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Dolphins and the Tuna Industry (1992)

Chapter: 7 Techniques for Reducing Dolphin Mortality

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Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
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7

Techniques for Reducing Dolphin Mortality

The effectiveness of fishing gear is a function of its design and the way it interacts with the animals. Therefore, any successful effort to develop gear that is effective at catching tuna without killing dolphins must involve knowledge of tuna and dolphin behavior as well as good engineering and design.

SMALL MODIFICATIONS OF CURRENT METHODS

Several small modifications to the current methods of purse seining for tuna have immediate potential for reducing dolphin mortality. Each of these changes could have an incremental effect. The cumulative effect of these and other innovations could significantly reduce the impact of purse seines on dolphins.

Medina Double Corkline

In spite of dolphins' characteristic trait of jumping well clear of the water, they do not leap over the corkline to freedom. 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. To sink the corkline under this condition, a large number of floats must be submerged. Only at the apex of the backdown channel and only when there is sufficient flow through the channel, does the corkline begin to submerge.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
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FIGURE 7-1 The conventional purse-seine corkline. Drawing by C. Goudey, Massachusetts Institute of Technology, Cambridge.

The use of a double corkline has been suggested as a way to ease the dolphins' escape (H. Medina, personal commun., 1990). Unlike a conventional corkline, shown in Figure 7-1 , which runs through the center of each float, the double corkline has each float tied on individually. As seen in Figure 7-2 , this rigging allows each float to move independently, accommodating dolphins' attempts to push their way over the corkline.

Rigging a seine in this way seems to present no hardship to the net builder. Hanging the netting to the corkline, in fact, would be much easier and could be done more precisely. The method has the additional advantage of allowing easy replacement of damaged floats.

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

FIGURE 7-2 The double corkline. Drawing by C. Goudey, Massachusetts Institute of Technology, Cambridge.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

the threat posed by the propeller. The introduction of the jet boat as a recreational watercraft now allows an alternative to the unpowered raft in controlling dolphin movements within the backdown channel and aiding in dolphin rescue. A Japanese manufacturer has introduced a fiberglass craft with a 52-horsepower engine that is propelled by a jet pump. This craft is 9 feet long and 5 feet wide, large enough for three occupants, and has a hull that sits low in the water. It can turn within its own length. A demonstration of the craft in San Diego in April 1989, sponsored by the Porpoise Rescue Foundation, indicated that it might be useful both inside and outside the net and before, during, and after backdown as a mobile platform for hand release of captured dolphins. The craft produces noise and bubbles, which may also act as a deterrent to dolphins swimming toward the vessel and swimming away from the release area during backdown. With no exposed propeller, the craft could quickly travel in and out of the net, passing over the corkline without risk to the dolphins of entanglement.

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. Two units being tested by the U.S. fleet have undergone some structural and mechanical modifications by both the dealer or manufacturer and by the vessels using them. This has improved reliability but some problems remain. Increasing the crew's familiarity with the craft and storing a suitable inventory of spare parts should yield better reliability. An aluminum version or one patterned after a rigid-hulled inflatable also may increase the utility and durability in this application. A Mexican company is currently developing a jet boat for use in dolphin rescue (M. Hall, personal commun., 1990). In addition, several manufacturers of outboard motors produce propellerless jet models.

Current Profiler

The Doppler current profiler is a hull-mounted sensor that measures the speed and direction of currents at various depths below the surface. Commercial versions might be useful in providing subsurface current information to vessel operators before setting the seine. One product, designed for commercial fishing, provides subsurface current values at any selected depth (Summers, 1990). Another product, designed for oceanographic applications, provides a profile of the current over a continuous range of depths, plotting the results on a video screen (M. Hall, personal commun., 1990).

