, DC 20240
This report should be cited as:

Inskip, P. D. 1982. Habitat suitability index models: northern pike. U.S.

Dept. Int., Fish Wildl. Servo FWS/OBS-82/10.17. 40 pp.

\
PREFACE

The habitat use information and Habitat Suitability Index (HSI) models

presented in this document are intended for use in impact assessment and

habitat management activities. Literature concerning the habitat requirements

and preferences of northern pike is reviewed and then synthesized into HSI

models, which are scaled to produce an index between 0 (unsuitable habitat)

and 1 (optimal habitat). Assumptions used to transform habitat use information

into these quantitative models are noted, and guidelines for model application

are described. Other models found in the literature which may also be used to

calculate an HSI are cited.

Use of these models for impact assessment requires the setting of clear

study objectives and may require modification of the models to meet those

objecti ves. Methods for modifyi ng HSI model s and recommended measurement

techniques for model variables are presented in Terrell et al. (in press).l A

discussion of HSI model building techniques is presented in U.S. Fish and

Wildlife Service (1981).2

The HSI models presented herein are hypotheses of species-habitat relationships, not statements of proven cause and effect relationships. The

models have not been tested against field population data. For this reason,

the U.S. Fish and Wildlife Service encourages model users to send comments and

suggestions that may help us increase the utility and effectiveness of this

habitat-based approach to fish and wildlife planning. Please send comments

to:

Habitat Evaluation Procedures

Western Energy and Land Use Team

U.S. Fish and Wildlife Service

2625 Redwing Road

Ft. Collins, CO 80526

lTerrell, J. W., T. E. McMahon, P. D. Inskip, R. F. Raleigh, and K. W.

Williamson (in press). Habitat suitability index models: Appendix A. Guidelines for riverine and lacustrine applications of fish HSI models with the

Habitat Evaluation Procedures. U.S. Dept. Int., Fish Wildl. Servo

FWS/OBS-82-10.A.

2U.S. Fish and Wildlife Service.

habitat suitability index models.

Serv., Div. Ecol. Servo n.p.

1981. Standards for the development of

103 ESM. U.S. Dept. Int., Fish Wildl.

iii
CONTENTS

Page

PREFACE iii

ACKNOWLEDGMENTS v

HABITAT USE INFORMATION 1

Genera1 1

Age, Growth, and Food 1

Reproduct ion 3

Speci fi c Habitat Requi rements 4

HABITAT SUITABILITY INDEX (HSI) MODELS ~..... 12

Model Description - Lacustrine and Riverine 12

Model Applicability 15

Suitability Index (SI) Graphs for Model Variables 17

Development of Suitability Index Graphs: Rationale

and Assumptions................................................. 21

Field Use of the Model 25

Interpreting Model Outputs........................................ 28

ADDITIONAL HABITAT MODELS 28

REFERENCES 29

iv
ACKNOWLEDGMENTS

F. M. Atton, R. F. Carline, J. M. Casselman, E. J. Crossman, J. S. Diana,

M..R. Haedrick, R. P. Johnson, D. W. Rondorf, S. L. Serns, and H. E. Snow

reviewed a draft of this document and offered many constructive suggestions.

Their assistance in improving the manuscript is greatly appreciated, but the

reader should not assume that they endorse its contents. D. Fago, S. L. Serns,

H. E. Snow, and 1. L. Wi rth commented on a draft set of habi tat eva1uation

criteria developed by K. A. Twomey. The present document is an independent

synthesis and interpretation of the literature, but the preliminary draft and

accompanying comments provided useful raw material. C. A. Chapman and W. C.

MacKay allowed me to cite an unpublished manuscript, and L. R. Aggus provided

listings of harvest and standing crop data for northern pike in U.S.

reservoirs. C. 1. Short provided editorial assistance. Word processing was

by C. J. Gulzow and D. E. Ibarra. J. R. Shoemaker drew the cover illustration.

v
NORTHERN PIKE (Esox lucius Linnaeus)

HABITAT USE INFORMATION

General

Northern pike (Esox lucius) occur worldwide in fresh waters north of

40° N (Toner and Lawler 1969). It is the only species in the esocid family

native to both North America and Eurasia (Scott and Crossman 1973). The

native North American range included Alaska, most of Canada south of the

Arctic Circle (excepting most of British Columbia), the Missouri River

drainage, the Mississippi River drainage upstream of the confluence with the

Missouri, the Ohio River drainage in Pennsylvania and New York, and the Great

Lakes drainage basin; the southern limit was probably in central Missouri

(Crossman 1978). Northern pike distribution has expanded with their introduction into impoundments outside of their native range, most commonly in Great

Plains and Rocky Mountain States (McCarraher 1961; Carlander et al. 1978).

Lakes account for most of the North American habitat of northern pike

(Carlander et al. 1978; Crossman 1978).

Age, Growth, and Food

Growth rate of northern pike is highly variable, both among and within

populations. There is a general inverse relationship between growth rate and

north latitude over a latitudinal gradient spanning the United States and

Canada (Miller and Kennedy 1948; Crabtree 1969; Vasey 1974), but the trend is

not always strongly defined over limited geographic areas (Diana, pers. comm.).

Temporal variability in growth rate, associated with changes in density of

northern pike and likely prey species, has also been noted (Kempinger and

Carline 1978). Individual growth rates can vary considerably within a single

cohort. Young of the year northern pike, stocked as fry in a Michigan rearing

pond in early May and recovered the following October, ranged in length from

8.3 to 44.6 cm (Carbine 1945).

The age at whi ch northern pi ke become sexually mature depends on thei r

growth rate. Male northern pike are typically 34 to 42 cm (total length) and

females 40 to 48 cm when they spawn for the first time (Frost and Kipling

1967; Priegel and Krohn 1975). In Great Bear Lake (Northwest Territories),

this translates to age 5 or 6 for males and age 6 or 7 for females (Miller and

Kennedy 1948). In Wisconsin, age 2 (males and females) is more typical

(Priegel and Krohn 1975). One-year old spawners are not unusual in Kansas

(Schryer et al. 1971), Missouri (Vasey 1974), and Texas (Crabtree 1969)

reservoirs, where growth is rapid.

1
Longevity and age-specific fecundity are also related to growth rate.

Ten-year old and older northern pike are not uncommon in northern Canadian and

Alaskan lakes, where growth is slow, and individuals as old as 24 years have

been recorded (Miller and Kennedy 1948; Rawson 1959; Alt 1969). Populations

in Wisconsin (Van Engel 1940) and more southern waters (Buss and Miller 1961;

Schryer et al. 1971) are composed mostly of 4-year old and younger fish.

Fecundity increases exponentially with length and proportionally with weight

(Carbine 1944; Frost and Kipling 1967; Priegel and Krohn 1975). Carbine

(1944) reported an average fecundity of 32,200 eggs for 30 pike between 40 and

89 em (total length).

Northern pike become piscivorous very early in life. Fry begin to feed

approximately 10 days after hatching, on reaching a length of 10 to 12 mm

(Hunt and Carbine 1951; Franklin and Smith 1963). The diet initially consists

of zooplankton but quickly broadens to include aquatic insect larvae and then

fish (Hunt and Carbine 1951; Frost 1954; McCarraher 1957; Fago 1977). By the

time the fry reach 50 to 60 mm, typically within 4 to 5 weeks after hatching

in the northern Mi dwest (Frankl in and Smith 1963; Fago 1977) and Northeast

(Forney 1968), fish comprise most of the diet (Hunt and Carbine 1951; Frost

1954). Fry are cannibalistic at sizes as small as 21 mm (Hunt and Carbine

1951).

Fish continue to predominate in the diet throughout the remainder of the

life history, but northern pike occasionally prey on leeches, aquatic insects,

crayfish, waterfowl, and small mammals (Frost 1954; Lagler 1956; Seaburg and

Moyle 1964; Lawler 1965; Mann 1976). Northern pike feed opportunistically,

exploiting seasonally abundant food sources (Frost 1954; Lawler 1965; Wagner

1972). Soft-rayed, cylindrical fishes are apparently mor-e easily swallowed

than are spiny and/or laterally compressed species (Frost 1954; Hoogland et

al. 1957; Beyerle and Williams 1968; Johnson 1969; Mauck and Coble 1971;

Wagner 1972; Weithman and Anderson 1977; Wolfert and Miller 1978). Gizzard

shad (Dorosoma cepedianum) (Wagner 1972), alewife (Alosa pseudoharengus)

(Wolfert and Miller 1978), yellow perch (Perea flavescens) (Lawler 1965;

Johnson 1969; Diana 1979), and trout-perch (l5ercopsis omiscomaycus) (Lawler

1965) have been noted as being especially common in the diet. White suckers

(Catostomus commerson i) are common ly eaten by 1arge northern pi ke (Lawl er

1965; Koshinsky 1979).

Reports of cannibalism among juvenile and adult northern pike are not

unusual, but, in most cases, it accounts for a minor part of the diet (Frost

1954; Lawler 1965; Johnson 1969; Wagner 1972; Mann 1976). Alt (1968) described

a notable exception; he found that the diet during summer and early fall

consisted primarily of northern pike in several Alaskan lakes. Dryden and

Jessop (1974) reported a high incidence of cannibalism among northern pike

captured during a spawning run in a stream in the Northwest Territories. It

appears that cannibalism occurs more frequently in waters with few fish species

than in those where the fish community is diverse.

