13

NOAA Technical Report NMFS 13

Guidelines for Reducing

Porpoise Mortality in

Tuna Purse Seining

James M. Coe, David B. Holts, and Richard W. Butler

September 1984

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service
NOAA TECHNICAL REPORTS NMFS

The major responsibilities of the National Marine Fisheries Service (NMFS) are to monitor and assess the abundance

and geographic distribution of fishery resources, to understand and predict fluctuations in the quantity and distribution of

these resources, and to establish levels for optimum use ofthe resources. NMFS is also charged with the development and implementation of policies for managing national fishing grounds, development and enforcement of domestic fisheries regulations, surveillance of foreign fishing off United States coastal waters, and the development and enforcement ofinternational

fishery agreements and policies. NMFS also assists the fishing industry through marketing service and economic analysis

programs, and mortgage insurance and vessel construction subsidies. It collects, analyzes, and publishesstatistics on various

phases of the industry.

The NOAA Technical Report NMFS series was established in 1983 to replace two subcategories of the Technical

Reports series: "Special Scientific Report-Fisheries" and "Circular." The series contains the following types of reports:

Scientific investigations that document long-term continuing programs of NMFS, intensive scientific reports on studies of

restricted scope, papers on applied fishery problems, technical reports of general interest intended to aid conservation and

management, reports that review in considerable detail and at a high technical level certain broad areas of research, and

technical papers originating in economics studies and from management investigations.

Copies of NOAA Technical Report NMFS are available free in limited numbers to governmental agencies, both Federal

and State. They are also available in exchange for other scientific and technical publications in the marine sciences. Individual copies may be obtained from: Publications Services Branch (E/AI 13), National Environmental Satellite, Data, and Information Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, 3300 Whitehaven

St., Washington, DC 20235.
NOAA Technical Report NMFS 13

Guidelines for Reducing

Porpoise Mortality in

Tuna Purse Seining

James M. Cae, David B. Holts, and Richard W. Butler

September 1984

U.S. DEPARTMENT OF COMMERCE

Malcolm Baldrige, Secretary

National Oceanic and Atmospheric Administration

John V. Byrne, Administrator

National Marine Fisheries Service

William G. Gordon, Assistant Administrator for Fisheries
The National Marine Fisheries Service (NMFS) does not approve, recommend or

endorse any propriety product or proprietary material mentioned in this publication.

No reference shall be made to NMFS, or to this publication furnished by NMFS, in

any advertising or sales proIrotion which would indicate or imply that NMFS

approves, recommends or endcrses any proprietary product or proprietary material

mentioned herein, or which ha~ as its purpose an intent to cause directly or indirectly the advertised product to be used or purchased because of this NMFS publication.
CONTENTS

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

Prevention of prerelease mortality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Towing with speedboats to prevent net collapse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Net depth effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

The mid-net zipper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Downwind sets .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Using speedboats to move porpoise inside the net. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Equipment malfunctions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Porpoise release. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Condition of net and equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Corkline hangings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Holes in webbing 8

Bow bunch failure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Vessel control systems 8

Configuration of the net for backdown "............................ 8

Dynamics of backdown 8

Tie-down point formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Porpoise safety panel "......................................... 10

Super apron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Executing backdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Porpoise behavior during backdown 13

Trouble shooting backdown performance ". . . . . . . . . . . . . . 13

Porpoise release without backdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Porpoise rescue , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Equipment for raft rescue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Techniques of raft rescue 15

Role of captain and crew. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Literature cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figures

1. Average porpoise kill per observed set 2

2. Percent of observed porpoise sets with zero porpoise killed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3. Purse seine towing points for holding net open using speedboats. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4. Idealized configuration of net under tow by speedboats. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

5. Rigging for speedboat towing bridle 4

6. Deeper nets increase surface area 4

7. A two-strip addition in the center of a 10- or 12-strip net can be accomplished with 5, 8, or 10 bales of web, depending on

the length of net .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

8. Mid-net zipper and fathometer trace. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

9. Always set after turning into the wind. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

10. Procedure for turning vessel around after making a downwind set. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

11. Use speedboats to prevent porpoise from entering dangerous areas of the net and from entering the area where the corks come

out of the water during net roll. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

12. Functional distribution of webbing in a stabilized backdown channel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

13. To ensure the proper amount of corkline for backdown, clearly mark tie-down points for the net and last bow bunch

pulled ... "............................................................................................ 9

14. Backdown channel cross section showing development of stern sway during backdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

15. Determining proper tie-down points for backdown .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

16. Configuration of porpoise safety panel and super apron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

17. Porpoise safety panel must cover the perimeter of the backdown channel "........ 11

18. Multiple tie-down points for backdown in nets without aprons when using 2, 3, or 4 bow bunches. . . . . . . . . . . . . . . . . . . . . . 11

19. Straighten out the channel apex before backdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

20. Proper backdown arc insures efficient waterflow to maintain a wide and deep channel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Tables

1. Summary statistics of porpoise sets from NMFS observer trips, 1971-80 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Numbers of porpoise killed by general cause in sets with greater than 15 porpoise killed, 1977-80 . . . . . . . . . . . . . . . . . . . . . . 3

iii
3. Major malfunction categories, 1977-79 7

4. Calculated distance in fathoms along the corkline from the apron apex to the stern tie-down point 10

iv
Guidelines for Reducing Porpoise Mortality in

Tuna Purse Seining

JAMES M. COE,! DAVID B. HOLTS,2 and RICHARD W. BUTLER3

ABSTRACT

More than a decade has passed since the passage of the Marine Mammal Protection Act of t972. During that

time the U.S. tuna purse seine neet reduced its incidental porpoise mortality rate more than to·fold. This was

made possible through the development of gear and techniques aimed at reducing the frequency of many low

probability events that contribute to the kill.

Porpoise are killed by becoming entangled or entrapped in folds and canopies of the net and suffocating.

The configuration of the net, both before and during the backdown release procedure, is a major determinant of

the number of porpoise killed. Speedboats can be used to tow on the corkllne to prevent net collapse and also to

adjust the net configuration to reduce net canopies prior to backdown. Deepening a net can reduce the probabili·

ty of porpoise being killed by prebackdown net collapse. The effects of environmental conditions and mechanical

failures on net configuration can result in high porpoise mortality unless mitigated by skilled vessel maneuvers or

prevented by the timely use of speedboats to adjust the net.

The backdown procedure is the only means to effectively release captured porpoise from a purse seine. It Is

also the time during the set when most of the mortality occurs. The use ofsmall mesh safety panels and aprons in

the backdown areas of nets reduces porpoise entanglement, and Increases the probability of an effective release.

The tie·down points on the net for preparing the backdown channel must be properly located in order to optl.

mize porpoise release. A formula uses the stretched depth of the net to calculate one of these points, making It a

simple matter to locate the other. Understanding the dynamics of the backdown procedure permits a thorough

troubleshooting of performance, thus preventing the repetition of poorly executed backdowns and thereby

reducing mortality.

Porpoise that cannot be released must be rescued by hand. A rescuer in a rigidly Innated raft can rescue porpoise effectively at any time during a net set. Hand rescue can make the difference between above average kill

and zero kill sets. In all circumstances, the skill and motivation of the captain and his crew are the final determinants in the prevention of incidental porpoise mortality in tuna seining.

INTRODUCTION

The purse seining method of catching yellowfin tuna, Thunnus

albacares, in association with schools of porpoise4 in the eastern

tropical Pacific has been described by Perrin (1969) and Peters

(1979). The primary target species, in order of importance, are the

spotted dolphin, Stenella attenuata, and the spinner dolphin, S.

longirostris, with occasional net sets made on schools of the

common dolphin, Delphinus delphis. Schools of these porpoise,

ranging in size from 50 to several thousand, are herded with

speedboats and encircled with a purse seine net that is from 900 to

1,400 m long and as much as 130 m deep. After the porpoise are

encircled, the bottom of the net is pursed, entrapping the mammals and any tuna that are associated with the school.

'Southwest Fisheries Center La Jolla Laboratory, National Marine Fisheries Service, NOAA, 8604 La Jolla Shores Drive, La Jolla, Calif.; present address: Northwest

and Alaska Fisheries Center, National Marine Fisheries Service, NOAA, 2725

Montlake Blvd. E., Seallle, WA 98112.

'Southwest Fisheries Center La Jolla Laboratory, National Marine Fisheries Service, NOAA, 8604 La Jolla Shores Drive, La Jolla, CA 92038.

'Southwest Fisheries Center La Jolla Laboratory, National Marine Fisheries Service, NOAA, 8604 La Jolla Shores Drive, La Jolla, Calif.; present address: 3327

Somerset Way, Carson City, NY 89701.

'Porpoise, in this paper, is used as a general term referring collectively to all small

cetaceans impacted by the fishery. While dolphin is the more scientifically correct

term, porpoise continues to be the term used by fishermen and researchers alike. The

three major species involved are the spOiled, spinner, and common dolphins: Slenella

attenuata, S. longirostris, and Delphinus delphis.