Such information could be valuable to the captain in determining whether subsurface currents are likely to cause a distorted set with collapses or canopies. If the current is not too strong, a set can be made with much less chance of a malfunction. The presence of a mild subsurface current, if its direction and speed

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

are known ahead of time, does not necessarily spoil a properly oriented set. An excessive subsurface current would indicate the need to move from the area and fish elsewhere. With time, these devices may provide evidence for a correlation between tuna abundance and subsurface currents. Such an understanding could assist in locating tuna not associated with dolphins.

Pear-Shaped Snap Rings

An innovation that has been adopted by almost half of the U.S. fleet is the pear-shaped snap ring. Introduced primarily as a time-saver and a way to avoid the dangers of a conventional ring stripper, the snap ring helps to reduce the time before backdown begins. Instead of a simple steel ring, the new design is pear-shaped and has a spring-loaded gate portion that can be opened for inserting or removing the purse line. 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. This innovation can save up to 15 minutes, reducing the time that the dolphins are exposed to canopies or net collapse.

Small-Mesh Medina Panel

It has been suggested that placing extra slack in the large-mesh (4 1/4-inch) netting below the Medina panel would force the small-mesh (1 1/4-inch) netting above to open more during the backdown and make a better backdown. Thus, 10–15% more large-mesh netting would be distributed along the boundary with the small-mesh panel when the net is assembled.

Third Fine-Mesh Strip

The Porpoise Rescue Foundation is putting a third fine-mesh strip aboard two vessels. Adding this strip below the second fine-mesh strip may help save any dolphins that dive deeply as the backdown is being performed. This strip is made with 2-inch mesh. The 2-inch mesh is being used because it has less drag than finer meshes do while being pulled through the water during backdown.

Safety Crook

The safety crook is the same aluminum pole that is used in swimming pools to save swimmers in trouble. One end of this pole has a double open-end arc. During the backdown, this safety crook can be used in a speedboat to help guide the dolphins toward the opening that will let them free.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

MAJOR MODIFICATIONS OF CURRENT METHODS

In the previous section, modifications and techniques were discussed that could be incorporated into present purse seines with little cost. In most cases, the potential benefit of the small modifications is the reduction in dolphin mortality during the backdown process. The basic freeing process remains unchanged. This section considers more sweeping changes in gear and methods. Most of the concepts presented have not been tested adequately to assess their impact on dolphin mortality and fishing effectiveness.

This section is organized around modifications to reduce two fundamental problems—canopies and roll-ups—and then continues discussion of four classifications of modifications—barriers, species selectivity, backdown-channel changes, and other purse-seine variations—that the committee believes show promise for reducing dolphin mortality.

The committee believes that the most promising major alterations in purse-seine gear are the following:

  • Modifications in netting material.

  • Modifications in hang-in ratio.

  • Modifications to the purse cable.

  • Development of lifting surfaces in critical parts of the net.

Modifications to Reduce Frequency of Canopies

Canopies are billows of netting along the perimeter of the pursed net and backdown channel; they are a common cause of dolphin mortality. These formations are caused by slackness in the upper portions of netting as the corkline is pulled sideways through the water. 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.

Canopies form when the downward pull on the net is overpowered by the drag forces due to water flow through the plane of netting. For most of the setting process, the weight of the rings, purse line, chain bridles, and leadline keeps the netting near the surface essentially vertical. As the net is pursed and the rings become supported by the purse line, only the weight of the netting itself remains to keep it hanging vertically below the corkline. The net configuration changes easily at this stage.

Some modifications to the purse seine singly or together would help prevent canopies. The most promising are the following:

  • Netting of reduced hydrodynamic drag.

  • Netting material with increased underwater weight.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

The following also might help:

  • A wider hang-in ratio (the ratio of the length of the corkline to the length of the webbing attached to it, typically around 80%).

  • Netting twine of less stretch.

  • 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. Individually or together, these changes could reduce the tendency of the corkline to precede the netting when flow is normal to the netting panel. In general, a low-drag, fast-sinking net is advantageous in purse seining. With the possible exception of the wider hang-in ratio, these modifications should produce a more effective net to capture tuna.