Northern pike are visual predators (Polyak 1957; Braekevelt 1975) and are

primarily day active (Carlander and Cleary 1949; Diana 1980). Their ambush

style of hunting requires cover, usually sought in the form of aquatic plants.

Tree stumps and fallen logs may also be used (Crossman, pers. comm.).

2
Consumption rate varies seasonally. Maximum rates usually occur in

spring or early summer (Johnson 1966a; Weithman and Anderson 1977; Diana

1979), following the spawning period. Weithman and Anderson (1977) observed

peak feeding at temperatures between 15 and 18° C for yearling northern pike

held under experimental conditions; there was a summer reduction in consumption rate between May and September highs.

Reproduction

Northern pike spawn in spring, shortly after ice-out, when the water has

warmed to 8 to 12° C (Embody 1918; McNamara 1936; Clark 1950; Fabricius and

Gustafson 1958; Franklin and Smith 1963; June 1971; Priegel and Krohn 1975).

Movement to spawning grounds often begins before all ice has melted. Upstream

spawning migrations out of lakes in Michigan (Carbine 1942) and Minnesota

(Franklin and Smith 1963) occurred as soon as there was sufficient clearance

between inshore ice and the bottom to permit passage. Robertson (1969)

observed that upstream movement of northern pi ke was interrupted when the

water temperature dropped below 5.5° C. Isolated spawning may occur at temperatures less than 8° C (McNamara 1936; Clark .1950; Schryer et al. 1971).

Most females have completed spawning by the time temperatures exceed 13° C

(Schryer et al. 1971).

Spawning occurs over vegetation in areas of calm, shallow water

(Williamson 1942; Clark 1950; Fabricius 1950). There is a strong tendency to

migrate up tributaries to flooded marshes, wetlands, or shallow pools among

both lake (Carbine and Applegate 1948; Schultz 1955; Robertson 1969; Koshinsky

1979) and river (Harrison and Hadley 1978) populations. Flooded terrestrial

vegetation (McCarraher and Thomas 1972) and shallow, weedy bays or backwaters

may also be used (Jarvenpa 1962a; Frost and Kipling 1967). The absence of

inundated vegetation can inhibit or delay spawning (Fabricius 1950; Fabricius

and Gustafson 1958). The timing of spawning has been linked to water level

changes (Fabricius 1950; June 1970, 1971, 1977).

Spawning groups, consisting of a female plus one, or more typically,

several, males move continually. Only a few eggs, 5 to 60 according to

Svardson (1949), are released at anyone spot (Svardson 1949; Clark 1950;

Fabricius and Gustafson 1958). Gametes are broadcast (Fabricius and Gustafson

1958), and no parental care is provided (Eddy and Underhill 1974). Eggs

adhere to vegetation (Fabricius and Gustafson 1958; Frost and Kipling 1967).

Both laboratory (Fabricius and Gustafson 1958) and field (Koshinsky 1979)

observations indicate that spawning occurs in distinct series of mating acts,

between which the fish are stationary, presumably to rest. Spawning groups

may cover considerable distances before all eggs are released (Koshinsky

1979). Distances between egg releases are likely to be greater when the

distribution of spawning habitat is patchy (Fabricius and Gustafson 1958).

Northern pike do not spawn at night (Clark 1950; Fabricius and Gustafson

1958). Spawning may be interrupted by cold weather, water level drawdowns

(Clark 1950; Fabricius and Gustafson 1958; June 1970), strong wind, or rain

(Koshinsky 1979). Prolonged interruptions may result in resorption of eggs

(June 1970).

3
Areas used for spawning can be far away from areas inhabited during the

rest of the year. Northern pike tagged on spawning grounds in a Missouri

River reservoir were later recaptured up to 322 km away; most, however, were

caught withi n 32 km of the site where they were tagged (Moen and Henegar

1971). Pike tagged during the upstream spawning run out of a Michigan lake

were subsequently recaptured up to 15 km away in the lake (Carbine and

Applegate 1948).

The degree to which northern pike home to particular spawning grounds is

unclear. Tagging and meristic studies of northern pike in Lac La Ronge,

Saskatchewan, suggested the presence of separate populations within the lake,

but. it is not necessary to postulate a homing instinct to account for this

observation (Koshinsky 1979). The northern pike did not disperse far from the

mouths of tributaries used for spawning. In spring, they may simply have

moved up the nearest suitable stream (Koshinsky 1979). Frost and Kipling

(1967) also gave evidence of repeated use of a spawning area in a lake in

England, but their results did not conclusively demonstrate homing. Miller

(1948) reported movements of several northern pike among widely separated (2.4

to 3.2 km) spawning areas within a la-day period. He did not note whether the

fish were ripe at each capture location. Franklin and Smith (1963) concluded

that northern pike in a Minnesota lake do not home to particular spawning

grounds.

Specific Habitat Requirements

Northern pi ke are not adapted for 1i fe in strong currents. Throughout

their range, they occur more frequently in lakes than in rivers (Crossman

1978), where they inhabit backwaters and pools (Christenson and Smith 1965;

Kamyshnaya and Tsepkin 1973; Paragamian 1976; Kallemeyn and Novotny 1977).

They avoid channelized reaches (Kallemeyn and Novotny 1977). Strong currents

(> 1.5 m/s) can block spawning migrations (Dryden and Jessop 1974).

The availability of suitable spawning habitat is the factor which most

often excludes northern pike from lakes, reservoirs, and slow moving rivers

and whi ch 1imi ts abundance where the speci es does occur. Shallow vegetated

areas, such as flooded marshes, flooded terrestrial vegetation, or weedy bays

provide suitable habitat, provided that high water level s are maintained

throughout the embryo and fry stages (Hass1er 1970). Strong year-cl asses

often occur as a newly created reservoir fills and in years when exceptionally

high spring water levels flood previously unflooded terrestrial vegetation

(Crabtree 1969; Cooper 1971; June 1976; Groen and Schroeder 1978; Nelson

1978). Conversely, weak year-classes in impoundments and natural environments

are associated with low water levels (Johnson 1957; Cooper 1971; Schryer et

a 1. 1971; Rundbe rg 1977; Ne 1son 1978; Stewa rt 1978). Northern pike do not do

well in reservoirs with widely fluctuating water levels (Wajdowicz 1964)

because nearshore vegetation does not develop. Long term reductions in the

abundance of northern pike have accompanied the draining and filling of wetlands (Brynildson 1958; Threinen 1969; Baumann et al. 1974; Forney 1977),

reductions in the density of aquatic plants (Hurley and Christie 1977;

Ciepielewski 1981), and blocking of access to spawning grounds (Trautman

1957). Fishery managers in Michigan were able to substantially increase

4
populations of adult northern pike in a set of lakes by enhancing the quality

and quantity of spawning habitat (Williams and Jacob 1971).

Young northern pike eventually disperse from spawning areas, but pike of

all ages continue to frequent shallow areas with vegetation. Small pike are

especi ally dependent on thi s habi tat. Growth and food conversi on effi ci ency

of young pike kept in tanks with no vegetation were erratic; pike kept in

ponds with no vegetation resided in any areas that provided cover (Johnson

1960). The minimum size of northern pike captured in gill nets in Lac La

Ronge was positively associated with depth, but the maximum size was not,

indicating that large pike use a wider range of depths than do small pike

(Koshinsky 1979). Nine tagged northern pike (1.6 to 4.1 kg) in an eutrophic

Alberta lake generally remained in areas with submerged and emergent aquatic

vegetation, in water shallower than 4 m, and within 300 m of shore (Diana et

al. 1977). Northern pike in a Michigan lake, that were tagged at spawning

time and subsequently recaptured by anglers, were caught almost exclusively in

weed beds (Carbine and Applegate-1948). Ninety percent of the pike captured

in a gill netting survey of Great Slave Lake in northern Canada were caught

withi n 400 m of shore, and very few were taken at depths greater than 10 m

(Rawson 1951). Fisheries surveys in lakes of the Churchill River basin in

northern Saskatchewan also indicated that pike mostly inhabit water shallower

than 10 m, although they occur at least as deep as 27 m (Dean 1975; Liaw and

O'Connor 1975; Koshinsky 1979).

Depth per se does not appear to be the factor determining depth distribution of northern pike. Depth distribution appears to be in response to differences in temperature, di sso 1ved oxygen, vegetation, and food. Northern

pike rarely venture below the thermocline (Reighard 1915; Pearse 1922; Rawson

1951). In Douglas Lake, Michigan, a well-defined thermocline typically forms

at a depth of 12.2 to 13.7 m; 13.7 m was the maximum depth at which northern

pike were captured (Reighard 1915). The other northern pike collected in this

study were caught at depths 7.9 m, which coincides with the lower (lakeward)

depth limit of aquatic vegetation. Eleven of these 21 pike were caught at

depths between 7.6 and 7.9 m, perhaps indicating that pike prefer the interface

between vegetation and open water (Reighard 1915). Underwater observations of

northern pike in an Alberta lake also indicated an attraction to this boundary

zone (Chapman and MacKay, in prep.). The edges of vegetated areas would

provide good cover for feeding activities.