The incidental mortality rate of these marine mammals was

estimated to exceed 40!setS in 1972, the year the Marine Mammal

Protection Act was passed (Table I, Fig. I). The U.S. tuna fleet,

with assistance from both private and government fishery specialists, succeeded in reducing their kill rate to 4!set by 1980. During

this period the development and utilization of new gear and procedures, coupled with increased operator skill, reduced the frequency of high kill sets (Table I) and increased the number of

zero-kill sets (Fig. 2). The technology leading to these results is in

varied used in both the domestic and foreign tuna fleets.

Individual captains have repeatedly demonstrated that over

99.9% of all captured porpoise can be released unharmed when

the gear and techniques described in this report are skillfully

employed. In the absence of future innovations to reduce mortality, further improvements in fleet performance may only be gained

by heightening the skill and motivation levels of the vessel

operators and by improving the durability and reliability of vessel

equipment. This is no easy task. Vessel operators must be constantly prepared to prevent, avoid, or remedy a collection of very

low probability events, anyone of which may result in porpoise

being killed. They must attempt to minimize the negative affects

of environmental conditions, equipment failures, and porpoise

behavior on the outcome of each set. This diligence must be maintained in the face of the risk of losing part or all of the tuna catch

'Mortality or kill refers to porpoise mortality incidental to the calch of yellowfin

tuna, Thunnus albacores. The "set" is defined as a single deployment of a purse seine

net around an aggregation of porpoise andlor tuna. Tuna catches are expressed in

short tons.
50

I- 40

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° ;gd~G'g~~;:-~;g~ Q, 00 r- o.. e:;-g _.. ~~ :::..e~ ...I 20 '" 0 r- r- '" ~~q~~~;::;~:~~

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76

YEAR

1972 73 74 75

Figure I.-Average porpoise kill per observed set.

Figure 2.-Percent of observed porpoise sets wltb zero porpoise killed.

100

while attempting to release every captured porpoise. The annual

kill quotas for the U.S. fleet are set at 20,500 animals/yr (NOAA

19806 ) for 1981 through 1985. At this level, the performance of

individual vessels can have a major impact on the length of the

fishing season, justifying extreme diligence by vessel operators.

The intent of this paper is to bring together information relevant to the occurrence and prevention of porpoise mortality in

tuna purse seining as a record of the present state of the art and as

a baseline for future research and development. It should be noted

that there are many events which may increase the probability of

porpoise being killed for which no specific preventive or remedial

measures exist. The most important of these are discussed.

Porpoise are killed when confined by the net in such a way that

they are unable to rise to the surface to breathe. Aside from occasional shark attacks and encounters with vessel power equipment

(speedboats, power block, net skiff, brailer, etc.), death by suffocation is the rule. Physically, the animals are either entangled

individually in the meshes or are entrapped singly or in groups by

folds or "canopies" of webbing. The probability that animals will

become entangled or entrapped depends upon the configuration of

the net, the number of porpoise and amount of tuna captured, their

behavior, and the skill of the vessel operator. Table 2 shows the

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-

"NOAA. 1980. Draft Environmental Impact Statement: Proposed regulations and

conditions for the issuance of a general permit governing the taking of marine mammals associated with tuna purse seining operations. Department of Commerce. National Oceanic and Atmospheric Administration, NMFS. November 1980, 88 p.

2
Table 2.-Numbers of porpoise kllIed by general cause in sets with greater than IS porpoise kllIed; NMFS observed trips 1977-80.

Number of Total Backdown canopies

Year sets (%) kill (%) Bow Stem Other

1977 135 (4.0) 5,277 (51.6) 749 1,539 774

1978 73 (4.6) 4,932 (68.0) 478 486 872

1979 54 (3.5) 2,879 (47.1) 960 173 43

1980 30 (2.9) 2,647 (65.7) 106 206 220

Total 292 (3.9) 15,735 (67.0) 2,293 2,404 1,909

Percent of

total kill 100 15 15 12

Backdown

Prebackdown channel Malfunction All other

Total net collapse collapse mortalities causes Unknown

3,062 440 346 329 339 780

1,836 1,897 338 397 243 221

1,176 284 326 84 925 84

532 29 1,504 303 140 139

6,606 2,650 2,514 1,113 1,647 1,224

42 17 16 7 10

primary causes of kill for observed sets with 16 or more porpoise

killed ("disaster sets") for 1977 through 1980. Note that the

primary causes are related to net configuration (Le., backdown

canopies, prebackdown net collapse, backdown channel collapse,

etc.). Most of the malfunction-related kill is also due to resultant

net-configuration problems. An example of an "other" cause of

mortality is when the dolphins are "sacked-up" in the net with the

tuna and suffocated. The unknown causes of kill are usually

unobserved but are probably related to the behavior of the captured porpoise, e.g., for no apparent reason some animals dive

under the boat and become entrapped in the net when trying to

return to the surface.

Prevention of incidental porpoise mortality involves three

distinct activities during the set: 1) From the time of encirclement

until its release, the porpoise school must be safely maintained

within the net, 2) the school must be released from the net without

allowing the tuna to escape and, 3) individual porpoise must be

rescued whenever possible.

PREVENTION OF PRERELEASE

MORTALITY

The captain must safely maintain the captured porpoise in the

net until he is able to release them using the backdown maneuver

(Coe and Sousa 1972), normally a period of about 1 h.

Table 2 indicates that a less-than-optimum net configuration is

a primary cause of porpoise mortality. Chief among the various

configuration problems is prebackdown net collapse, which can

be caused by strong currents, changes in wind direction and

strength, major equipment malfunctions, failure of the captain to

orient the set properly with the wind, or any combination of these.

Regardless of the cause of prebackdown net collapse, the outcome

is the same, i.e., a substantial portion of the captured porpoise

school may be killed and the tuna may be lost as well. Realizing

that the harvest of tuna will always involve occasional sets under

adverse environmental conditions and that equipment malfunctions cannot always be prevented, the main technique for preventing prerelease mortality addresses the symptoms of net collapse

rather than its causes.

Towing with Speedboats to Prevent Net Collapse

Use of speedboats can prevent or delay net collapse during

pursing and hauling operations (Everett et al. 19767). Net collapse

occurs when the surface area of the net is decreased, forcing ~he

'Everett, J., J. E. Powers, R. McNeely, J. M. Coc, and R. Butler. 1976. The use of

speedboats in reducing incidental porpoise mortality in tuna purse seining. Southwest Fisheries Center Admin. Rep. LJ-76-35, 29 p.

3

porpoise into contact with the webbing and preventing them from

surfacing to breathe. The effectiveness of speedboats in preventing net collapse is totally dependent on acting before the net has

collapsed around a school of porpoise. The captain conducting the

fishing operations must understand that once a net is collapsed,

speedboats will not be able to reopen it.

Four effective methods have been demonstrated for towing on

the net; each adapted to a particular set of circumstances. The four

methods (Fig. 3) are: 1) Towing directly on the corkline, 2) towing

on the end of one bunchline, 3) towing on the middle of an openended bunchline and, 4) towing on the ends of two adjacent

bunchlines (one reversible and one regular which together create a

towing point).

Towing directly on the corkline is best used to clear small

collapsed sections or small canopies in which the corkline is tight.

This method (Method 1) is particularly useful for eliminating a

partially collapsed backdown channel just prior to backdown. The

best towing method when the corkline is not under tension is to

tow on the bunchlines so the excess slack in the corkline is

gathered together into bunches (Fig. 4). Here the speedboat is

hooked- up to the ends of two adjacent bunchlines (Method 4), one

regular and one reversible, which together create a single towing

point specifically designed for this purpose. Since the net takes

many different shapes while it is pursed and/or collapsed, it may

not always be possible for the speedboat to pull on the towing

points. In this case the fish captain may require the speedboat to

APRON

ALIGNMENT

BUOY

6ACKOOWN APEX

Figure 3.-Purse seine towing points for holding net open using speedboats. The

three reversed bunchlines should be strategically located around the perimeter

of the net but should not interfere with the porpoise apron (Method 4). Speedboats may at times Ond it beller to pull only one regular bunchline from Its end

(Method 2), or from its center If Its open-ended (Metbod 3). Speedboats may

also pull directly on the corkllne (Method I) to remove small net collapses and

canopies.
10 FATHOMS OF

Y,' POLYPROPYLENE

Figure 4.-Idealized configuration of net under tow by speedboats.

tow on the middle (Method 3) or the end of a single bunchline

(Method 2). Whenever the corkline is slack, towing must be

accomplished by towing on the bunchlines at some point which

will allow the corkline to be gathered together.

Towing bridles are required on the stern of all speedboats for

pulling (towing) on the net when the speedboat is engaged in the

forward mode (Fig. 5). The stern towing bridle is made to hang

loosely (but not so that it interferes with the propeller) and has a

sliding ring for hooking-up with the towline. The towline should

either be permanently attached to the sliding ring, or be made to

hook-up to the ring easily with a snap hook. The opposite end of

the towline should have a snap hook for quick attachment to the

ends of the bunchlines or to the corkline.