At present, nylon netting is used for purse-seine construction because of its low cost, durability, flexibility, and sinking characteristics. Alternative netting materials such as Dacron * (polyester), Kevlar (aramid), and Spectra (ultra high molecular weight polyethylene) may offer advantages, but Kevlar and Spectra are more expensive than nylon. Dacron, in particular, has proved advantageous in other commercial fisheries. Because of its higher density, greater strength, and reduced stretch, its consideration in tuna purse seining may be appropriate.

One California netting company has built a purse seine in polyester, which, during use by one vessel, was found to be productive (H. Medina, personal commun., 1990). The company reports a weight advantage, cost advantage, and a sinking rate twice that of a nylon seine of equivalent strength (Kirkland, 1990). The reluctance of seiners to change netting material remains unexplained. Through attrition, the cost of such a change would be small since seine nets have a useful life of only 2–5 years (H. Medina, personal commun., 1990).

Modifications to Reduce Frequency of Roll-Ups

Roll-ups happen when the lower portion of the seine becomes twisted around the purse cable. Specific data on the frequency of occurrence of roll-ups are not available because roll-ups typically are categorized as a “major malfunction.” In general, sets in which major malfunctions occur have nearly four times the normal rate of dolphin mortality (DeBeer, 1980).

In the following discussion of roll-ups, the related event of netting or bridles becoming snagged on the purse cable is also included. Roll-ups typically occur during the setting of the net when the cable is close to the netting. The bad effects of a roll-up begin during the pursing operation when a portion of the cable, entangled with netting, is pulled through the purse rings. The cable invariably becomes jammed and when it comes within reach of the crew, the hauling process must stop while the purse line is cleared. During this delay, net collapse and dolphin entanglement can occur.

* Dacron and Kevlar are trademarks of the DuPont Corporation. Spectra is a trademark of Allied Signal, Inc.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

The fishing industry has been strongly motivated to reduce the occurrence of roll-ups not only because of dolphin mortality but also because of the effort and danger involved in clearing roll-ups. Industry has eliminated some of the causes, such as reducing the occurrence of broken wire strands through proper maintenance of the purse cable. The use of longer chain bridles has helped to increase the space between the purse cable and the lower edge of the netting. 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. Experiments with cables that were constructed to be torque balanced were begun aboard commercial tuna seiners. Though notable successes were experienced, cost and difficulty of splicing this unconventional wire rope prevented adoption. In addition, twist induced by the low-helix, three-strand torque-balanced cable passing over the purse block sheaves continued to cause occasional roll-ups.

Today, the delays during the hauling process remain an important cause of dolphin drownings. Despite some lingering problems, however, the torque-balanced cable can reduce significantly the frequency of roll-ups. Fortunately, alternatives to conventional wire rope are now available.

The successful introduction of Spectra braided line as a replacement for conventional wire rope in a variety of commercial fishing applications suggests its use as a purse cable. It has been reported that the Spectra line can last over 10 times as long as wire rope (Nye, 1990). Because of strength/diameter ratios similar to steel, few changes are needed to accommodate Spectra. Winch drum capacity would remain adequate. In addition to being torque-balanced, none of the problems associated with broken strands would exist, and splicing would be much easier.

The use of lead-core synthetic line as a replacement for chain along the lower edge of the seine may also be of value. This material was used by many of the San Diego boats in 1976, just after its introduction by Sampson Ropes (H. Medina, personal commun., 1990; T. Bergen, Sampson Ocean Systems, Inc., personal commun., 1990). Sink rates were adequate but its stiffness caused problems during stacking of the net on deck. The synthetic leadline was abandoned in favor of chain.