Northern pike are a coolwater species (Casselman 1978). Their occurrence

over a broad latitudinal belt, circa 40 to 70° N in North America (Karvelis

1964), however, demonstrates their adaptability to a variety of thermal

regimes. For example, the average maximum summer temperature in the limnetic

zone of Great Bear Lake (Northwest Territories) is 5 to 7° C, and the ice free

season usually lasts less than 19 weeks (Johnson 1966b). Temperatures may

reach 16° C, at 1east bri efly, in protected bays, where most northern pi ke

occur (Miller 1947). Lake Mendota, in southern Wisconsin, typically warms to

about 24° C, with an ice free season of 36 to 40 weeks (Juday 1940).

5
Summer habitat for northern pike is limited in some lakes by a combination

of high surface temperatures and low oxygen concentrations in cooler, deeper

strata. Krohn (1969) reported a mid-August die-off in a Wisconsin lake following several very warm days. Temperatures ranged from 320 C at the surface to

240 C at a depth of 3.7 m, the lower limit of oxygenated water. High surface

temperatures (320 C) coincided with the beginning of a northern pike die-off

ina Mi ssouri reservoi r (Goddard and Redmond 1978). Dead fi sh were found to

have bacterial, fungal, and protozoan infections. High surface temperatures

can contribute to high incidences of infection by lowering resistance to

disease and by concentrating the pike in a narrow depth stratum. Substantial

mortality of northern pike occurred in an Iowa marsh when temperatures reached

35.6 0 C at the surface and 31. 70 C at the bottom (Ridenhour 1957). In thi s

case, oxygen concentrations were adequate, but temperatures were too high at

all depths.

It seems 1ike ly that hi gh summer and/or wi nter temperatures 1imit the

southern distribution of northern pike. Although present in the Mississippi

River drainage, pike are absent from waters south of the 32.20 C isotherm for

normal July daily maximal air temperature, except in thermally stratified

reservoirs where they have been introduced (Visher 1954; Crossman 1978).

Northern pike are absent from all but the upper reaches of the Ohio River and

its tributaries, although muskellunge ([sox masguinongy), a closely related

species with similar spawning habitat requirements, are widely distributed in

the drainage (Forbes and Richardson 1920; Gerking 1945; Trautman 1957; Crossman

1978). Much of the Ohio River is south of the 31 0 C isotherm for mean maximal

July air temperature (Nat i ona1 Oceanographi c and Atmospheric Admi nistration

1974). Surface water temperatures of 29 to 31 0 C are not unusual for Ohi 0

lakes (Tressler et al. 1940). In the laboratory, young northern pike do not

grow at temperatures above 280 C, even when fed ad libitum (Casselman 1978).

Temperatures greater than 320 C can cause death within 2 days (Scott 1964).

High temperatures at other times of the year may 1imit reproductive

success. In a 1aboratory study, the percentage of yellow perch that spawned

was highest when they were exposed to temperatures s 60 C for a minimum of 185

days, beginning October 30 (Jones et al., unpub1ished data cited in Hokanson

1977). No successful spawning occurred among perch held at a minimum temperature of 120 C or higher. Northern pike, which occur farther north than do

yellow perch (Scott and Crossman 1973), are presumably also adapted to a

seasonal temperature cycle involving cold winters.

Di sso 1ved oxygen concentrati on is usually the most important habitat

variable affecting overwinter survival of northern pike. Shallow, heavily

vegetated lakes and rivers with low discharges - habitats that might be quite

favorable for northern pike at other times of the year (Johnson and Moyle

1969) - frequently develop low di sso 1ved oxygen concentrations duri ng wi nter.

Laboratory and field observations indicate that pike are more tolerant of low

dissolved oxygen conditions during winter than are many other temperate

species, including black crappie (Pomoxis nigromaculatus), largemouth bass

(Micropterus salmoides), bluegill (Lepomis macrochirus), walleye (Stizostedion

vitreum vitreum), and yellow perch (Cooper and Washburn 1949; Moyle and

Clothier 1959; Patriarche and Merna 1970; Petrosky and Magnuson 1973). If the

6
change to low dissolved oxygen is gradual, northern pike appear able to tolerate concentrations as low as 0.1 to 0.4 mg/l for at least several days (Cooper

and Washburn 1949; Magnuson and Karlen 1970; Petrosky and Magnuson 1973). A

prolonged period with dissolved oxygen concentrations less than 1.0 mg/l,

however, appears to cause partial or complete winterkill (Johnson and Moyle

1969; Stewart 1978). The critical oxygen concentration also depends on temperature. Requirements for dissolved oxygen increase exponentially as tem- . perature increases, but the lower incipient lethal concentration [concentration

below which pike cannot survive for an indefinite time (Fry 1947)J is still

relatively low « 1.5 mg/l) at 28° C (Casselman 1978).

To 1erance of low di sso1ved oxygen condi t ions appears to be inverse ly

related to size for juvenile and adult northern pike. Overwinter mortality in

an Ontario lake with low oxygen levels was total for pike older than 2 years

and/or longer than 39 cm (fork length), but many smaller, younger individuals

survived (Casselman and Harvey 1975). Similarly, very few northern pike older

than 2 years were present in a Montana stream in which winter dissolved oxygen

concentration dropped to low levels, although young fish were common (Stewart

1981).

Fish kills due to low dissolved oxygen concentrations can also occur

during summer in shallow, nutrient-rich waters. The cause is often the

collapse of a heavy bloom of blue-green algae (Barica 1975). Northern pike

were among the many dead fishes observed in a small Wisconsin river when the

decomposition of a large algal mass temporarily depleted the dissolved oxygen

(Mackenthun et al. 1945).

An extensive review of limnological characteristics of Ontario lakes

containing northern pike, walleye, smallmouth bass (Micropterus dolomieui),

and lake trout (Salvelinus namaycush), singly and in various combinations,

provides a useful overview of northern pike habitat in this region (Johnson et

al. 1977). Nearly 70% of the lakes that contain northern pike contain either

only northern pike or northern pike and walleye. It appears that lakes with

northern pi ke typi ca lly have mean depths of 2 to 6 m, 1ittora1 (~ 6.1 m deep)

areas which are 60 to 80% of the total surface area, Secchi disk transparencies of 2 to 4 m, total dissolved solids (TOS) levels between 50 and

125 mg/l, and a near neutral or slightly acidic pH, based on average values of

these habitat variables for groups of lakes with different species combinations. These ranges could reflect availability of habitat conditions as well

as habi tat requi rements of northern pi ke, parti cul arly for water qual i ty

parameters. In those lakes where both northern pike and lake trout occur,

pike are likely to be largely confined to the perimeter or to island/shoal

areas.

Although northern pike occur in oligotrophic waters, they are more typical

of mesotrophic or borderline eutrophic conditions (Casselman 1978; Ryder and

Kerr 1978). The mean total phosphorus (TP) concentration in Minnesota lakes

containing northern pike in association with walleye and yellow perch was

0.034 ppm, compared to 0.02 ppm for lake trout-cisco (Coregonus artedi) lakes,

0.058 for bass-panfi sh 1akes, and a.126 for "rough fi sh" 1akes; 1akes in whi ch

northern pike accounted for a significant fraction of the fish population

7
typically had TP levels 0.10 ppm (Moyle 1956). Moyle noted that temperature

may also explain patterns of fish distribution in Minnesota and cautioned

against overemphasis on edaphic factors alone.

Ionic concentrations appear to be directly limiting to northern pike only

in coastal and arid environments. Ryder et al. (1974) noted that TDS level s

are rarely so low that ions essential for fish metabolism and growth are

limiting; these authors speculated that such a limit is near or below 3 ppm,

and very few natural waters are that dilute. Waters with high ionic concentrations apparently do limit the distribution of northern pike, but the critical

concentration is unknown. Northern pike apparently do not utilize brackish

hab i t at.s in North America, but there are reports of pike occurring in the

Baltic Sea at salinities of 10 ppt and reproducing at salinities of 7 ppt

(Svardson, personal communication cited in Toner and Lawler 1969), and pike

have been commercially harvested from brackish waters in Fi~land (Lind 1977).

Rawson and Moore (1944) found northern pike in saline Saskatchewan lakes only

where TDS levels were less than about 3,500 ppm (roughly equivalent to a

salinity of 3.5 ppt). Northern pike are said to have "some to l er-ance" to TDS

levels of 2,200 to 3,800 ppm in alkaline Nebraska lakes (Schoenecker 1970),

but pike did not spawn in a Nebraska lake with a TDS level of 1,024 ppm

(McCarraher 1962). High concentrations of particular ions may be harmful at

salinities which do not cause general osmoregulatory stress. For example,

mortality of northern pike embryos and fry is extensive when total alkalinity

exceeds 950 mg/l (McCarraher 1971). Until effects of individual ions are

clarified, it seems reasonable to assume an upper TDS limit of 3,500 ppm.