Webbing shoaling near the surface may prevent a speedboat

from towing with the stern towing bridle. In these circumstances

the only alternative is to use the speedboat's bowline and tow in

the reverse mode. This method is not nearly as efficient as towing

with the stern towline, but it is useful under certain circumstances

(such as small canopy removal).

Three towing points should be located on the corkline at

approximately 1/4, 1/2, and 3/4 net. Each towing point is made of

TOWING BRIDLE (Synthetic Line)

-------~~~~~~~ ------ -- ----- ----- ----- Figure 5.-Rigging for speedboat towing bridle. Note that the galvanized rhg is

free to slide the entire length of the towing bridle.

4

two adjacent bunchlines, one reversed and one normally oriented

(Fig. 3). The length of these towing point bunchlines should be

about 30-55 m (20-30 fathoms) for each line. These bunchlines

should be positioned so that their use will not interfere with formation of the backdown channel. Use of a towing point located on

the porpoise safety panel could cause major canopies during

backdown resulting in entrapment instead of release.

Net Depth Effects

As soon as a net is set and pursing begins, the net starts collapsing (i.e., the enclosed surface area decreases). The degree of net

collapse that occurs between encircling and releasing the porpoise

determines, to a large extent, the probability of prebackdown

mortality. The rate of collapse, among other things, is dependent

on the depth of the net: The deeper the net, the greater surface area

and volume during pursing and throughout backdown. Increased

surface area allows the net to remain open longer and reduces the

likelihood of porpoise being forced into contact with the webbing

due to net collapse.

As part of an experiment conducted in 1979,8 a 12-strip net

was deepened to 14 strips and the increased surface area estimated

(Fig. 6). The two-strip addition in depth gave rise to an 11.5% increase in the surface area when pursed and increased fishing depth

by 17.2 m (9.4 fathoms). The deepened net could be pursed faster

and had a larger backdown channel than the original.

Figure 7 shows a method of deepening the center section of a

purse seine by two strips using six equal-length bales of webbing

and a 4-mesh, 2-bar taper. This method may be applied to the

addition of any even number of bales of equal length to give a

two-strip increase in depth. If the bales are each 183 m (100

fathoms) long, the top of the tapered insert will be 220 m (120

fathoms) longer than the bottom side. For example, six 183 m

bales, each 100 meshes deep, produce an insert 659 m (360

'Unpublished cruise reports: M/V Cabrillo May 19. to July 19, 1979 and August

18, to October 7, 1979. On file at the Southwest Fisheries Center, Oceanic Fisheries

Resources Division, P.O. Box 271, La Jolla, CA 92038.

14 strip net 11.5% more area when pursed

12 strip net

Figure 6.-Deeper nets increase surface area, Actual measurements made during local and at·sea tuna hoat charters have demonstrated an 11.5% increase in

surface area when pursed by adding two strips to the center of a 12-strip net.
Two .trlp odclltlon with ,I.

100 fathom balh 0' 4* Inch web

~--:::ron 0

STRIP NO. I I Fine mesh sof.ty ponti 2 I I 1

3 I 4

5

6

7

8

9

10 "' ./ "If "If "If ...........1 I 1\ ~'if 'if 'if 'if 4mtsh,2 bar toptr t 100 Fathom.--!

\

\ "If

Figure 7.-A two-strip addition in the center of a 10- or 12-strip net can be accomplished with S, 8, or 10 bales of web, depending on the length of net. The addition is

installed between the last strip and the strip above. When only six loo-fathom bales are used, the top edge ofthe addition will measure 360 fathoms while the shorter bottom edge measures 240 fathoms. Note: New tie-down points must be determined for backdown anytime the depth of the net is changed.

The Mid-Net Zipper

It has long been suspected that unobserved porpoise mortality

occurs in deep areas of the seine prior to backdown. Research

divers have observed that the mid-net zipper may hold up a wall

of webbing 10m or more above the floor of the net. This wall, or

"mid-net zipper ridge" forms during the pursing operation and

persists until that part of the net is retrieved (Fig. 8). Captured

porpoise ordinarily concentrate over the deepest area of the net.

Consequently, when large numbers of porpoise and tuna are captured they may be forced to cross over the zipper ridge, increasing

Downwind Sets

probability of entanglement or entrapment. Divers have observed

porpoise entangled in the overhanging wall of webbing and lesser

folds associated with this area (Holts et al. 19799) but, the extent

of mortality attributable to the zipper ridge has not been

determined.

The zipper ridge forms when the "stopper" webbing (a vertical

strip of webbing of heavy construction designed to stop rips in the

net) is prevented from extending down as far as the adjacent

lighter web in the main body of the purse seine. Its formation most

commonly results from improper installation of the heavier and

stronger "stopper" webbing. Less common causes for the ridge include a short zipper line and incorrectly installed guide rings.

A close look at zipper installation techniques has shown a

definite advantage for those seines in which the stopper was installed with an appropriate amount of hang-in slack. While other

factors may contribute to restrict full extension of the stopper, one

fairly simple adjustment can eliminate them. This adjustment is

most easily made at dockside but can be done at sea. The 127 mm

(5-in) mesh stopper must be rehung to the 108 mm (4~-in) mesh

with enough slack to permit full extension of the stopper. Here the

existing 127 mm mesh stopper webbing is cut out of the seine and

rehung, mesh-for-mesh, to the 108 mm mesh. This provides

approximately 30% slack in the stopper webbing. An additional

one to three meshes of stopper web should be "killed-in" to each

strip of the 108 mm mesh. For example, about 4 m of additional

slack are provided when only two 127 mm meshes are killed-in

for each strip of a 14-strip net. This procedure will ensure a

problem-free zipper line by providing for differential shrinkage

rates between the 127 and 108 mm mesh, should it occur (Holts

1980).

A Mld-ntt zipper

fathoms) long on top, 439 m (240 fathoms) long on the bottom,

with two tapered edges of 110 m (60 fathoms) each. If eight 183

m bales are used, the top will be 842 m (460 fathoms) long and

the bottom will be 622 m (340 fathoms) long. Nets that are

already tapered or are going to be deepened by more than two

strips should be examined carefully to decide on the most suitable

and economical deepening plan. When deepening a purse seine, it

should be kept in mind that nets with a depth-to-length ratio

higher than 1:7 may require a slowdown of pursing to avoid excessive strain on the webbing (Fridman 1969).

c o

en 6

e:i 12

w 18

:E

:I: 24

Ii:: 30

36

42

mld-ne' rldg. B ,

"~/

Occasionally, an operator will set the net downwind of the

vessel, that is, with the wind blowing the vessel into the net.

"Downwind sets" occur very infrequently but can result in a large

portion of the captured porpoise being killed. These sets are

highly susceptible to net collapse and tend to be more frequent

with less experienced captains. Experienced captains will circle

around the porpoise school and set when the wind is on the port

bow (Fig. 9), insuring that the net circle will be upwind of the

vessel, reducing the risk of net collapse.

Figure 8.-A. Mid-net zipper ridge when fully pursed. Note Indentation of corkline opposite vessel. B. Zipper ridge with one-fourth net retrieved aboard vessel.

C. Fathometer trace when fully pursed.

"Holts, D. B., R. McLain, F. G. Alverson, and J. DeBeer. 1979. Summary of

research results from the fifth cruise of the dedicated vessel. November 11 to

December 9, 1978. Southwest Fisheries Center Admin. Rep. LF-79-20, 60 p.

5
Tow Line

If the net is accidently set downwind of the vessel, the captain

must pay particular attention to the developing bow and stem

bends in order to maintain near-equal surface areas in each (Fig.

lOa). As soon as pursing is completed, the skiff should tow the

seiner's stern out and clear of the stern bend in order to avoid

possible entanglement on the propeller. As the stem bend is rolled

on board (Fig. lOb, c, d), the skiff should begin towing the vessel

around until the wind is on the port side. Using the skiff and bowthruster to move the vessel around to the wind should provide a

good net configuration before backdown (Fig. IOf). The biggest

danger with these sets is to act too late or not at all.

Using Speedboats to Move Porpoise Inside the Net

Speedboats have long been used to herd and concentrate porpoise in order to encircle them. This is effective because porpoise

have a keen acoustical sense and tend to avoid the speedboats.

Speedboats are also used during a set to influence the location of

porpoise within the net. Circling just outside of the corkline

A

Figure It.-Use speedboats to prevent porpoise from (A) entering dangerous

areas of the net and (B) from entering the area where the corks come out ofthe

water during net roll. Corks make good sonar targets for the porpoise while t08

mm (414·in) mesh may not.

WIND

/

d

d. Skiff continues

b. Skiff pulls stern free

f. vessel ready to backdown

...

WIND

c. Skiff pulls vessel around

a. Down wind set: rings up

8. Vessel oriented to wind

Figure to.-Procedure for turning vessel around after making a downwind set.

Figure 9.-Always set after turning into the wind. This will keep the net upwind

of the vessel.

6
discourages porpoise from approaching that area. This tactic is

useful for keeping porpoise out of potential danger areas such as

bow and stern bends, the bow bunches, and where the net rises out

of the water to the power block (Fig. II). Speedboats should be

used whenever possible to discourage porpoise from entering

hazardous areas. Speedboats attached to the backdown apex during backdowns, however, should have their motors turned off.