The stiffness of the leadline during initial use of the product was caused by the type and construction of the lead used in the core. The lead used had to be soft enough for extrusion and was notched to give it flexibility. However, under strain, this lead core would tend to deform, fusing the notches and

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

causing stiffness. Recently, the manufacturer has introduced a harder lead, in cast ingot form, which has excellent flexibility that will not degrade under strain. Benefits that might accrue from the use of this new material would be reduced purse-line jamming, fewer roll-ups, faster sinking rates, and quieter setting while school fishing.

Modifications by Insertion of Barriers Between Tuna and Dolphins

One of the significant drawbacks among the present methods used to remove dolphins from purse seines is the loss of tuna. Increasing the opportunities for dolphins' escape also increases the risk of losing tuna. Certain techniques that might be appealing because of their possibilities in reducing dolphin mortality are unacceptable commercially. 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. Their approach was to use a line passing under the net partway up the backdown channel. During backdown and when the dolphins were beyond the line, the line would be pulled up, choking off the channel. With the two species separated, efforts to provide escape opportunities for the dolphins could be increased. Unfortunately, the dolphins were not always found on the right side of this zipper line, and the shallow channel would collapse. Experiments were discontinued. It is possible that attempts to learn how to maneuver dolphin herds inside the net would alleviate this problem.

The motivation for such gear modification remains sound and deserves continued thought. Opportunities may exist during the set when the two species are spatially separated. At the completion of pursing, dolphins are typically at the end of the net away from the seiner (Norris et al., 1978). During backdown, the dolphins are swept toward the apex of the channel. In both cases, placing a barrier between the species might be feasible. Deploying such a barrier is a challenge, because both species can react adversely to the disturbances caused by such activity.

Though still entirely conceptual, the design shown in Figure 7-3, Figure 7-4 through Figure 7-5 can serve as an example of a barrier method. A netting panel of suitable size and shape can be buoyed into place at the proper time. Before deployment, the panel would be secured to the seine netting itself as shown in Figure 7-3. At the appropriate time, a release cord would then allow the panel 's false corkline to pull the barrier to the surface, thereby separating the dolphins in their own netting compartment as shown in Figure 7-4. The main corkline and the netting below it would then be released and the dolphin herd would have an obvious escape route as shown in Figure 7-5.

The design and incorporation of this barrier net into the purse seine

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

FIGURE 7-3 A full, deployed purse seine after pursing. The seiner would be to the left. Drawing by C. Goudey, Massachusetts Institute of Technology, Cambridge.

without excessively complicating the normal net-handling procedures would require careful thought. The principal advantages of this method are that (1) it could be done early in the set immediately after pursing, and (2) the backdown process would be eliminated.

Another approach is placing such a barrier net in the area of the backdown

FIGURE 7-4 The barrier net has been released and has buoyed to the surface. Drawing by C. Goudey, Massachusetts Institute of Technology, Cambridge.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

FIGURE 7-5 The main corkline and net are opened to release the dolphins. Drawing by C. Goudey, Massachusetts Institute of Technology, Cambridge.

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.

Essential to the success of these methods would be an awareness of the movements of both species in the seine and knowing the right time to activate the raising of the barrier. As discussed in Chapter 5, our knowledge of the tuna-dolphin bond and the movements of both is far from complete. The rational development of such gear modifications awaits the development of a better understanding of species behavior.

Species Selectivity

The tuna-dolphin problem is a gear-selectivity problem. When netting is involved, the typical approach to such selectivity problems is the use of a specific mesh size that will retain certain fish based on size or herding traits. The situation is simpler when the by-catch species is smaller or is herded less easily than the target species. In such cases, a mesh size is used that retains the desired species while allowing the smaller or more elusive species to escape. The inverse problem—e.g., fish by-catch in a shrimp fishery—can also be solved using mesh size, but less directly. A typical approach is the use of a false net or barrier that deflects the larger or easily herded species away, while the smaller or less easily herded species passes through into the portion of the gear that ultimately retains it. The turtle excluder device is an example of a solution to the inverse problem.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

Dolphins are far more easily retained in a net than tuna. The problem is therefore inverse: The dolphins must be separated from the tuna using a barrier through which the tuna will want to pass. Once the species are separated, further steps can be taken to release the dolphins, while precautions are taken to prevent tuna from recrossing the barrier. Implementing such a technique in a practical tuna purse-seining operation is far from trivial.