Self-sustaining populations of northern pike can persist at pHis as low

as 5.0 (Harvey 1980). Lower pHis are damaging to fry. Cumulative mortality

of fry held at a pH of 4.2 (range 4.0 to 4.3) was 96.5%, compared to 25.6%

mortality at a pH of 5.2 (4.7 to 6.0), and 17% mortality at a pH of 6.8 (6.5

to 7.2) (Johansson and Kihlstrom 1975). Towards the opposite extreme, northern

pike occur in lakes with a pH of 8.9 (Rawson and Moore 1944; Priegel and Krohn

1975). Fingerling and larger northern pike can survive pHis in the 9.0 to 9.5

range, but it is not clear whether successful reproduction can occur at the

upper limit of this range (McCarraher 1962).

Northern pike grow through a broad size range; tolerances, requirements,

and preferences change as they grow. Altho~gh most developmental changes are

gradua l , it is useful to descri be habi tat requi rements for northern pi ke in

terms of four stages: spawning/embryo; fry; juvenile; and adult. These life

stages, for purposes of this discussion, are defined as follows:

1. Spawning/embryo. Includes spawning requirements and requirements of

the developing embryo.

2. Fry. Immediately after hatching to size .at which they assume adult

proportions; i.e., approximately 6.5 em, according to Franklin and

Smith (1960).

3. Juvenile. From 6.5 cm to onset of sexual maturity (beginning of

gonadal maturation).

8
4. Adult. From sexual maturi ty until death.

Spawning/embryo. Optimal spawning substrate for northern pike is a dense

mat of short vegetation (Fabricius and Gustafson 1958) in a shallow, windsheltered area. The type of vegetation does not appear to be critical

(Fabricius 1950; Forney 1968), although grasses or sedges seem to be preferred

(Jarvenpa 1962b; Franklin and Smith 1963; Robertson 1969; June 1971; McCarraher

and Thomas 1972). In Nebraska, the greatest densities of eggs were found in

flooded prairie grasses at depths of 0.2 to 0.45 m (McCarraher and Thomas

1972). Mowed hay and flooded hay bales were used when flooded natural grasses

were not available. Similarly, Forney (1968) found that pike would spawn over

prepared plots of winter wheat (approximately 15 cm tall), as well as over

natural grasses (Spartina ~pp.), sedges (Cyperaceae), and water plantain

(Alisma sp.). Spikerushes (Eleocharis spp.) and canary grasses (Phalaris

spp.) are also commonly used (Franklin and Smith 1963; Forney 1968; Mississippi

River Work Unit 1978). Scattered vegetative debris may be used when more

preferred substrates are not available (Priegel and Krohn 1975). Crossman

(pers. comm.) reported that northern pike in Lake Simcoe (Ontario) often spawn

over "finely shredded remains of dead cattails loosely scattered on black

muck", and over deciduous leaves from the previous autumn. Most spawning

occurs in water shallower than 0.5 m (Williamson 1942; Clark 1950; Fabricius

1950). Northern pi ke have been observed spawni ng wi th thei r backs out of

water (Clark 1950).

The vegetation mat should provide abundant surface area for eggs to

adhere to and yet allow the ci rcul ati on of water. Eggs whi ch fa 11 to the

bottom are unlikely to hatch due to anoxic conditions in the organic-rich

sediments typical of pike spawning grounds. Northern pike embryos are sensitive to high siltation rates (~ 1 mm/day), such as occur in waters with extensive wave action and bank slumping (Hassler 1970). The rate of embryo development, the percent normal hatch, and larval vigor are dependent on oxygen

availability during incubation (Gulidov 1969; Siefert et al. 1973). A

di sso 1ved oxygen concentration of 4.5 mg/l is adequate for embryo development

and survival at temperatures up to 19° C (50% saturation) and a low flow rate

(30 ml/min); concentrations of 3.2 mg/l (33% saturation) or less appear to be

unsuitable for embryos and larvae (Siefert et al. 1973). Gulidov (1969)

reported a much higher percent normal hatch for northern pike embryos incubated

at dissolved oxygen concentrations greater than 35 to 40% saturation (approximately 4 mg/l at a temperature of 12 to 12.5° C) than for those incubated in

lower oxygen tensions.

Hydrogen sul fi de may accumul ate when the di sso 1ved oxygen concentration

is low. Exposure to hydrogen sulfide levels greater than 0.014 to 0.018 ppm

for 96 hours decreases the viable hatch of northern pike eggs (Adelman and

Smith 1970a).

Incubation time is inversely related to temperature (Swift 1965; Lillelund

1966; Walker 1968). Average length of incubation is approximately 26 days at

6° C, 17 days at 8° C, 12 days at 10° C, 9 days at 12° C, 6 days at 14° C, and

5 days at 16 to 20° C (Swift 1965; Walker 1968).

9
Temperatures at which development is most rapid are also associated with

reduced embryonic and larval survival. Mortality of eggs is high at incubation

temperatures greater than 16° C (Swift 1965). Larvae from eggs exposed to

temperatures greater than 18° C early in development appear unhealthy

(Lillelund 1966). Lillelund (1966) reported maximum hatching success for

incubation temperatures between 9 and 15° C and noted that the tolerance of

embryos to temperatures outside of this range increases quickly after fertilization. Hokanson et al. (1973) gave an optimal range for incubation of 6.4 to

17.7° C. Most eggs die when temperatures drop to, or remain near, 5° C, but

embryos can tolerate diel temperature fluctuations of up to 4° C provided the

temperature stays in the suitable range (Hassler 1970).

Fry. The habi tat of the embryos becomes the in iti a1 habi tat of the fry

(Franklin and Smith 1963). Dense vegetative cover provides several advantages

to larval northern pike (Frost and Kipling 1967). Yolk sac fry have small

papillae on the front of their heads with which they can attach to vegetation

and remain suspended above the sediments. This keeps the fry removed from

dangerously low levels of oxygen, and high levels of H2S, on the bottom.

Hydrogen sulfide concentrations greater than 0.004 to 0.006 ppm (96-hour

exposure) decrease growth and survival of sac fry (Adelman and Smith 1970a).

Once the yolk sac is absorbed and the fry are ready to feed, approximately

10 days after hatching in Minnesota (Franklin and Smith 1963), the invertebrate

fauna associated with the vegetation provides a suitable food base. Thick

vegetation also provides refuge from potential predators. Young northern pike

are vulnerable to predation from a variety of fishes, including other northern

pike (Hunt and Carbine 1951; Alt 1968).

Immediately after hatching, fry are very active (Frost and Kipling 1967;

Howard and Thomas 1970). Within the first day, however, most attach to vegetation, where they remain for several days while the yolk sac is being absorbed

(Frost and Kipling 1967). In hatchery jars, with no vegetation, fry became

quiescent after the initial burst of activity and sank to the bottom (Howard

and Thomas 1970).

Fry average 7 to 9 mm in 1ength at hatchi ng (Frost and Ki p1i ng 1967;

Forney 1968). They begin to emigrate from spawning sloughs when 15 to 20 mm

long (Carbine 1942; Franklin and Smith 1963; Forney 1968). The time required

to grow to this size is variable. Emigration began 10 to 13 days after hatching in New York (Forney 1968) and 16 to 24 days after hatching in Minnesota

(Franklin and Smith 1963).

Emigration may be concentrated or prolonged. Forney (1968) reported that

82, 99, and 37% of the fry left a controlled marsh within 20 days after emigration started, in 3 successive. years. Data of Franklin and Smith (1963) indicate similar variability in the length of time for emigration. Most young-ofthe-year pike emigrated from a Saskatchewan marsh within a 1-week period,

6 weeks after the completion of spawning (Koshinsky 1979). The reasons for

this variability are unclear. Emigration is inhibited by low light intensity;

pro longed peri ods of overca st weather mi ght extend the emi grati on peri od

( Frankl in and Smi th 1963; Forney 1968). Other factors, such as water 1eve1

and food supply, may also determine when the fry depart (Royer 1971).

10
Growth and survi va1 rates of northern pi ke fry depend on temperature.

Survival is poor at temperatures less than 5.8° C (Lillelund 1966). Growth

rate is positively related to temperature for pike fry held at constant temperatures between 7.5° C and 25.6° C, but mortality of I-day old yolk sac fry

increases significantly at temperatures greater than 20.8° C; thus, the rate

of net biomass gain is highest at 20.8° C (Hokanson et al. 1973). The rate is

high for temperatures from 18.0 to 25.6° C.

Northern pike are spring spawners, and early developmental stages experience a progressively warmer average temperature. Tolerance of hi gh temperatures increases within the embryo and fry stages (Lillelund 1966; Hokanson et

al. 1973). The optimal temperature for growth and survival of fry increases

from about 21° C shortly after hatching to 26° C after 1 or 2 weeks (Hokanson

et al. 1973).

Juvenile. Young of the year and yearling northern pike fed ad libitum

grew in weight most rapidly at 19° C; maximum growth in length occurred at

21° C (Casselman 1978). Growth rate increased sharply at temperatures greater

than 10° C but was positive (about 4% of the maximum) even at 3 to 4° C. No

growth occurred above 28° C. The growth rate of yearling northern pike held

in small Mi ssouri ponds (average depth = 1.0 m) approached zero duri ng a 3 to

5 week peri od in mi dsummer when water temperatures exceeded 27° C (Wei thman

and Anderson 1977).