Equipment Malfunctions

The nature of purse-seining operations and the environmental

conditions in which they are carried out can lead to gear and

equipment breakdowns. These malfunctions do and will continue

to occur somewhat routinely on all tuna seiners. They cause porpoise mortality directly through critical breakdowns and indirectly through delays in the net-retrieval procedures. During the

period 1977-79, an average minimum of 5.6% of the observed

mortality was directly due to equipment breakdowns and gear

malfunctions. During this 3-yr period, observed sets with delaycausing malfunctions averaged 19 min longer than sets with no

malfunctions.

The role of malfunctions as an indirect cause of mortality may

be more important than that of direct causes. The direct influence

of hydraulic failures, broken lines, and cables can be readily

assessed. But when a malfunction causes an operation slow-down,

or change in the net configuration, it is difficult to assess the

subsequent contribution to mortality. Therefore, mortality in sets

with malfunctions may be attributed to other, more directly observable causes of kill (e.g., net collapse). The percentage of the total

observed kill directly caused by malfunctions (5.6%) must therefore be considered a bare minimum.

Table 3 lists the major malfunction categories, frequency of

occurrence, percent of malfunction-related kill, and the average

delay associated with each. While elimination of all malfunctions

is not a reasonable expectation, the frequency and severity of

some may be minimized through careful maintenance and inspection schedules.

The proper functioning of a seiner's mechanical equipment is

essential both for catching tuna and for safe maintenance and

effective release of porpoise. While no specific technology has

been developed to reduce the incidence of mechanical failures,

machinery critical for porpoise fishing should be inspected more

regularly while on the fishing grounds. Maintaining a routine

visual inspection and repair policy for all net retrieval systems and

speedboats will reduce porpoise kill rates. The most common

equipment malfunctions involve speedboats, hydraulic systems,

the seine skiff, the various winches, and cable or line failures.

Another class of malfunctions does not necessarily involve the

failure of a mechanical system; rather, they result from an illdefined combination of vessel net characteristics, operator actions, and environmental conditions. These problems include rollups (webbing wrapped tightly around the purse cable, preventing

pursing) and the entangling of the purse rings in the webbing

(preventing net retrieval, after pursing). The propensity for rollups seems to be vessel specific and, although a wide variety of informal information on their cause and prevention exists, no technological solution has been devised. In fact, the specific physical

causes of roll-ups are not yet fully understood. Tactical methods

for avoiding entangling the purse rings in the net may be successful. However, those tactics would be as variable as the environmental circumstances (winds, current profile) which cause

the problem. These circumstances and their specific effects have

yet to be fully defined.

PORPOISE RELEASE

Backdown is the only proven technique for the wholesale

release of captured porpoise. It is also the activity which causes a

large share of the porpoise mortality (42% of the kill in sets with

kill greater than 15, Table 2). The procedure forces porpoise into

contact with the webbing of the floor, side walls, and apex of the

channel. Here they may become entangled individually in the

meshes, hangings, and holes, or they may become entrapped, en

masse, in folds and canopies of loose webbing. Primary factors

affecting porpoise mortality during backdown are the configuration of the backdown channel prior to and during backdown, the

skill and effort of the captain and crew, the condition of the net,

and the behavior of the fish and porpoise. A carelessly executed

backdown virtually guarantees high porpoise kill and increases

the probability of accidental fish loss.

Condition of Net and Equipment

Failures of hardware components cause loss of valuable fishing

time and can lead to the loss of tuna and even other pieces of

equipment or the net itself. Similarly, gear failures contribute to

kill rates both prior to and especially during backdown. Some

commonly occurring gear and equipment problems that contribute

Table 3.-Major malfunction categories showing frequency of occurrence, percent of minimum malfunction-related mortality, and average delay of set for each of the 3 yr of this study.

1977 1978 1979

Avg. Avg. Avg.

No. Kill delay No. Kill delay No. Kill delay

Malfunction sets % (min.) sets % (tilin.) sets % (min.)

Speedboats 120 9 1 78 2 I 82 13 I

Purse cable 94 13 18 27 15 12 22 24 16

Hydraulic problems 73 5 22 44 5 28 17 5 18

Skiff 63 18 23 35 8 16 29 21 18

Winch and ring stripper 12 2 18 6 3 42

Roll-up 151 12 34 59 12 47 39 17 53

Net tangled in rings 129 13 16 42 29 19 31 5 15

Bow bunches 41 5 6 37 2 16 22 6 15

Other 144 25 20 44 ~5 12 23 6 16 -

Total 815 100 19 378 100 19 271 100 19

7
Figure 12.-Functional distribution of webbing in a stablUzed b.lckdown

channel.

to backdown-related mortality are set out below with discussions

and recommendations where applicable.

Holes in Webbing.-Shark bites, porpoise and fish entanglement, snags during setting and retrieval, as well as many less common abuses contribute to the creation of holes of varying size in

the porpoise safety panel and apron. Holes in the backdown area

of the net allow fish to escape, porpoise to be drowned, and,

because they cause uneven distribution of loads in adjacent areas,

tend to get bigger or contribute to the formation of other holes.

Aside from the bunt end and the rolling strips, the webbing in the

backdown apex bowl (apron and first safety panel strip, Fig. 12)

receives the most wear and tear and should get maximum attention for marking and mending of holes. Careful attention should

also be given to the lacings between the webbing strips on a vertical line from the backdown apex to the chain since a good deal

of the strain of backdown is along this line (Coe and Butler 1980).

The age and degree of use of the webbing in the safety panel

and apron greatly affect its resistance to tearing. Three to four

years seems to be about the operational life of an apron. It is

preferable to replace worn out aprons with new webbing.

Segments (bales) in the safety panel can be exchanged from areas

of high wear to areas of low wear, to help prolong the service life

of the entire panel.

Vessel Control Systems.-These systems are vitally important

to the efficient execution of all phases of purse seining, and failure

or malfunction in any of them wastes valuable time and contributes to high porpoise kill. Control systems of particular importance for backdown execution are: I) The net skiff and its towline,

2) the bowthruster and its controls, 3) the vessel gear selector and

throttle control, and 4) the rudder control.

Configuration of the Net for Backdown

Dynamics of Backdown.-Any systematic effort to reduce

porpoise mortality requires an understanding of the dynamics of

the backdown procedure. While each net is different in one

respect or another, all nets respond similarly during backdown

because they are all constructed of physically similar units (rhombic meshes) oriented in the same direction, with similar external

forces working on them. Backing down essentially tows the net by

the two ends of the corkline and by the entire bottom of the net

(the chain and ring bridles). To achieve the backdown channel

Bow Bunch Failure.-One of the most rapidly wearing components of the backdown system is the outermost bow bunchline.

These lines suffer serious abrasion along most of their length by

being hauled under load through small diameter steel rings and by

being wrapped on a capstan to achieve this hauling power.

Typically, bunchlines are 28.6 mm (1-1/8 in) three-strand,

twisted nylon rope. After being hauled into its proper position the

bunchline is bent sharply over the port rail and secured around a

305 mm (12-in) cleat. The outboard end of the bunchline is usually fastened to a pear link or swivel link and chain by means of a

clove-hitch and back-splice. These practices break several basic

rules of safe rope use and virtually guarantee a short working life

of the outermost bow bunchline.

When the bow bunchline breaks during backdown, backdown

must be stopped, the channel collapses immediately, and canopies

develop everywhere leading to unavoidable mortality. Under these

circumstances the bunchline must be temporarily retied or replaced on the spot, a procedure which is always difficult and often impossible depending on the location of the break. If the bunchline

cannot be quickly refitted, the alternative then becomes to release

the tie-down, roll an amount of net equivalent to the length of the

broken bunch, tie-down again, and back out whatever animals are

still left alive in the net.

The probability of breaking the outermost bunchline can be

reduced to near zero by handling it slightly differently than is the

most common practice. First, use an eye splice with a thimble to

connect the bunchline to the swivel or pear link at the outboard

end (this may require an increase in the diameter of the outermost

bunchline rings). Second, provide a fairlead, roller, or bullnose on

the port rail with a diameter three to four times that of the bunchline (10-13 cm; 4-5 in). Third, secure the outermost bunchline on

the bow mooring bitt rather than to a cleat. Any sharp bend in a

rope under load decreases its strength and may cause premature

damage or failure. Most port-side rails and bow cleats in the U.S.

fleet create sharp bends and thus contribute to bunchline failure.

Even with these precautions, ropes can fail and, so, should be

regularly inspected for signs of wear and damage. Further, the

terminal chain, its component parts, and bridle splices to the corkline should be examined for weakness or damage on a regular

basis. Making the modifications suggested above and frequently

inspecting the bunchline and connections will virtually eliminate

the infrequent but usually costly consequences of bunchline

failure during backdown.

@ APEX BOWL

/

MID-NET TIE-DOWN

ZIPPER POINT

Corkline Hangings.-During backdown the weight of the net

pulling down on the hangings forms openings between the webbing and the corkline. The 20 fathoms of corkline on either side of

the backdown apex are subjected to the greatest downward pull

during backdown and at the same time are subjected to maximum

porpoise contact. The porpoises' beaks and flippers get caught in

these openings, holding the animals securely until they drown or

are rescued.