Information on tuna-dolphin behavior is inadequate for devising or implementing purse-seine modifications involving mesh selectivity as a means of mitigating the tuna-dolphin problem. More information is needed on the response of each species to various types of barriers. Many attempts to modify gear are based on behavioral differences between tuna and dolphins, as described in Chapter 5 and Chapter 6.

One possible approach would be to allow the tuna, which are more active in the net than dolphins, to escape the main body of the net into a secondary enclosure through a passage that discourages their return. The development of such a tuna “check valve” or “diode” has not been attempted, but there are many tools that could be brought to bear. Light, sound, mechanical flaps, or some combination of these could be incorporated into an opening, which would represent an inviting escape opportunity in one direction but prevent passage in the other.

Conceptually, a short tunnel with flexible reeds pointed in radially but biased in one direction would function to allow tuna passage only one way. A simple cone of netting as used in a fyke net or fish trap might also work. Determination of the size and clearance requirements of such a device would need some experimentation. Incorporating it into a commercial operation would require some ingenuity.

If, on the other hand, a directional light or sound source were discovered that repelled tuna, its placement around a hole in a panel of netting and pointed “downstream” might be an elegant approach. The principal appeal of this concept is that the dolphins would not pass through such an opening, and after the tuna had passed through, the main body of the purse seine could be opened and the dolphins could leave without running the backdown gauntlet.

The passage that the tuna would take would lead into a netting chamber that could be hauled aggressively as it would contain no dolphins. This chamber could take the form of a small pursed seine or simply a large sack of netting. Its design would require consideration of the quantities of tuna that would be encountered and their weight.

Modifications to Improve 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. Changes

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

or additions that add to the predictability or controllability of the backdown channel's geometry could assist in the reliable release of all dolphins.

Ideally, the backdown channel would remain open and functional, regardless of the behavior of the dolphins, the strength of the currents, the length of the release process, or the skill of the crew. The use of special components in the backdown channel that have controllable flotation, stiffness, or dimension but that still pass through the power block may have merit. Flexible hose that could be pressurized with air or water to provide near-rigid structural support may be of use. Control lines that change the shape of the netting or leadlines that are strategically placed might help form a more enduring backdown channel. Such an engineering solution is complicated by the poorly understood behavior of both dolphins and tuna. It is safe to say that few ideas representing truly novel approaches have been tried, much less properly evaluated in commercial conditions.

One concept that has been tested is the use of a backdown board to prevent the collapse of the channel. This board is a hydrodynamic lifting surface rigged and positioned to pull the corkline outward with the flow of water during backdown. The design was tested by IATTC, was found ineffectual, and no further experiments have been done (Bratten and Guillen, 1981). However, based on such limited experience, the concept of using hydrodynamic force to keep the backdown channel open cannot be dismissed.

The backdown board suffers from the fact that its performance depends on proper rigging and careful deployment. In addition, the device needs to be large—too large to be practical—to provide the force necessary. A more appealing alternative to the backdown board is a series of smaller flexible devices, attached to the corkline or the netting itself, that would remain in place on the seine. Flexibility and small size would be required since passage through the purse block is essential. Recent developments in headrope kites suggest that such flexible hydrodynamic lifting surfaces are compatible with the rigors of commercial fishing (Goudey, 1987).

Figure 7-6 shows a device that uses small annular lifting surfaces that would be attached to the portion of the corkline needing to be pulled out. Though flexible, these devices would retain their shape. Their size and spacing would be based on the lift forces required. They would be attached to the corkline so that they would align on the outside of the net, pulling outward because of backdown flow.