Dissolved oxygen concentrations below 7 ppm (77% saturation) resulted in

reduced growth rates of juvenile northern pike at a temperature of 18.6° C and

a flow rate of 0.24 ml/sec (Adelman and Smith 1970b). The decline in growth

rate was gradual down to concentrations of 3 to 4 ppm (33 to 44% saturation)

and sharp at levels below 3 ppm.

Young of the year northern pike (12 to 38 g) showed signs of stress when

held for 20 to 24 hours at dissolved oxygen concentrations less than 6 ppm

(temperature 21 to 22° C) (Petit 1973). Feeding ceased at concentrations of

2 ppm, and none of the test fish survived more than 20 hours at 1 ppm. Test

fish held at 1 ppm until they appeared close to death recovered within 24

hours when transferred to water wi th a di sso 1ved oxygen concentration of

6 ppm.

Adults. Temperature requi rements of adult and juvenil e northern pi ke

appear to be similar. Maximum growth in length of age 2 and 3 northern pike

in an Ontario lake occurred at approximately 20° C; fish of similar age reared

in the lab grew best at approximately 21° C (Casselman 1978).

Northern pi ke in Lake Wi ndermere (Engl and), where water temperatures

rarely exceed 21° C (Macan 1970), grew faster in warm than in cold years

(Frost and Kipling 1967). Surface water temperature, indexed as the cumulative

number of degree-days over 14° C, accounted for 86% of the variation in weight

of 4-year old pike, for 16 year-classes. Comparison of scale patterns for

northern pike from Waskesiu Lake in northern Saskatchewan with those for pike

from several shallow Michigan lakes (Williams 1955) suggests that high summer

temperatures may interrupt the growth of northern pike in more southern

regions.

11
Temperatures below 4° C do not appear to be stressful for juvenile or

adult northern pike when cooling is gradual (Casselman 1978). In fact, male

northern pike in an Alberta lake achieved about 35% of their total annual

growth during the winter at temperatures near 1° C (Diana and MacKay 1979).

Sudden temperature drops can, however, be 1etha1. A s i zeab1e northern pi ke

kill occurred when water temperatures in an Alberta 1ake recei vi ng heated

effluent dropped from 21.8° C to 4.9° C within 30 minutes after a power plant

shutdown (Ash et a1. 1974).

HABITAT SUITABILITY INDEX (HSI) MODELS

Model Description - Lacustrine and Riverine

The Habitat Suitability Index (HSI) models which follow are attempts to

condense the preceding observations into a manageable set of habitat evaluation criteria, structured so as to produce an index between 0.0 and 1.0 of

overa 11 habitat quality for northern pi ke. A positive re 1ati onshi p between

HSI and carrying capacity of the habitat is assumed (U.S. Fish and Wildlife

Service 1981).

Separate HSI models are presented for lacustrine (Fig. 1) and riverine

(Fig. 2) habitats, but the two are very similar and are more appropriately

described as variations of a single general model. The only differences are

that the riverine model contains two additional variables (V. and Vg ) to

account for the availability and accessibility of low velocity habitat, and

TDS (V..) can sometimes be excluded from the riverine model. It is implicit

that V. and Vg are not limiting in lentic environments; that is, both would

have suitability indices equal to 1.0.

The model includes habitat variables believed to be of general importance

in limiting the occurrence of northern pike. The assumed functional relationship between each habitat variable and habitat suitability for northern pike

is represented in a suitability index (SI) graph. An SI graph associates SI

ratings (on a 0.0 to 1.0 scale) with different levels of an environmental

variable. It is assumed that SI ratings for different habitat variables can

be compared. Overall habitat suitability is assumed to be determined by the

variable with the lowest suitability index; that is:

HSI (lacustrine) = minimum value for suitability indices Vl to V7

HSI (riverine) = minimum value for suitability indices Vl to Vg

12
Habitat variables Suitability indices

Ratio of spawning habitat to

summer habitat [area that is

less than 1 m deep and

vegetated (spring) divided

by total midsummer area] (VI) ------------ SI 1

Drop in water level during embryo

and fry stages (V2 ) --------------------- SI 2

Percent of midsummer area with

emergent and/or submerged

aquatic vegetation or remains

of terrestrial plants (bottom

debris excluded) (V) ---------- SI)

Log 1 o TDS in epilimnion during

midsummer (V 4

) ---~---- SI 4

Least suitable pH in spawning

habitat during embryo and

fry stages (Vs) ------------- SIs

Average length of frost-free

season (V 6 ) --------------------------- SI 6

Maximal weekly average

temperature of the

epilimnion (1 to 2 m

deep) (V 7 ) ----------....:...--- SI 7

Figure 1. Diagram showing habitat variables included in the lacustrine

HSI model for northern pike and the aggregation of the corresponding

suitability indices (SIl s) into an HSI. HSI = the lowest of the seven

suitability index ratings.

13

HSI
Habitat variables Suitability indices

Ratio of spawning habitat

to summer habitat [area that

is less than 1 m deep and

vegetated (spring) divided

by total midsummer area] (VI) ------------ S1 1

Log 1 o TDS during midsummer (V4 ) ----------- SI 4

Stream gradient (Vg ) --------------------- SI g

Average length of frost-free

se ason (V6) --------------------------- S16

~._ HSI

SIs

Least suitable pH in spawning

habitat during embryo and

fry stages (Vs ) ------------

Maximal weekly average

temperature (1 to 2 m

deep) (V 7 ) -------------SI 7

Area of backwaters, pools, or

other standing/sluggish

« 5 em/sec) water during

summer, as a percent of

total area (V s) ------------ SIs

Percent of midsummer area with

emergent and/or submerged

aquatic vegetation or remains

of terrestrial plants (bottom

debris excluded) (V3 ) --------- S1 3

Drop in water level during embryo

and fry stages (V2 ) -----------S1 2

Figure 2. Diagram showing habitat variables included in the riverine

model for northern pike and the aggregation of the corresponding

suitability indices (SI's) into an HSI. HSI =the lowest of the nine

suitability index ratings. V4 should be eliminated from the model under

certain conditions (see p. 24).

14
The model is designed to assign the highest HSI's to systems capable of

producing the most northern pike biomass per unit time on a sustained basis,

regardless of how that biomass is apportioned among individual fish. Few long

term data are available on production, yield, standing crop, or other measures

of populati on performance for northern pi ke under di fferent envi ronmenta1

conditions. Therefore, behavioral observations and measures of individual

performance, such as growth and survival, were also used to develop suitability

index curves for model variables. The rationale and assumptions used to

develop the curves are presented in a following section.

Modifications to the model, such as including different or additional

variables, may be necessary in some situations. The model is structurally

simple, and modifications are easily made.

Model Applicability

Geographic area. The model is applicable to lakes, reservoirs, rivers,

and streams throughout North America.

Season. The mode 1 is structured to account for sea sona1 changes in

weather and habitat requirements of northern pike and, thus, to indicate the

ability of a given habitat to sustain a population on a year-round basis.

Overwintering requirements may not be adequately addressed for northern pike

in shallow lakes or low discharge rivers that experience extended ice cover

(see below).

Cover types. The model is applicable to permanent lakes, ponds, and

reservoirs with a mean depth 1 m during midsummer. It is of doubtful reliability for shallow lakes (even those with mean depths> 1 m) that are covered

with ice for long periods. The suitability index graph for V3 (curve A) is

the only part of the model that addresses the possibility of winterkil1 of

northern pike due to low dissolved oxygen concentrations. It is recognized

that any single variable is an inadequate indicator of the occurrence and

severity of this phenomenon. However, an accurate, general model for predicting winter dissolved oxygen concentrations, based on easily measured or

predicted habitat characteristics, does not exist. [The empirical techniques

developed by Barica and Mathias (1979) for predicting winterkill in shallow

prairie lakes may be useful in some cases.] Because some of the factors that

affect the likelihood of winterkill are highly variable from year to year,

dissolved oxygen measurements for a single winter do not adequately indicate

the likelihood of occurrence of winterkill in a given lake over a series of

years. The fact that northern pi ke can fi nd and use pockets or 1ayers of

water with re lati vely hi gh oxygen concentrations (Magnuson and Karlen 1970;

Scidmore 1970) further complicates the modeling problem. Habitats that

experience occasional, partial winterkill of northern pike may nonetheless be

highly productive of northern pike. Rather than include additional variables

with no guarantee of improved model accuracy, it was decided to keep the model

simple and emphasize its questionable applicability to potential winterkill

lakes. Information presented by Greenbank (1945) and Schneberger (1970)

15
should help with the identification of such lakes. Material presented in the

Habitat Use Information section of this document should allow the construction

of a 51 graph for winter dissolved oxygen concentration, if such a variable is

appropriate for a particular model application.

The model app1i es to permanent ri vers and streams with base fl ows > 50%

of the average annual daily flow and assumes an average depth 0.5 m in pools

and backwaters. Those parts of intermittent tributaries that provide spawning

habitat for northern pike residing in a permanent river (or lake) shoul d be

included in the determination of V1 , Vz , and Vs '

It is common for northern pi ke to spend most of the year ina 1ake or

reservoir, but migrate upstream to spawn. In such cases, only the lacustrine

model need be applied. The utilization of upstream spawning sites is accounted

for in V1 , which includes all accessible potential spawning areas. If,

however, both lake and stream habitat are to be evaluated as summer habitat,

the ri veri ne model shoul d be used for the stream and the 1acustri ne model for

the lake. The fact that northern pike may utilize spawning grounds at considerab1e di stance from areas i nhabi ted duri ng the rest of the year can make it

difficult to establish study area boundaries for rivers, large lakes, or

chains of lakes.