All hangings above the porpoise safety panel and porpoise

apron should be tight enough so as not to permit a cylinder of 35

mm (1-3/8 in) diameter to be inserted between the webbing and

the floats or corkline. As the fishing season progresses, knots

tighten, floats are abraded and compressed, and hangings break,

thus producing an increasing number of dangerous openings during backdown. Porpoise caught in these openings increase the job

of the rescuers and add to the length of the set. It is strongly

recommended that operators ensure that the corkline hangings

around the backdown apex are regularly checked by the raft man

and maintained in the best possible condition. This task can be

simplified by placing the backdown area floats on a lighter auxiliary corkline "Medina corkline" and lacing the webbing directly

to the main corkline.

8
configuration that is least likely to kill porpoise, the length of

corkline in the water from the outermost bow bunch to the stern

tie-down point must be just right. That length is solely determined

by the stretched depth of the individual net.

Iftoo little corkline is left in the water, a large apex canopy will

form, drowning many porpoise (Fig. 13a). If too much corkline is

left in the water for backdown, the apex will fold and wrinkle, and

the corks will be difficult to sink (Fig. 13 b; McNeely and Holts

197710). When the proper bow and stern tie-down points are used,

the backdown apex forms a bowl of fairly taut, open meshes that

slopes gently away from the vessel and has no surface canopies.

The apex bowl is developed as the water flows against the mesh

(apron) at the end of the loop. This bowl is held in place by its

connection to 1) the corkline, 2) the vertically stretched webbing

between the rings and the apex, and 3) the stern-going net bars

from the apex to the chain (to the left in Fig. 12). This 3-point

connection is the reason why the stretched depth of the net is so

critical in calculating the length of corkline necessary for an optimum backdown channel.

As backdown begins, the net is pulled through the water, causing the meshes to elongate and the net floor to rise. The resistance

to the lift being exerted on the net floor results in a downward

force sufficient to sink the corkline at the channel apex. This

localized sinking is augmented by the weight of porpoise against

the webbing below the apex corkline, facilitating porpoise release.

The net floor usually reaches its shallowest point after about 11

min of backing down (Coe and Butler 1980). At this time the subsurface configuration of the backdown channel becomes very

stable and the apex corkline becomes more difficult to sink. Every

effort must be made to release the captured porpoise as early in

backdown as possible. Figure 12 shows the approximate distribution of functional areas of webbing in a stabilized backdown

channel (Coe and Butler 1980). Note that the line of force pulling

the net through the water causes a large triangle of webbing below

the apex and the stern-going net bars to "roll-up" into a long thin

sausage shape which spreads out to form the apex bowl. To the

bow side of this sausage is the channel floor, which is flat and taut

and reaches over to the deeper pocket formed by the slack webbing under the bow bunches. From the stern side of the sausage

rises the stern wall which is forced into the channel by the oblique

flow of water. The stern wall thus overlaps the sausage and part of

the net floor, forming the subsurface canopy known as "stern

sway"" (Figs. 12, 14). Stern sway disappears where the sausage

opens out to form the apex bowl. At present, stern sway is an

unavoidable feature of backdown, and may entrap porpoise that

stay deep in the channel and fail to move to the apex area as

backdown progresses. The faster and more deliberately the early

stages of backdown are executed, the less likely it is that animals

will become trapped in the stern sway.

Tie-Down Point Formula.-All vessels installing aprons

should perform a water haul (trial set) to locate the proper bow

and stern tie-down points for backdown. This insures that the

apron apex coincides with the actual backdown channel apex. In

order to assist vessel captains and net makers who cannot make

trial sets, a series of net measurements for approximating the optimum tie-down points for their nets has been carried out. The

results of this work show that the optimum apex to stern tie-down

distance is equal to the stretched net depth plus the length of one

ring bridle plus the distance from the ring stripper (center) to the

point on the port rail where the net is tied during backdown (Fig.

15).12 It is strongly recommended that the net be tied down as far

aft of the ring stripper as possible. The error for this calculation

can be up to 3 fathoms, so some minor adjustments to suit individual nets and captains may have to be made after setting the tiedown point.

Table 4 lists the approximate distance from the backdown apex

(center of the apron) along the corkline to the correct stern tiedown point for various net depths. When this measurement has

been made and the stern tie-down point marked, the outermost

bow bunchline can then be adjusted to align the balloons at the

apron ends, thereby establishing the bow bunchline mark for a

properly aligned backdown channel. From this point, minor

adjustments can be made based on net performance and the cap11 UnpUblished Cruise Report: M/V Margaret L., May 19 to September II, 1977.

On file at the Southwest Fisheries Center, Oceanic Fisheries Resources Division,

P.O. Box 27, La Jolla, CA 92038.

12Unpublished Cruise Report: M/V Maria c.J., September 12 to December 28,

1980, 25 p. On file at the Southwest Fisheries Center, Oceanic Fisheries Resource

Division, P.O. Box 271, La Jolla, CA 92038.

STERN STERN

2

STERN STERN

Apex canopy

.b) Too much corkline in water

'OMcNeely, R. L., and D. B. Holls. 1977. Methods of reducing porpoise mortalily

in the yellowfin tuna purse seine fishery. Southwest Fisheries Center Admin. Rep.

U-77-13, 19 p.

Figure 14.-Backdown channel cross section showing development of stern

sway dUring backdown.

Figure B.-To ensure the proper amount of corkllne for backdown, clearly

mark tie-down points for the net and last bow bunch pulled. Never estimate

these points. The proper amount of corkllne Is directly related to the depth of

the net. Too lltlle corkllne (a) In the water will result In apex canopies. Too much

corkllne (b) will produce a collapsed and wrinkled corkllne.

3 4

9
Stretched net

depth

(calculated) "

Figure IS.-Determlnlng proper tie-down points for backdown: The sum of the

calculated stretched depth (In fathoms), the length of one ring bridie, and the

distance from the ring stripper to the point on the vessel where the net Is tied

down, gives the tie-down point within 4 or 7 m (2 or 3 fathoms).

Table 4.-Calculated distance In fathoms along the

corkllne from the apron apex to the stern tie-down

point. This table Is based on nets with 7 to 16 horizontal

strips (not counting apron) of 108 mm (414 In) stretched

mesh web 100 meshes deep plus 2 strips of 32 mm

stretched mesh web of 340 meshes with an apron and a

.Ivage 10 meshes deep of 127 to 203 mm (S to 8 In)

stretched mesh and a single purse ring bridle leg of 4 m

(2 fathoms).

where n the number of webbing strips of any size or depth

between the apron and the chain,

Yj the number of meshes deep in the jth strip,

Zj the stretched mesh size in the ith strip in inches,

A the stretched depth of the apron in inches (500-550

or, 0 if no apron),

B = the length of one leg of a ring bridle in inches.

For example, the SD of a hypothetical net 15.5 strips deep with

no apron and varying mesh sizes would be calculated by the

following steps. Starting at the corkline and working down to the

leadine (note as a convenience to net makers these calculations

are presented in fathoms and inches):

1. Apron depth (A) no apron oin

2. Two strips, 114-in mesh, 340 meshes deep

(2 x 340 x 1.254) = 850

3. Three strips, 3-in mesh, 140 mesh deep

(3 x 140 x 3) = 1,260

4. Four strips, 414-in mesh, 100 meshes deep

(4 x 100 x 4.25) = 1,700

5. Six and one-half strips, 5-in mesh, 100 meshes deep

(6.5 x 100 x 5) = 3,250

6. One salvage strip, 8-in mesh, 20 meshes deep

(I x 20 x 8) = 160

7. Single ring bridle length (B), 3 fathoms

(3 x 72 in/fathom) = 216

8. Adding items 1 to 7 = 7,436 in

9. Dividing 7,436 in by 72 in/fathom yields: SD = 103.28 fathoms

By following this method, the stern tie-down point to backdown

apex distance on the corkline can be approximated for any net and

vessel combination by adding the distance, in fathoms, from the

center of the ring stripper to the tie-down location on the port rail

(X in Table 4 and Fig. 15). It is absolutely essential that the net

structure (strips, meshes deep, and mesh size) between the backdown apex and the chain be known precisely.

I X = distance in fathoms from center of ring stripper to

the tie-down location on the port rail (e.g., X = 0 if tiedown is under the ring stripper).

tain's preference. This method of establishing tie-down points for

backdown is independent of the number of bow bunches pulled. It

is based solely on the stretched depth of the net and a known

backdown apex location.

The formula for calculating stretched net depth (SD) in fathoms

as presented in Table 4 takes the form:

Stretched Distance; apex to

No. of net depth stern tie-down

strips (fathoms) (fathoms)

9 62.8 62.8 + X'

10 68.7 68.7 + X

11 74.6 74.6 + X

12 80.5 80.5 + X

13 86.4 86.4 + X

14 92.3 92.3 + X

15 98.2 98.2 + X

16 104.1 104.1 + X

17 110.0 110.0 + X

18 115.9 115.9 + X

SD

n

(A + B) + :r YjZj

j=1

72

10

Porpoise Safety PaneI.-Since 1977, Federal regulations have

required a 32 mm (l't4-in) stretched-mesh safety panel in the

backdown area. The employment ofsmall mesh protection against

entanglement (Barham et al. 1977) has been a major factor contributing to the drastic reduction of average kill rates from 1976 to

the present.

