Dolphins and the Tuna Industry (1992) / Chapter Skim
Currently Skimming:
7 Techniques for Reducing Dolphin Mortality
7 Techniques for Reducing Dolphin Mortality
Pages 72-109
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From page 72... ...
When they make an effort to escape over the corkline, typically during the backdown process, they may push against the corkline, deflect it downward, and swim out. During backdown, the corkline surrounding the backdown channel is tight and resists being submerged.
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From page 73... ...
Jet Boat In the present backdown process, use is often made of an inflatable raft with an occupant trying to direct the dolphins toward the end of the backdown channel. In the past, no motorized craft could be used for this task because of Corkline Floats \ ~ ~ Wateri i no ~838~ FIGURE 7-2 The double corkline.
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From page 74... ...
Since the demonstration, several vessels of the international fleet have tried the jet craft as a dolphin rescue platform. Early trips revealed mechanical and structural problems with the recreational craft due mainly to the rigorous conditions of commercial purse seining.
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From page 75... ...
In conventional purse seining, once the rings are up, the rings must be transferred from being supported by the purse line to being supported by the cantilevered ring stripper. With the snap rings, this process is unnecessary and sacking up can begin immediately after the rings are up.
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From page 76... ...
Causes of canopies can be subsurface currents, improper orientation of the backdown channel with respect to the wind, or improper control of the vessel during backdown. Although these causes cannot always be controlled, modifications to the purse seine that would make it resistant to canopies would be advantageous.
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From page 77... ...
· Netting with additional weight in the form of a false leadline partway down the net. These modifications are most useful when incorporated into the portions of the purse seine that become the backdown channel.
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From page 78... ...
However, these longer chains can themselves cause problems by wrapping around the purse line if they become slack. In 1972, the Southwest Fisheries Center identified an important cause of roll-ups: the rotation of the purse cable during changes in its tension.
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From page 79... ...
It is well established that tuna are far more likely than dolphins to escape from pursed nets. With this in mind, NMFS's Southwest Fisheries Center experimented with a "backdown zipper" system that would separate the dolphins in the backdown channel from the tuna remaining in the main body of the net.
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From page 80... ...
the set immediately after pursing, and (2) the backdown process would be eliminated.
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From page 81... ...
channel to block the movement of tuna out of the main body of the net. With such a barrier in place, far more obvious escape opportunities could be provided for dolphins without risk of losing tuna.
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From page 82... ...
Its design would require consideration of the quantities of tuna that would be encountered and their weight. Modifications to hnprove Escape of Dolphins from the Backdown Channel The portion of the purse seine that forms the backdown channel is a relatively small part of the overall net; therefore, design modifications may be feasible for the channel that would be impractical for the entire seine.
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From page 83... ...
Both devices are conceptual and would require considerable development and testing before their value could be determined. In addition to maintaining the integrity of the backdown channel, controlling the escape route at its apex would help to reduce dolphin mortality.
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From page 84... ...
~ A,,, , ,:,, it. ~~L i, i_ by; 1~ FIGURE 7-6 Annular lifting surfaces to maintain channel width during backdown.
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From page 85... ...
Since its introduction in the early 1970s, most of the effort spent on reducing dolphin mortality has concentrated on perfecting the backdown and the portions of the net that make up the backdown channel. However, the procedure is difficult and has opportunities for mishap- indeed, most dolphins die as the result of mishaps.
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From page 86... ...
POTENTIAL BEHAVIOR-BASED RELEASE OF DOLPHINS Pre-Backdown Release of Dolphins There is evidence that the "protective structure" of a dolphin group and the dolphins' unwillingness to cross a gate such as the one provided by the backdown channel are closely correlated. Dolphins could be released more efficiently from the net if such a correlation were understood better (Norris and Dohl, 1980b; Norris and Schilt, 1987~.
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From page 87... ...
A gate of sorts is now used for dolphin release when the backdown channel is pulled hard by the seiner, depressing the corkline at the end of the channel 1-2 meters beneath the surface. The effect of this transitory opening is not to produce a gate through which dolphins will rush, but rather to provide an opening through which dolphins are tumbled willy-nilly, many moving out backwards, and sometimes swing back into the net.
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From page 88... ...
· The reluctance of dolphins to escape from the backdown channel on their own may also be due to the fact that the space provided is too small to allow the intact passage of the basic school unit. · The predicted minimum gate for oceanic dolphins is about 7 meters wide and 7 meters deep.
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From page 89... ...
BREAKING THE TUNA-DOLPHIN BOND Any program that seeks to reduce dolphin mortality in the ETP yellowfin tuna fishery by understanding and subsequently modifying the behavior of tuna or dolphins must be based on the answers to a number of fundamental questions.