Water qual i ty.

sewage, those known

particular solutes,

thermal regime.

The model is not appl icable to waters overburdened with

to contain toxic substances or extreme concentrations of

or where heated discharge significantly alters the normal

Minimum habitat area. The mt mmum area required for a self-sustaining

population of northern pike is not known. They occur in Ontario lakes as

small as 0.4 to 1.4 ha (Johnson et al , 1977).

Other. The model assumes the presence of at least one species of fish

that feeds primarily on macrophytes, plankton, macroinvertebrates, and/or

detritus. It is not necessary to specify abundance of potential prey species

because it is assumed that prey abundance is correlated with general system

productivity, as indicated by other model variables.

Verification level. The model represents the author's interpretation of

how specific environmental factors combine to determine overall habitat suitability. The model has not been field tested.

16
Suitability Index (51) Graphs for Model Variables

Suitability indices for variables in the lacustrine (L) and riverine (R)

HSI models should be determined from the following set of curves.

Use the appropriate curve, according to the density of vegetation or

plant debris within the area included as spawning habitat. Do not include

areas covered with filamentous algae, living or dead, as suitable spawning

habitat.

A. Vegetation obscures most (> 80%) of the bottom; plant material dense

throughout the 15 cm of the water column above the substrate. When

submersed, plant material not tightly compacted. Vegetation provides

extensive surface area to which eggs may adhere, abundant cover for

eggs and fry, and allows for circulation of water around the eggs

(e.g., flooded sedge or grass meadow).

B. Plant growth less lush than in A, but covers much of the substrate

and occupies much of water column directly above the sediments.

More than 60% of the bottom obscured in vertical projection from

depth of 15 cm above the bottom. Plant material not compacted when

under water.

C. Vegetation or debris covers much of the bottom, but plant material

does not occupy much of the water column immediately above the

substrate. This category may include compacted vegetation, branches

of woody plants, and leaves of deciduous trees.

17
D. Thinly scattered vegetation or debris only. Would provide little,

if any, shelter for eggs and fry.

L,R V2 Drop in water level 1.0

during embryo and

fry stages. 0.8

"'0

s::

A. Embryo and early .....

>,0.6 fry stages (until .jJ

yolk sac absorbed). 'r-

..... B. Fry stage, after :c 0.4

yolk sac absorbed. .jJ

.:; 0.2

51 for V2 = A or B, (/)

whichever is the 0.0

lowest. 0.0 0.5

m

1.0

L,R V3 Percent of midsummer 1.0

area with emergent or

submerged aquatic x 0.8

vegetation or remains Q)

"'0

of terrestrial plants s::

(bottom debris ..... 0.6

excl uded). 'r-

=;: 0.4

A. Max. depth < 3 m ..0

and lake ice- +-'

covered > 2 .:; 0.2

(/)

months.

B. Max. depth > 3 m 0.0

or 1ake i ce- O 25 50 75 100

covered S 2 months,

or both. %

18
L,R

V4 Logarithm (base 10) of 1.0

total dissolved solids

concentration of surface

x

(1) 0.8

waters (1 to 2 m deep) "t:I c during midsummer. ..... 0.6

.....

t-

.....

..c 0.4

to

+J

.....

:;, 0.2

V')

0.0

0 1 2 3

4

log ppm

L,R

Vs Least suitable pH in 1.0

spawning habitat during

I

,

embryo and fry stages.

x

(1) 0.8

"t:I C

.....

>, 0.6

+-'

.....

t-

.....

..c 0.4

to

+J

.....

:;, 0.2

V')

J 0.0

\

357

9 11

pH

L,R V, Average length of frost- 1.0

free season (average

number of days between 0.8 last spring occ~rrence x

(1)

and first fall occurrence "t:I c

of an air temperature of ..... 0.6 00 C). .....

::: 0.4

..c

to

+J

.; 0.2

V')

0.0

0 100 200 300

days

19
L,R V7 Maximal weekly average 1.0 ,

temperature of the ,

surface layer (1 to x 0.8 ,

2 m deep). Q) , -o

s:: , A

0.6 A. Strat ifi ed 1ake ,

0 , wi th 1. 5 ppm .r- ,

dissolved oxygen r-

.r- 0.4 ,

in metalimnion. .D , ttl

B. River, stream, , .r- 0.2 unstratified lake, , V'l , or stratified lake

with < 1. 5 ppm 0.0

dissolved oxygen 0 10 20 30 40

in metalimnion.

°c

R V. Area of backwaters, 1.0

pools, or other

standing/sluggish x 0.8 « 5 em/sec) water Q)

-o

during summer, as s::

a percent of total >, 0.6

surface area. .rr-

.r-

.D 0.4 ttl

.r- 0.2

V'l

0.0

0 25 50 75 100

%

R Vg Stream gradient. 1.0

x 0.8

Q)

-o

s::

0.6

0.rr-

.r- 0.4 .D

ttl

.r- 0.2

V'l

0.0

0 1 2 3 4 5

m/km

20
Development of Suitability Index Graphs: Rationale and Assumptions

The preceding suitability index graphs should be regarded as tentative

and open to modifi cation. The prospective user shoul d understand that the

curves are not the products of extensive laboratory or field investigations.

Rather, they reflect the author's subjective integration of the literature,

personal experience, and reviewer comments. The following discussion documents

some of the thought process that went into constructing the curves. Some

curves are better documented than others. In many cases, there is information

about preferred and limiting or unsuitable conditions, but little information

on which to base ratings of intermediate conditions. No particular significance should be attributed to inflection points unless specifically noted in

the text. Straight line segments can be substituted for the sigmoid portions

of the curves. The model is offered as a starting point, with the hope that

refinements will be made as additional information, including results of

testing the model, becomes available.

Ratio of spawning habitat to summer habitat (VIl. Spawning habitat availability is a logical starting point in the development of a habitat suitability

index model for northern pike. High spring water levels can create spawning

habitat if they flood terrestrial and wetland vegetation. The amount of new

spawning habitat that results depends on shoreline topography and the amount

of adjacent vegetation. The relative availability of spawning habitat can be

estimated by the ratio between the amount of surface area in the spring that

is shallower than 1 m and vegetated and the entire surface area of the water

body in summer.

The minimum ratio, below which spawning habitat availability limits

population size, depends on the carrying capacity, or productivity, of summer

habi tat. Other model vari abl es account for factors whi ch 1imit post-fry

stages. Therefore, the minimum ratio is estimated for the theoretical maximum

density of northern pike. Snow (1978) presented data suggesting a maximum

density of 40 pike greater than 35 cm in length per ha (20 kg/ha). Higher

densities could not be maintained even though food was plentiful. This maximum

density value was used to develop the suitability index curve for spawning

habitat availability.

It is assumed that spawning habitat suitability is positively, but

asymptotically, related to the area of spawning habitat per spawning female.

This means that an increase in the amount of spawning habitat per female is

more beneficial when spawning habitat is relatively scarce than when it is

plentiful. Limited evidence in support of this assumption is available.

Female northern pike have typically been stocked in managed spawning marshes

at densities between 5.4 and 23.2/ha (1 fish per 1,847 m2 and 431 m2 , respectively) (McCarraher 1957; Forney 1968; Royer 1971; Fago 1977). Increasing the

density of females from 11.4 to 22.4 fish/ha (5.9 to 17.6 kg/ha) only slightly

increased the yield of fingerlings from an experimental marsh in Saskatchewan

(Royer 1971). Max imum production from managed spawni ng marshes in Mi nnesota

was obtained at stocking rates of 11.2 kg of females/ha (Jarvenpa 1962b).

21
This assumed relationship between suitability and spawning habitat availabil ity must be sca1ed if it is to be used as the basis for a sui tabil ity

index curve. Two different scaling procedures were used, one based on spawning

behavior and a second based on what appeared to be representative values for

fingerling production from managed spawning marshes and subsequent survival

through adulthood.

Observations made by Fabricius and Gustafson (1958) under laboratory and

natural conditions provide a basis for estimating the area over which eggs of

a single II average" female are dispersed. These authors recorded the following

observations for pike « 50 em) spawning in an aquarium (bottom dimensions:

7.0 mx 0.8 m; depth: 0.4 to 0.5 m) at a temperature of 18° C:

mating frequency, within a series of mating acts: 1.5-2.6 mating acts/min

distance traveled between mating acts: 1 m

average duration of a series of mating acts: 5 min

average interval between successive series: 8 min

(At this time, the pike are at rest.)