The required safety panel for most seiners is a minimum of 330

m (I80-fathom) stretched length (before installation) by two full

strips of 32 mm mesh. Typically, the safety panel consists of six

bales of webbing, each 110 m (60 fathoms) long by 340 meshes

deep, laced together in a rectangle 3 bales long and 2 bales deep

(Fig. 16). Because 340 meshes of 32 mm stretched mesh webbing

is equal in depth to 100 meshes of 108 mm stretched mesh (a

standard production size), the safety panel is conveniently installed by replacing the top two strips of 108 mm mesh directly below

the corkline around the backdown channel (Fig. 17). The specific

location of the safety panel in the net is partially determined by

Federal regulations in that it must cover at least two-thirds of the

stern side of the backdown channel and extend around the channel

to a point 110-183 m (60-100 fathoms) from the bow end of the

net (ortza). The bow end of the safety panel should completely

cover the outermost bow bunch that is pulled for backdown. Further, all operators must understand that the length of corkline

around the backdown channel is dependent on and determined by

the stretch depth of the net (see Tie-Down Point Formula). This is

true even if the net does not have an apron.
PORPOISE APRON

n-_---'":-..--,ALlGNMENT IlUOYS (4)

3nl BOW BUNCH 2nd BOW BUNCH

t----60fa------..j

,. 180 FATHOMS

BEFORE INSTALLATION ·1

Figure 16.-Configuratlon of porpoise safety panel and super apron.

Figure 17.-Porpolse safety panel (PSP) must cover the perimeter of the backdown channel, at least from the last bow bunch puUed, around the.apex and

sternward at least two·thirds of the distance back along the sternside of the

channel.

along the sides of the backdown channel near the apex, and it adds

another strip-and-a-third of small mesh to protect the floor and

sides of the channel from porpoise entanglement. Another positive feature of the apron is that its required use has forced vessel

operators to locate and use their optimum tie-down points. This

has certainly resulted in far fewer porpoise deaths due to unnecessary canopies in poorly configured backdown channels.

The construction of an apron requires one 110m (60-fathom)

by 340-mesh bale of 32 mm webbing. Instructions for building

and installing an apron are provided by Holts (1977 13 ).

Executing Backdown

For those seiners without aprons, having more than one set of

tie-down points can be useful. In order to backdown successfully

with varying numbers of bow bunches pulled, pairs of clearly

marked tie-down points must be established for each apex location desired. The safety panel should be lengthened to ensure complete coverage of the backdown channel's perimeter in each backdown configuration. The pairs of marks (stern-side corkline and

outermost bow bunchline) must be clearly distinguishable

(numbered or color coded) to avoid serious errors in tie-down

combination (Fig. 18).

Super Apron.-The super apron, or simply the apron, as it has

come to be known, is a triangular-shaped section of 32 mm

(11/.i-in) webbing approximately 165 m (90 fathoms) long at the

base and 450 meshes high at its peak (Fig. 16). It is installed

between the corkline and the upper edge of the porpoise safety

panel in a location that places the peak of the triangle at the

desired backdown channel apex. The apron has several advantages

in that during backdown it forms an inclined ramp which helps

deliver the porpoise to the release area at the backdown apex, it

reduces the tendency for slack webbing to develop into canopies

The use of right rudder, thrusting to port, and briefly towing the

stern to starboard with the skiff at the start of backdown can help

to quickly set up the channel in the correct position for porpoise

release. The skiff and bowthruster should be used as needed prior

to tie-down to gently begin stretching the net and positioning the

vessel for the start of backdown. This pre-forming of the channel

will help ensure that porpoise are at or near the apex when backdown begins, thus contributing to rapid, en masse release.

Care mu~t be taken to ensure that the net will not fold in on

itself when backdown begins because this malformation of the

channel can result in high kills." Therefore, the captain should

use a speedboat to pull the corkline out whenever it is folded into

the channel (Fig. 19). Bunchlines must be arranged so that the

bow bunches are evenly spaced and fall in the natural line of the

bow corkline from the apex to the breastline attachment. Improperly aligned bunches will either reduce the volume of the bow

"Holts, 0, B. 1977, The super apron system: Construction and installation, Southwest Fisheries Center, Admin, Rep, U-77-30, 12 p.

"Unpublished Cruise Report: M/V Bold Contender, September 22 to December 4,

1975, 13 p, On file at the Southwest Fisheries Center, Oceanic Fisheries Resource

Division, P,O. Box 271, La Jolla, CA 92038,

41" MESH SIZE 200 MD 200 MD

TIE DOWN (T)

AND APEX (A) FOR

4 BOW BUNCHES

TIE DOWN (T) AND APEX(A)