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From page 90... ...
In all the world's oceans, other methods are used. Even in the eastern Pacific, much small yellowfin tuna is found and caught in ways other than rounding up dolphin herds.
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From page 91... ...
Awbrey, San Diego State University, personal common., 1990~. Another acoustical method for locating tuna is sonar, which is used very successfully in the western Pacific tuna fishery, as well as in fisheries for other species.
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From page 92... ...
The manufacturer is working with Honor Manne Communications of San Diego and some vessel owners to test whether one of these devices, adapted to work from a small helicopter, can detect tuna schools in the western Pacific (R. McCloskey, Honor Marine Company, personal common., 1990~.
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From page 93... ...
Purse seiners catch substantial amounts of tuna every year by setting nets on floating objects, but no one knows why tuna associate with floating objects or how strong or long-lasting the attraction is. History and Perspective Before World War II, Philippine fishermen started using anchored FADs, which they call "payaos," to attract pelagic fish.
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From page 94... ...
that landed fish from the nearly pure schools of yellowfin tuna caught in sets on dolphins in the ETP are large (>80 cm or 23 lb)
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From page 95... ...
Dolphin Sets Log Sets FIGURE 7-8 Size distribution of yellowfin tuna caught in dolphin and log sets during 1974-1985 in the eastern Pacific. Data from Figure 41, IATIC, 1989b.
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From page 96... ...
A FAD-deployment experiment began in July 1991 conducted by IATTC, NMFS, and Mexico and supported by Bumble Bee Seafoods. Potential The chance of success in the endeavor to attract tuna away from dolphins or to develop an alternative aggregator of large yellowfin tuna would be much higher if we understood why tuna associate with dolphins and with flotsam.
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From page 97... ...
1 Peso . 1 _ I I ~ I I I ~ ~ ~ 1 30° 1 20° 11 0° 1 00° 90° 80° , , , 97 Boo 20° loo _ Do ~ nal5 ~ ~ ~ _ 1 Do ~ \ 20° 30° Too FIGURE 7-9 Size distribution of flotsam-assoc~ated pure schools of yellowfin tuna in 10-degree squares In the eastern Pacific.
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From page 98... ...
The productivity of longline fishing does not compete boat for boat with ETP purse seining. As a result, the use of the present tuna seiner as a conventional longline vessel would not be economically feasible.
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From page 99... ...
An economic assessment of this technique would also be essential. Midwater Trawls Few alternative methods of fishing offer the productivity rates enjoyed by toga purse seining in the ETP.
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From page 100... ...
Catch rates of 3-8 metric tons per day have been reported. Catch rates of purse seiners are considerably higher.
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From page 101... ...
If the passage to the cod end of the net is viewed by the tuna as a possible escape route, high towing speeds may not be required. The same speedboats that are used in purse seining may be useful in tuna midwater trawling.
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From page 102... ...
The first set of options centers on regulatory alternatives that would further prohibit, directly or indirectly, dolphin mortality. Immediate Prohibition of All Dolphin Mortality from Fishing This alternative would consist of an immediate prohibition of purse seining on tuna associated with dolphins.
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From page 103... ...
in the eastern Pacific. (1 short ton = 0.907 metric ton.)
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From page 104... ...
6.2 32.9 Age (yr) 1 88.2 1 57.2 2 3 4 FIGURE 7-11 Percent maturity of female yellowfin tuna as a function of average weight and age.
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From page 105... ...
Incentives for Tuna Fishing with Alternative Gear Under this alternative, positive incentives for switching to alternative fishing gears would be created, probably in the form of subsidies for purchasing or constructing vessels or gear designed for fishing methods other than purse seining. Pace Incentives for Fishing on Non-Dolph~n-Associated Tuna The argument has been made that one of the factors associated with dolphin mortality is the premium price paid for larger yellowfin tuna, which are more likely to associate with dolphins.
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From page 106... ...
Five general alternatives present themselves: 1. Fishing on school fish not associated with dolphins outside the 200-rnile Exclusive Economic Zones of the Resource Adjacent Nations in the ETP.
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From page 107... ...
· Meetings with individual captains to discuss their performances. Measuring the Captains' Performance Some of the factors affecting dolphin mortality, such as gear malfunctions or subsurface currents, are beyond the control of captain and crew.
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From page 108... ...
This development resulted in a large number of relatively inexperienced captains who were unfamiliar with fishing on dolphins and who are now in various stages of learning. Because of these problems, IA11C is providing the technical expertise for the assessment and correction of captain performance for the international fleet.
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From page 109... ...
Both NMFS and IA11C offer services to the fleets for testing dolphin safety gear. These services involve a trial set to evaluate the condition and operation of the net and the other gear.
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