Figure 7-7 shows another device that could be attached to the panel of netting below the corkline. These small, flexible, kite-like panels would produce incremental amounts of lift, forcing the panel of netting outward. 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. Systems that include inflatable corklines and operable openings in the netting

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

FIGURE 7-6 Annular lifting surfaces to maintain channel width during backdown. Drawing by C. Goudey, Massachusetts Institute of Technology, Cambridge.

have been proposed and attempted. A device called the “apex flapper” was tested with no success in 1977 (Coe et al., 1984). In related tests, a downhaul gate was rigged, but malfunctions prevented its evaluation. The limits of this approach have not been reached either in concept or in practice.

A U.S. patent has been issued on a modification to the corkline of a seine

FIGURE 7-7 Netting panel kites for preventing channel collapse. Drawing by C. Goudey, Massachusetts Institute of Technology, Cambridge.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

that could be used to allow the escape of dolphins while preventing the loss of tuna (McKnight, 1979). It incorporates hoses that would be selectively pressurized with air or water. This invention is not known to have been built or used in the fishery.

No Backdown

The backdown procedure is an effective way of getting most of the dolphins out of the seine most of the time. 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. Dolphin release may be accomplished in ways that do not require such precise coordination of seine handling, maneuvering, and the full cooperation of the weather.

One reason for an apparent preoccupation with backdown is that the purse seine will not remain open indefinitely. Unless its internal volume is continually decreased, it does not retain the desired hemispheric shape. Wind, waves, currents, and gravity conspire to collapse the seine and eventually folds and canopies of netting will begin to take their toll on the dolphins inside.

Intervention by the fishermen is necessary to prevent such net collapse. The sideways pull on the seine by the net skiff and the use of the speedboats to tow the seine back to its proper shape can delay collapse. There are reports of a seine being held open overnight by three speedboats while mechanical problems on the seiner were being repaired (D. Cormany, IATTC, personal commun., 1990). Weather conditions had to be favorable for this to be possible.

The backdown procedure does not normally require the intervention of speedboats. Sacking up the net and pulling in the corkline bunches keep an outward flow of water through most of the seine netting. Once a proper backdown begins, the netting remaining in the water tends to be held open by the flow.

If a simple means of maintaining the shape of a fully pursed seine were found, alternative techniques for the removal of dolphins could be developed and employed. Such techniques are currently impractical because there is not enough time to accomplish individual release of dolphins before collapse begins. However, the force required to maintain the shape of a seine must not be underestimated. Even the strong pull of the speedboats is insufficient when weather or current conditions are adverse.

The use of a flexible tubular device to replace or supplement the conventional corkline has been suggested as a way to produce a net perimeter with variable rigidity (D. Cormany, personal commun., 1990). Such a tube could be pressurized to prevent flexing of the tube and collapse of the seine. If the tube

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

were inflated with air, the structure could actually replace the corks. If it were inflated with water, the corks would be retained and the tube would be hung parallel to the corkline. The diameter and pressure needed to achieve the required rigidity over a net perhaps 4,000 meters around have yet to be determined. Whether such a tube could be run through a power block is yet another issue.

Another approach would be developing a seine float that allows the corkline to flex in only one direction. If the perimeter of the net could take only a straight or convex shape, the inversions of the corkline associated with net collapse would be impossible. Passing such a corkline through the power block and stacking it on deck would require special handling techniques and might be impractical.

Other Purse-Seine Variations

Other methods have been proposed for separating tuna and dolphins. They range from bubble barriers to acoustic signals to helicopter-deployed extraction devices and have varying degrees of potential. However, few of these ideas have received more than a cursory examination.

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).

A sharp behavioral difference exists between tuna and dolphins that might be used to design release methods. Tuna schools will flow through rather small openings in a net. Dolphins instead balk at passing through openings three times larger or more than a tuna school would use (Norris and Dohl, 1980b). H. Medina (personal commun., 1990) reports that tuna will sometimes flee the net through as slight an opening as a portion of the corkline that is locally depressed.