FOR 3 BOW BUNCHES

~~~~'':':::~::'"''

-:-:----.,.-T, T --T2-A3r----Az--f:::;;:~=r::::::orrlI:::c::::L:r=b=~==i==:: 4fMESH SIZE

Additional II.. "'"" nKellQry to ccwer boQdown channel perlmet.r when 4 bow buncha ore puilid

Figure 18.-Multlple tie-down points for backdown In nets without aprons when using 2, 3, or 4 bow bunches.

Remember to avoid placing bunchUnes within 18 m (10 fathoms) of any of the apex markers.

11
"Coe, J. M. 1976. The effectiveness of the porpoise apron in improving the backdown prodecure. Southwest Fisheries Center, Admin. Rep. LJ·76·38, 17 p.

Backdown arc too great

minimum as it temporarily blinds the speedboat operator and any

rescuers in the area. Floodlighting of the entire net using a diffuse

light source may eliminate some of the problems caused by poor

visibility during night backdowns.

Another speedboat can be used during backdown to circle in the

area of the bow bunches and discourage porpoise from swimming

upstream, away from the apex release area. The backdown operation should be continued until all the porpoise are released. Since

the apex corks become progressively harder to sink after the channel stabilizes (about 11 min), aggressive, nonstop backdown

should be continued as long as there are enough porpoise in the

net to plug the apex and prevent fish release. As the channel narrows, the release area gets smaller, and requires fewer porpoise to

fill in the release area and prevent fish loss (Coe 1976t5). Thus

rapid backdown can continue until very few porpoise are left in

the net. These are general recommendations and must be tailored

to particular circumstances in the field.

Rescue by hand should be continued to ensure that zero-kill sets

occur as frequently as possible. Diving and depth recording instruments show that within I min after the vessel stops backdown,

the channel floor sinks from 4-6 to 20-25 m. This rapid increase

in net volume gives any live porpoise remaining in the net a

tremendous amount of space in which to avoid rescue attempts.

For this reason, backdown should be continued, at least slowly,

until rescuers have removed all live porpoise from the net.

When all the captured porpoise have been released it is common practice to release or partially release the outermost bow

bunchline. This action takes the tension off the apex area of the

channel and helps to ensure that the corkline will not accidentally

sink and release fish. The vessel should continue to backdown

slowly until the wind is on the port bow. At this point backdown

can stop and net retrieval can be resumed.

Backdown arc too tight

Figure ZO.-Proper backdown arc insures efficient waternow to maintain a wide

and deep channel. Maintain correct radius of turn by changing the pulling position of the net skiff, rudder angle, and direction of bow thruster as necessary.

A

8

side pockel for the fish or create canopies along the bow wall of

the channel, both of which increase the difficulty of backdown

and increase the probability of porpoise kill. The outermost bow

bunch should be positioned no farther out in the channel than onethird of the distance from the bow to the apex and should be fully

protected by the porpoise safety panel.

Maintenance of an open and canopy-free backdown channel

requires careful control of the arc that the vessel makes while towing the net (McNeely and Holts footnote 10). This natural arcing

tendency can be reduced or augmented depending on its suitability for channel operation. If the arc is too tight (i.e., the channel is

too curved), bow side canopies will be present and the corkline

won't sink at the apex (Fig. 20a). If the arc is too straight, the

channel will quickly collapse and porpoise will create canopies by

being overcrowded (Fig. 20b).

Not infrequently the lack of adequate communication between

the captain and the speedboat operator at the backdown apex has

resulted in either accidental fish loss or the failure to release all of

the captured porpoise. This problem is particularly apparent during night backdowns when hand or flashlight signals are difficult

to see and interpret. To relieve this problem, two-way radio communication may be set up between the apex-tending speedboat

operator and the captain. During night backdowns a powerful

floodlight should be continually shown in the area of the bow

bunches in order to attract and possibly hold some of the fish in

the net pocket below the bunches. A hand-held spotlight should be

used only as needed by the speedboat driver at the apex to determine how many porpoise remain in the net. Intermittent spotlighting of the apex area from the main vessel should be kept to a

Figure 19.-Straighten out the channel apex before backdown. Do not attempt

backdown when the apex is folded into the net. Use a speedboat to pull out

collapsed section before attempting backdown.

12
Porpoise Behavior During Backdown

There are several aspects of porpoise behavior that are relevant

to their release or rescue during backdown. This information is

presented so that participants in the fishery may have a better

understanding of some of the causes of mortality that are uncontrollable as well as the behaviors that supplement the release and

rescue efforts.

Most porpoise (especially spotters) display a behavior termed

"rafting" (Norris et al. 197916). Rafting occurs when individuals

or groups of porpoise remain at the surface or just below it for

long periods of time with a minimum of swimming and virtually

no diving activity. Typically, the rafting animals are motionless at

the surface with the head up and tail down. Rafting groups can involve any proportion of the school and are nearly always composed of pure spotters. Rafting usually begins after the rings are up

and increases until backdown disrupts it.

Since rafting animals move very little, they can be positioned at

or near the apex for the start of backdown by using the skiff to

slowly stretch the bow corkline during net rolling and pull the net

to the porpoise. When backdown begins the rafting groups drift up

to the corkline where they may become more active in an attempt

to avoid the advancing wall of net. By this time it is too late for the

rafting animals to avoid the net because those nearest the vessel

keep drifting against the animals nearest the apex, forcing their

release over the corkline. It seems that as long as there are rafting

animals on the boat side of the porpoise at the apex corkline and

there are no major disturbances to cause these rafts to break up,

this "passive crowding" effect will operate and the porpoise will

be easily released. Unnecessary noise or other stimulation during

the set should be avoided in order to enhance the generally inactive state of the porpoise and get the maximum benefit from

passive crowding during backdown.

Backdown release becomes more difficult when the remaining

porpoise are aware of the advancing apex corkline. At this point

the animals will swim upstream, away from the apex where they

may fight the net at the surface (or at depth), passively allow

themselves to be dragged along with the net, or actively attempt to

get over the corkline.

Porpoise in the captured school either swim actively from the

advancing wall of the net, raft, or display what is termed

"passive" behavior (Coe and Stuntz 1980). During backdown

there are occasionally some animals that sink and lie on the webbing and many more that show signs of the sinking behavior and

drop out of the rafting groups. At first glance, the animals appear

to be struggling feebly, perhaps against the current being produced

as the seine is pulled through the water during backdown. Their

movements are weak and lack the grace that one expects in porpoise. After about 3-5. min, the passive porpoise begin to rise

singly or in two's and three's to breathe and are either backed out

of the net or hand-released if the backdown procedure has already

been terminated. Animals usually do not return to rest on the bottom of the net after surfacing.

When viewed from a speedboat tending the corkline in the

release area during backdown, animals may appear to be dead,

and as a result the release efforts may be prematurely terminated.

Porpoise not released during backdown have a high probability of

being killed. The reasons for passive behavior in purse seine-

"Norris, K. S., W. E. Stuntz, and w. Rogers. 1978. The behavior of porpoises and

tuna in the eastern tropical Pacific yeJlowfin tuna fishery-preliminary studies. Available U.S. Dep. Commer, Natl. Tech. Inf. Serv., Springfield Va., as PB 283970, 86 p.

13

caught spotters are not fully understood and this behavior has only

rarely been observed in spinners.

Porpoise cannot swim backwards. Once in a canopy they rarely

get out unless they either sink free due to exhaustion or passive

behavior, are rescued, or have the canopy pulled past them such as

in a bow or stern wall canopy during backdown.

A very large catch of fish may cause fragmentation of the porpoise school and result in groups being caught in the meshes deep

in the net prior to backdown. Large catches may also cause some

of the porpoise to enter deep canopies near the boat during

backdown or to get caught under the bow bunches. Little can be

done to avoid these occurrences, but increased rescue effort, such

as using a second raft, may help to minimize the resultant mortality. The number of porpoise killed in a set is highly correlated with

the amount tuna caught. That is, with increasing tuna catch one

can expect release to be more difficult, increased rescue effort to

be necessary, and a higher likelihood of unavoidable kills.

Trouble Shooting Backdown Performance

Skipper interviews, observer data, and field observations point

out several common and recurring problems with backdown performance. These are identified and discussed in this section.

1. Inability to sink the corkline at the backdown apex is one

of the most common and frustrating problems that occurs. It

causes higher than normal kill rates, protracted backdown times,

and high levels of hand-rescue effort. There are two causes for this

complaint. The most frequent is the stabilization of the channel

configuration after 10 or 11 min of backdown. Stabilization of the

channel is unavoidable due to the physical structure of the net and

its reaction to induced water flow. However, there are several

techniques to help reduce its consequences.

First, backdown faster and more aggressively before the channel stabilizes, so that most, or all, of the porpoise are out of the net

while the corks will still sink.

Second, slightly more downward force on the corkline can be

generated after stabilization by having a little less corkline in the

water. Shorten the corkline in the backdown channel by increments of 1 fathom at both the stern and bow bunchline tiedown points. Continue shortening the corkline in this manner, 1

fathom during each set until the desired results are achieved. If a

slight canopy forms at the apex, slack off a few fathoms from both

the bow and stern and discontinue this method.

Third, assuming the remedies outlined above failed to produce

the desired results, begin removing flotation from the apex area.

Do this carefully and only as a last resort, because removing floats

to enhance late backdown sinking will enhance the ease of sinking

and the difficulty of raising the corkline in early backdown.

2. Surface canopies are a common cause of porpoise mortality

during backdown. In a properly configured backdown channel,

they are ordinarily formed when the porpoise swim against the

webbing along the walls of the net. In the case of canopies on

either side of the channel, the captain's most immediate job is to

continue backing down as fast as possible to cause the animals to

slide down the channel toward the apex. If there is a canopy at the

apex of the channel, then the net is improperly tied down and

more corkline must be put into the loop. This should be done in

steps of 2-4 m (1-2 fathoms) from both bow and stern until the

apex canopy no longer forms. In any particular instance an apex

canopy can usually be relieved by slacking the outermost bunchline a few fathoms and continuing the backdown. However, adjustment of this bunchline during backdown is not a solution to
consistent apex canopy problems. Adjustment of the stern corkline mark and the outermost bow bunchline mark to put more

corkline in the water should permanently solve the problem.

Major bow-side canopies are usually the result of too tight an

arc for backdown, which causes an oblique water flow and pulls

the bow corkline over the surface of the channel. The use of more

right rudder to decrease the backdown arc is useful in straightening out the channel while continuing to backdown.

Consistent bow-side canopies may be due to differences in

hang-in rates caused by the improper installation or reinsta!lation

of a porpoise apron that required the "killing-in" oflarge amounts

of webbing. Check the hang-in in any areas with consistent

localized canopies and correct it so that the entire bow wall has

the same hang-in rate.

Stern wall canopies during backdown are almost entirely caused by the porpoise themselves due to crowding or in avoidance of

fish. At present there is no remedy other than to continue rapid

backdown to flush the porpoise to the apex and to check the hangin rates in areas with consistent localized canopies.

Once begun, backing down should never be completely stopped