Field observations of spawning pike indicated an average mating frequency of

1.7 acts/min, at a temperature of 12° C. This value corresponds to a net rate

of movement of 37.5 m/hr. If an average female carries 32,200 mature eggs

(Carbine 1944) and sheds an average of 60 eggs/spawning act [the upper limit

of the range reported by Svardson (1949)], 537 mating acts would be required

before all eggs were deposited. At a rate of 1.7 acts/min, spawning would

take at least 5.3 hr and cover a minimum distance of 200 m. If eggs are

scattered 0.5 m to either side of the spawning fish, a total area of 200 m2

(0.02 ha) would be involved. Assuming that spawning groups refrain from

spawning where eggs have already been deposited, the minimum ratio of spawning

habitat area to summer habitat area can be calculated as follows:

spawning habitat area = max. density x % females x minimum spawning area

summer habitat area female

= 40 pike 1 female 0.02 ha spawning habitat

ha summer habitat x 2 pike x female

=0.40 ha spawning habitat/ha summer habitat, for a

population with a balanced sex ratio.

The validity of this approach for estimating spawning habitat requirements

depends on the accuracy of several unproven assumptions about northern pike

spawning behavior, the density at which crowding occurs on spawning grounds,

and the consequences of crowding. An alternative procedure, based on estimates of fingerling production and survival, was tried as a check.

Fingerling production data are available for a large number of managed

marshes (Fago 1977). These data show production to be highly variable (7 to

22
72,000 fingerlings/ha), both among marshes and among years. The median

(2,717 fingerlings/ha) was used in the following calculations.

Survival of fingerlings through adulthood is also highly variable.

Estimated survival of fingerling northern pike through their first summer was

6%, 22%, and 63% in three successive years (mean = 28%) in a Michigan lake

(Beyerle and Williams 1972). Assuming 65% annual mortality thereafter [which

seems reasonable for lakes in the conterminous United States, on the basis of

data compiled by Carlander (1970)], 9.8% of the fingerlings would survive to

age 1+, 3.4~6 to age 2+, 1.2% to age 3+, and 0.4% to age 4+. Ages 2 through 4

usually account for most of the breeding stock for populations in the conterminous U.S. Assuming a stable age structure and that most mortality occurs

between autumn and spring, there would be a total of 0.034 + 0.012 + 0.004 =

0.050 adult fish in the population per fingerling produced in spawning habitat.

The minimum ratio of spawning to summer habitat can be calculated in the

fo 11 owi ng way:

spawning habitat area = max. density of adults x 1

summer habitat area spawning habitat productivity

x fingerlings per adult

=40 adults/ha summer habitat

x 1 ha spawning habitat/2,717 fingerlings

x 1 fingerling/O.OS adults

= 0.29 ha spawning habitat/ha summer habitat

If it is assumed that most mortality occurs between spring and autumn,

then the 1+ fish should be included in the calculation. There would be

0.098 + 0.034 + 0.012 + 0.004 =0.148 adult fish in the population/fingerling

produced, and the minimum ratio of spawning to summer habitat would be 0.10.

Both values are similar to that obtained using the method based on spawning behavior (0.40). An intermediate value, 0.25, is used to develop a suitabil ity index curve. It is assumed that optimal northern pike habitat must

have at least 0.25 ha of spawning habitat for each ha of summer habitat.

Habitats in which the relative availability of spawning habitat is considerably les~ than this can still have high suitability index ratings for V1 because of

the asymptotic shape of the suitability index curves.

Type of vegetation in spawning areas is also important. The probability

that fertilized eggs will settle on vegetation as they sink depends on vegetation density. Young northern pike also depend on vegetation for protective

cover and on the associated invertebrate fauna for food. Lush vegetative

cover is ideal; sparse cover is poor. The suitability index for V1 can be

maximal (1.0) only if proper vegetation is present.

23
Drop in water level during embryo and fry stages (V 21. Northern pike are

vulnerable to even slight water level changes during the incubation and nursery

periods because they usually spawn in water less than 0.5 m deep and often in

water as shallow as 0.1 to 0.2 m. Declining water levels can strand the

embryos, fry, and even adults. Embryos are particularly vulnerable because of

their immobility. Newly hatched fry become inactive after an initial burst of

activity and are, therefore, likely to be stranded by a decline in water

level. Once the fry begin to feed, they can probably follow a slowly receding

shoreline, provided that the pathway from the spawning area to summer habitat

is not blocked. The greater mobility of older fry is the reason for the

difference between curves A and B in the suitability index graph. Curve A has

a steep slope between 0.1 and 0.5 m because this is the depth range in which

most spawning occurs.

Percent of midsummer area with emergent and/or submerged aquatic vegetation or remains of terrestrial plants (bottom debris excluded) (V31.

The extent

of vegetative cover is an important component of habitat suitability for

northern pike for several reasons. Rooted macrophytes supplement planktonic

production and, presumably, increase the food supply for northern pike.

Vegetation provides a refuge from predation for young pike and hunting cover

for pike of all ages. Complete vegetative cover, however, is probably suboptimal. Northern pike are often associated with the vegetation-open water

interface (Reighard 1915). These presumed hunting stations would not be

available if the water body were totally covered with vegetation. Furthermore,

decomposing vegetation can deplete dissolved oxygen during winter in shallow,

ice-covered lakes and rivers with low discharges. It is assumed that optimal

habitat would have extensive submerged and/or emergent aquatic vegetation

interspersed with open water. At the present time, the model does not contain

an interspersion variable.

Logarithm (base 10) of TDS in surface waters during midsummer (V 41. Low

levels of TDS usually indicate low fertility, while high concentrations can

cause ion regulatory or osmoregulatory stress. A positive relationship between

TDS and food supply for northern pike is assumed for TDS values between 0 and

80 ppm. Habitat suitability is assumed to remain constant for TDS levels

between 80 and 800 ppm and to decrease at nigher levels, with zero suitability

for TDS values> 3,500 ppm.

TDS is not a reliable indicator of food supply in most rivers and streams.

This variable should be excluded from the riverine model unless TDS is

> 800 ppm.

Least suitable pH in spawning habitat during embryo and fry stages (Vsl.

Northern pike are tolerant of a wide range of pH values but pH can be a locally

or regionally important component of habitat suitability. There is no evidence

of any adverse effects over the pH range of 6.0 to 9.0. Northern pike reproduction is markedly impaired when pH falls below 5.0. An upper limit is not

well defined, but pH levels higher than 9.0 appear to interfere with reproduction. Embryos and fry seem to be more sensitive to pH than do other life

hi story stage s.

24
Average length of the frost-free season (V61. The length of the frostfree season correlates with the length of the growing season for northern

pike; however, if the cold weather season is too short, normal gonadal development may be impa ired. The suitabil ity index curve is based on growth data

for northern pike, laboratory observations on yellow perch spawning (see

text), the re 1at i ve position of i sop 1eths for 1ength of frost free season

(Visher 1954), and the southern limit to the native range of northern pike

(Crossman 1978).

Maximal weekly average temperature of surface water (1 to 2 m deep) (V71.

Summer water temperature affects growth and survival of northern pi ke. The

optimal temperature for growth of fingerling and older northern pike is 19 to

20° C (Casselman 1978). Temperatures will be in the optimal range during

warming and cooling phases if the maximal summer temperature is slightly

higher than the growth optimum. Maximal suitability of V7 is assumed for

peak weekly average temperatures of 20 to 25° C. Laboratory studies show that

temperatures over 32° C can kill northern pi ke wi thi n several days (Scott

1964). An oxygenated metalimnion, mouths of tributaries or inlets, or springs

may be used when temperatures elsewhere are too high. Their effectiveness as

thermal refugia depend on their size and the duration of hot spells.

Percent pools and backwaters during midsummer (Va) and stream gradient

lY91. Riverine habitat may be suitable to the extent that it resembles a lake

envi ronment. Northern pi ke requi re standi ng, or very slowly movi ng, water.

Two variables are included as indicators of the presence of low velocity

habi tat: percent pools and backwaters duri ng midsummer and stream gradi ent.

A direct, proportional relationship between the availability of standing water

habi tat (Va) and habi tat suitabil ity is assumed. However, a hi gh gradi ent

stream is likely to be of low suitability even if pools are present, because

movement between pools would be restricted. It is assumed that northern pike

are less well adapted to high gradient streams than are muskellunge, based on

patterns of distribution for the two species (Crossman 1978; Harrison and

Hadley 1978). Muskellunge occur in stream reaches with gradients as high as

6.9 m/km (Parsons 1959), but gradients less than 2 m/km seem to be more typical

of riverine muskellunge habitat (Brewer 1970; Miles 1978). It is assumed that

stream reaches with gradients greater than 5 m/km (0.5%) have little or no

habitat value for northern pike.

Field Use of the Model

Year to year variation in weather complicates the application of this

habitat model. Such variability is most pronounced for variables related to

temperature (V6 and V7 ) and water level (Vi and V2 ) . Between-year variation

does not pose a problem if the objective of an HSI model application is to

obtain an HSI rating for a single year. More often, however, the goal is to

obta in an average or representative HSI for aparti cul a r body of water that

can be compared with HSI's for other bodies of water or for the same body of

25
water at different times in the future. If it is known that the fluctuation

mostly occurs within a range over which the suitability index does not change,

then no further consideration of the variability is necessary. Problems

arise, however, when stochastic variation in a model variable can result in

drastically different HSI's for different years. Ideally, sufficient long

term data to estimate probabilities of occurrence of different levels of the

habitat variable would be available. The average length of the frost-free

season (average number of days between the last spring occurrence and first

fall occurrence of an air temperature of 0° C) between 1941 and 1970 is known

for many areas in the United States (National Oceanographic and Atmospheric

Administration 1978). Average values for selected areas in southern Canada

were compiled by Chapman and Brown (1966). Long term water temperature and

water level data are also available in some cases. It is recommended that

suitability indices used for determination of HSI1s be averages of suitability

indices corresponding to measured, estimated, or projected conditions over a

series of years. Even if long term data are not available, the degree to

which water temperature or water level measurements for a single year are

"representative" can be evaluated by comparing air temperature, precipitation,

and/or stream discharge data for the year in which the measurements were made

with corresponding values for preceding years. Climatological summaries and

records for U.S. Geological Survey stream gauging stations are possible sources

of such information.