because the tension goes off the meshes and animals can then push

against the webbing and form a canopy anywhere in the net. By

the time backdown is resumed, it may be too late to back the

animals out of their self-formed canopies. This same problem occurs with tuna and can result in pockets of dead fish that slow net

retrieval after backdown. Furthermore, if porpoise hit the net in

areas of 108 mm (4'/.i-in) mesh while it is slack, their beaks will

go through and when tension is reapplied, the mesh will stretch

and close, holding the animal in place.

3. Backdown-channel collapse is a fairly common occurrence

at the end of the maneuver and is usually only partial. If all or

nearly all of the porpoise are out of the net at the time of partial

collapse, it is of little consequence and, in fact, helps rescuers get

their hands on the stragglers for quick rescue. Once the wind is on

the port bow and net rolling begins, the channel should open immediately, providing room for the live fish to circulate.

If channel collapse occurs early and consistently in backdown,

the cause is usually too little arc in the path of backdown (i.e., it is

too straight) and is remedied by moving the rudder more to port

and, if necessary, thrusting the bow to starboard (McNeely and

Holts footnote 10). Channel collapse, especially at the apex, can

occur if too much corkline for the amount of webbing is in the

water. This can be remedied by shortening the corkline by increments of 2 fathoms. This must be done carefully since pulling too

much corkline, or pulling it unevenly, will create canopies at the

apex.

Ifincreasing the backdown arc and shortening the corkline loop

do not solve the early backdown-channel collapse problem, the

stern tie-down location may be removed as far aft on the port rail

as possible. If this is done, the stern tie-down mark will also have

to be moved an equal distance.

4. Sinking of the corkline on the bow side of the channel in

the early stages of backdown is usually a harmless though aggravating characteristic of the backdown procedure. In some instances, where porpoise start to leave the net in this area, problems can develop when the net straightens out and these corks no

longer sink. Some porpoise, expecting to follow those just released, can become entrapped under that area of the net. This problem

can be reduced by trying to stretch out the bow corkline and getting the boat in line with it before applying power for backdown.

The skiff can help orient the boat and stretch the bow corkline

during net rolling just before backdown so that no time is wasted.

14

5. When these remedies have been tried without success or

the solution to one problem gives rise to another, it is time to consider the balance of chain and corkline at the stern tie-down point.

Ifthe chain is 18-36 m (10-20 fathoms) ahead or behind the corkline at the tie-down point, the backdown channel may be seriously

affected. Problems stemming from the corkline or chain consistently falling behind can usually be remedied by adjusting the

angle of the power block. For example, when the corkline is falling behind the chain, adjust the power block to allow the chain to

drop into the bottom of the "Y" (inside) while the corkline is on

the outside riding over the top of the webbing. This allows the

corkline to come aboard at a faster rate than the chain. If the

chain/corkline cannot be balanced in this way then a sling can be

used to bunch 5-8 m (3-4 fathoms) of corkline at a time until the

proper balance is achieved. The original stern tie-down mark on

the corkline must be used to preserve the proper location of the

backdown apex. Adjusting the corkline/chain balance only' influences the amount of webbing and its configuration in the water.

Porpoise Release Without Backdown

On the rare occasion where backdown cannot be performed, the

only reasonable way to release the captured porpoise also releases

the fish from the net. However unsatisfactory this alternative may

be, it is recommended that it be carried out to avoid drowning

most of the ma~mals. After the contents of the net are released,

the cause for the inability to bac1cdown can be repaired and, if

enough daylight remains, the school may be recaptured.

To accomplish this release, roll the net until the area is approximately equal to or smaller than that for a normal backdown

(depending on the number of porpoise captured), while trying to

keep the net shape as rounded as possible. The bow ortza and

breastline are passed to the net skiff and the bottom six to eight

purse rings on the ring stripper are cut loose from their bridles. If

possible, the breastline should be reattached at the ortza before

transferring it to the skiff. If the bow bunches have already been

pulled in, the bunch lines should also be passed to the skiff at this

time. The skiff should then idle away from the bow to a distance

of about 55 m (30 fathoms) and try to hold that position. If the

skiff gets too far from the ring stripper, the chain (leadline)

between the stripper and the skiff will rise, reducing the size of the

opening for porpoise release. One or two speedboats should circle

slowly outside the corkline on the stern side of the net to help

move the porpoise toward the opening. It may take some time for

the animals to start to leave the net. A raft and rescuer should be in

the water on the stern side of the porpoise in case of entanglements or formation of canopies.

While the net is in this configuration, it is recommended that it

be rolled to squeeze the porpoise out only as the very last resort, in

which case more rings should be dropped. Since there is no skiff

to control the position of the boat, rolling the net will cause the

opening to become more shallow by pivoting the boat and may

also force the porpoise into canopies as the stern moves toward

them. Throughout this operation, the strength and direction of the

wind will determine specifically how the skiff, vessel, and net will

be maneuvered to maintain the release opening.

In the case where only a few porpoise have been captured, a raft

man or two can attempt to r~scue them by hand prior to opening

the net. Ifthey are successful in this attempt, any fish captured can

be retained.
PORPOISE RESCUE

Each tuna seiner engaged in fishing for tuna in association with

porpoise·should have at least one inflatable raft for use as a porpoise rescue platform during and after the backdown procedure

(Coe and Vergne 1977). The following guidelines are recommended for raft use.

Equipment for Raft Rescue

1. The raft should be at least 2 m long, with a double bladder

inflating system which can be rigidly inflated and durable enough

to withstand sun as well as deck and net abrasion. A back-up raft

and repair kit should be on board.

2. A mask and snorkel are required to give the rescuer a clear

view of underwater porpoise activity. Spotted porpoise occasionally "play dead" (passive behavior) by lying against the webbing during backdown and may be thought dead unless viewed

underwater with a mask and snorkel. The raft man and his alternate should personally acquire their masks and snorkels to ensure

proper fit. Low volume, three-window masks with a purge valve

seem best suited for this activity.

3. A snap hook attached to the bowline of the raft about 2-4

m (1-2 fathoms) from the bow will allow rapid attachment and

detachment from the corkline during rescue maneuvers. This hook

should be used at all times during backdown.

4. Cotton gloves should be worn to give the necessary grip on

the slippery beaks of the porpoise.

5. A wooden canoe paddle or its equivalent will allow the

rescuer to paddle to the corkline.

6. The rescuer should be an experienced swimmer, who is

individually motivated to get the live porpoise out of the net. One

rescuer per raft is more efficient than two, because stability and

mobility are maximized.

Techniques of Raft Rescue

During backdown, the snap hook on the raft bowline should be

attached to the corkline or webbing about 9-18 m (5-10 fathoms)

from the apex on the stern side. From this vantage point the

rescuer may observe the entire backdown and signal when the

channel is clear of fish and when all live porpoise have been

released. If the raft man is suffici.ently skilled, he may paddle to

the center of the channel and hold position, thereby helping to

keep the porpoise from swimming upstream. In doing this, he

should try to stay at least 10m upstream from the closest porpoise.

As backdown nears completion, the rescuer should detach the

raft, move to the apex of the net, and reattach so that he can disentangle and remove any remaining live animals. The rescuing process is made easier if the vessel continues slowly astern; this helps

keep the porpoise at the end of the net and within reach. Every effort should be made to remove all of the live porpoise before ceasing backdown. Should any animals remain in the net after backdown, the rescuer can sometimes move down the corkline and

drive the porpoise onto shallow webbing or back to the apex

where they can be caught and released. Rescue efforts should be

carried on until all porpoise are removed or the net skiff comes

around. Although it is commonly done, using the raft after dark or

leaving the raft to rescue porpoise by swimming is not recommended. At any time during a set, the raft may be launched to rescue porpoise or to clear fouled bunchlines. In sets where backdown is hampered or impossible, a rescuer, wearing a mask and

15

snorkel, operating from a raft can be instrumental in reducing an

otherwise certain high kill. Hand rescue efforts can make the difference between an above average kill and zero kill in many sets.

ROLE OF THE CAPTAIN AND CREW

The successful execution of backdown and, for that matter, the

entire purse seining operation, depends on the level of skill,

motivation, and effort of the captain and crew. The captain has

overall responsibility for initiating and directing the seining

operations but cannot personally supervise each specific action by

each member of his crew. Therefore, however broad or narrow

responsibilities may be, each functioning member of the crew

must 1) recognize circumstances requiring action, 2) know what

action is required or to whom to report the circumstances, and 3)

be able to execute the necessary action effectively. It is the captain's responsibility to ensure that his crew is competent and

replace or train those who are not. The proper blend of skill and

responsibility will inevitably lead to a highly competitive, selfmotivated, and successful team.

The high rate of exchange of captains and crew among vessels

detracts from the overall proficiency of execution of operations,

and reduces the level of equipment maintenance. The operation

and handling characteristics of each vessel and net combination

necessitate a learning period. During this period, hitches in the

operation, gear malfunctions, misinterpreted orders, etc. cause increased porpoise kill. Whenever a captain changes vessels, with or

without changing crew, at least one trial set, with backdown,

should be performed. By performing a trial set(s), the new captain

may be able to identify specific vessel control characteristic& for

backdown, generally review crew performance, alter any aspects

of the operation for greater efficiency, and have the chidengineer

and deck boss identify any gear or machinery in need of attention.

Utilization of the information presented here will help lead to

the lowest possible porpoise mortality rates achievable with the

present state of purse seining technology. The maximum possible

exchange of information, techniques, and ideas among the captains in the fleet is essential for the maintenance of low kill rates.

The Expert Skipper's Panel (ESP), established through the

American Tunaboat Association, is an excellent source of information for captains desiring to improve the efficiency of their

fishing operations, especially on schools of porpoise. The ESP can

help analyze the causes of problems in gear or procedures and

make practical suggestions for their remedy.

ACKNOWLEDGMENTS

The research, and therefore this paper, was made possible by

many more individuals than can ever be properly acknowledged.

W. F. Perrin, E. G. Barham, W. W. Fox, Jr., J. T. Everett, and

especially R. L. McNeely were the principal individuals in NMFS

responsible for the initiation, support, and direction of the

research. On the industry side, special thanks go to F. G. Alverson,

A. Felando, L. Brito, D. Burney, M. Zollezi, J. DeBeer, C. Peters,

and many other representatives of the tuna industry who recognized the need for our research. Many captains over the years have

participated in and advised this program. Consistent support and

valuable information were received from Captains G. Sousa, L.

Castagnola, A. Castagnola, J. Zolezzi, V. Guarassi, J. Scafidi, J.

Medina, Jr., J. Melussi, M. Silva, J. Gonsalves, L. Romani, M.

Jorge, and J. Jorge. We thank particularly R. Silva, who captained

the dedicated vessel, MV Queen Mary. Our thanks also go to the
NMFS porpoise observers and to every crew member who ever

put up with one.

Drafts of this manuscript were reviewed by W. Perrin, G.

Sakagawa, R. Guillen, D. Bratten, F. Alverson, J. Lambert, and M.

Brown. The manuscript was typed by G. Hostler, H. Becker, L.

Prescott, and M. DeWitt. The figures were prepared by H. Orr.

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