Several reviewers had reservations about including V6 (average length of

frost-free season) in the model. The variable is not useful for distinguishing

between water bodies that are close to one another and at the same elevation.

It is useful when considered over a wider geographic area. If the model is to

be applied over a limited area and comparisons of HSI's are to be made only

for water bodies within that area, then V6 can be deleted.

Daily variations in water temperature can be considerable. Daily means,

rather than extremes, shoul d be used in thi s mode 1, because northern pi ke can

to 1erate bri ef exposures to moderately hi gh temperatures. It is recommended

that V7 (maximum weekly average water temperature) be determined by averaging

the means of dai ly minimum and maximum water temperatures for the warmest

(water temperature) week of the year. The minimum daily temperature usually

occurs sometimes near dawn, and the maximum generally occurs in the early to

middle afternoon. Day-to-day monitoring of water temperature is not always

possible. Whatever procedure is used to estimate V7 should account for the

daily temperature cycle.

A measurement depth of 1 to 2 m was specified for water temperature (V 7 ) .

In cases where curve B is to be used (e. g., unstratifi ed 1akes or stratifi ed

lakes with < 1.5 ppm dissolved oxygen in the metalimnion), measurements should

be made in the littoral zone from 1 m below the surface to the bottom. The

most favorable temperature for northern pike, based on the SI curve for V7 ,

should be used to determine V7 •

26
Aerial or ground surveys can be used in conjunction with past or projected

water level data to estimate spawning habitat availability (VI) over a series

of years. Many plants have luxuriant above-surface growth during summer and

early autumn only to die back during late autumn and winter. Spawning habitat

quality depends on vegetative cover during spring. Also, if vegetation is

examined at a time when it is not under water, its characteristics when submersed should be estimated.

The time and duration of the embryo and fry stages must be determined in

order to properly estimate V2 and Vs . Spawning typically begins when the

wat~r temperature reaches 8° C and is usually complete when water temperatures

have remained at or above 13° C for several days. Allowing 2 weeks for incubation, 2.5 weeks for fry to grow to the size at which emigration begins, and

3 weeks for most of the fry to depart from the nursery grounds, there appears

to be a critical period of 7.5 weeks during which water level drawdowns can

affect reproductive success. Absorption of the yol k sac requires about 10

days in Minnesota (Franklin and Smith 1963). The first 3.5 weeks after spawni ng (i ncubat i on and sac-fry stages) woul d, therefore, seem to be especi ally

critical. These values can serve as general guidelines, but should be revised

if there is reason to believe that the critical intervals are different for a

particular study site.

The percent of midsummer area covered with vegetation (V 3 ) and the percent

of area as pools, backwaters, or other standing/sluggish water (VB) can be

estimated by following procedures outlined in Terrell et al. (in press).

Several of the variables included in these models do not require field

measurements. As noted above, average length of frost free season (V6 ) can

usually be obtained from a local weather station or from a published climatologi ca 1 summary (Nat i ona1 Oceanographi c and Atmospheri c Admi nistration 1978).

If it is necessary to interpolate a value, the relative elevations of the

study site and of reference stations should be considered.· Stream gradient

(Vg ) can be calculated from U.S. Geological Survey topographic maps. It is

not necessary to measure TDS (V4 ) or pH (Vs ) if it is known that the measurements would fall within the broad ranges in which suitability indices equal

1.0 or 0.0. Otherwise, the measurement techniques described by Lind (1974)

can be used.

Rivers, streams, and surface waters of lakes are usually well-mixed.

Horizontal variation in water quality variables, such as temperature, pH, and

TDS, are rarely so large as to affect suitability index ratings. Sheltered

bays, mouths of tributaries and inlets, and springs are possible exceptions to

this generalization. If measurements made at different sites result in different suitability indices, the suitability indices, rather than the raw

habitat measurements, should be averaged. Each suitability index should be

weighted according to the fraction of the total area over which it is assumed

to apply.

27
The user must decide what part of a body of water is to qual ify as "summer

habitat". To this point, it has been implied that the entire area would be

used. This is not necessarily so. In the case of a large, stratified lake,

for example, only that area shallower than the depth of the bottom of the

thermocline might be included as as " available habitat". In a river, areas

where the velocity exceeds some threshold value might be excluded. If such a

preliminary screening step is taken, the decision about limiting the area used

as available habitat applies for all model variables. The area included as

summer habitat for V1 should also be used as the total area (that is, the

divisor) for V3 and VB; the model output (HSI) applies only to this area.

Interpreting Model Outputs

The model described above can generate HSI's to any- desired number of

decimal places, but it would be misleading to present results to a level of

precision greater than ±O.l. The model cannot be expected to discriminate

among different habitats with high resolution at this stage of development.

It depends on a series of untested assumptions and known oversimplifications.

Interactions among model variables and species interactions both playa role

in determining habitat quality for northern pike, but these influences are

ignored. I recommend interpreting HSI outputs as indicators (or predictors)

of excellent (0.8-1.0), good (0.5-0.7), fair (0.2-0.4), or poor (0.0-0.1)

habitat.

Habitats with high HSI I s would, on average, be expected to have higher

standing crops of northern pike than habitats with low HSI's, but a close

correlation between population size and HSI is unlikely. Factors not included

in this habitat model can also limit northern pike populations.

ADDITIONAL HABITAT MODELS

Aggus and Bivin (in press) used angler harvest as the criterion of habitat

suitability and calculated a regression equation relating harvest to reservoir

habitat variables for 37 impoundments in the conterminous United States:

Log1 o harvest =3.7882 - 0.0177 (growing season) - 0.8447 (Log 1 o outlet depth)

R2 = 0.67

Units are kg/ha (harvest), days (growing season), and feet below a specified

elevation (outlet depth). These authors discuss procedures for converting

measured or predicted harvest values to HSI's.

28
Jenkins (1982) calculated the regression equation using mean depth (m),

rather than outlet depth:

LOg 1 D harvest = 3.918 - 0.017 (growing season) - 1.425 (LOg 1 D mean depth)

N = 35

R2 = 0.53

These regression equations are based on reservoirs with growing seasons

100 days and outlets between 0.3 m (surface) and 42.7 m deep (Aggus, unpubl.

data) and should not be applied to reservoirs that do not met these criteria.

It 'should also be noted that northern pik.e populations in many of these

reservoirs were supported by stocking, so reproductive requirements may not be

accounted for in the above equations.

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32
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33
Hunt, B. P., and W. F. Carbine. 1951. Food of young pike, Esox lucius L.,

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34
June, F. C. 1970. Atresia and year-class abundance of northern pike, Esox

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35
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36
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spawning success

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37
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38
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39
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50272 '101

REPORT DOCUMENTATION ! 1. REPOlfT NO. I~ \ J. Red...... Acc8uloft No.

PAGE i· - FWSjOBS-82jlO.17 i I '" Tltll .nd Subtltll I5. ReIlOIt 0_

Habitat Suitability Index Models: Northern Pike July 1982

I.

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Peter D. Inskip

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U.S. Department of the Interior 14-

Washinaton D.C. 20240

15. Suoo....._.ry Not..

. II. AllItraet (u,.,n: 200 _nIIl

Literature describing the habitat preferences of the northern pike (Esox lucius Linnaeus)

is reviewed, and the relationships between habitat variables and life requisites are

synthesized into a Habitat Suitability Index (HSI) model. HSI models are designed for use with the.Habitat Evaluation Procedures (HEP) in impact assessment and habitat

management activities.

17. 00F""'_ A".lysi. a. O....,;ocora Anlmal behavior Habit

Aquatic biology Fishes

Habi tabi 1i ty

O. Idlntlti'~/OlMfI·£ndld Tlrm.

Northern pike Impact assessment

Esox lucius Linnaeus Habitat management Habitat Suitability Index (HSI)

Habitat Evaluation Procedures (HEP)

c. COSAT1 Filld/GrauD

18. Av.ilaoility Statl,.,lnt lSI. security Clas. (Thi. Rloortl I 21. No. of P....

RELEASE UNLIMITED UNCLASSIFIED iii - v +40pp

20. Securtty CIa•• (Thi. P••II ; 22. PriCI .,; , . UNCI A~~i~9~_, ;: .

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CeDartmlnt ot Commlrcl
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Headquart...Office ()f BJoJogIcaJ

Servtces, Wuhlngton, D.C.

Natlona' Coastal Ecoeyatema Team,

Slidell, La

Western Energy and Land UseTeam.

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.~

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