While we make no attempt here to design such a separation and release system, any such pre-backdown release method seems to require the following:

  • The ability of fishermen to maneuver the trapped dolphin herd easily, which is apparently possible.

  • A net-based device using the difference between the escape patterns of

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

tuna and dolphins that could be deployed or incorporated in the net design within the larger seine and that could separate fish from dolphins.

  • When fish and dolphins are separated, the lowering of a gate that will allow release of dolphins at some point on the net perimeter.

Devising such a system would require a joint effort by knowledgeable fishermen, behaviorists, and gear designers.

Because of the possibilities of this pre-backdown release method, it is worth examining the data on dolphin and tuna gate behavior. 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 swimming back into the net. The fishermen must carefully guard such a temporary gate because tuna will use it at once if it is available to them.

The remarkable reluctance of dolphins to pass through even large gates is well known by dolphin trainers (DeFran and Pryor, 1980). Gates that are 2 meters across and deep are regarded by captive dolphins as frightening barriers. Only after concerted training will individual dolphins rush through such gates. Even then such a passage always seems to involve an element of fear.

Recently, Norris et al. (in press) suggested that the central problem is that such gates do not accommodate the predator avoidance structure of the dolphin herd. The smallest protective dolphin herd is postulated to be approximately 6–10 animals arranged in a fluid geometry, the spacing of which is based on maintaining maximum visibility of the signals of neighbors and minimum reaction time to them. Such a protective group (called the basic school (herd) unit) with its geometry intact presumably cannot be accommodated in a small gate.

The first experiments related to this idea were those of Perrin and Hunter (1972), who attempted to define the gate size through which captive Hawaiian spinner dolphins could be induced to pass. These tests were inconclusive and seemed to predict a gate size much smaller than that used willingly by intact herds. The experiment did not include an intact basic school unit, and hence the reactions seen were not the reactions of a herd.

The second tests were the “hukilau tests” by Norris and Dohl (1980a), who attempted to learn the minimum barriers and dimensions of openings that would contain or release entire wild Hawaiian spinner dolphin herds. These dimensions probably bear a better relation to the dimensions of gates that dolphins will pass voluntarily through in purse seines than those defined by the Perrin and Hunter tests.

The important features of this test history can be summarized as follows:

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×
  • Much smaller gates will allow passage of a tuna school than will allow passage of a dolphin herd.

  • The sinking and stacking of dolphins in the backdown channel may be due to their protective herd being compressed to such an extent that visual signaling between dolphins is no longer functional. The dolphins may be left without their protective geometry.

  • 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. These dimensions correspond well to the gate size that allowed escape of spinner dolphins from the original hukilau tests (Norris and Dohl, 1980a). No predictions appear to exist for tuna escape.

Backdown Release of Dolphins

Much attention has been given over the years to refining the backdown-related seine-handling technique and the gear associated with it. In skilled hands, with good weather, and with smoothly working gear, it can be operated with very low mortality rates per set. Most mortalities occur during problem sets or are caused by inexperienced captains attempting the backdown method. Therefore, efforts to teach its intricacies to all the captains in the fleet are clearly the most promising route for immediate reduction of kill during seining.

Since fishermen are under constant pressure to produce short trips, they want to use every available working moment to catch fish. This incentive means that sets will be started when schools of fish are found, which may be late in the day, and may extend into dusk or even darkness. In such sets, mortality of dolphins rises markedly; however, sundown sets are prohibited by NMFS regulations as well as those of other countries. Usually, this elevated mortality is blamed on the increased difficulty of net handling and of carrying out seamanship in low light levels. However, the dolphins in the net may experience even greater difficulties than the fishermen. At dusk their ability to avoid entrapment or to assist in their own release may be severely reduced, because the reduction in illumination beneath the sea surface during dusk is much more profound than in air (100 times or more) (McFarland and Munz, 1975).

Thus, in low light levels, dolphins may have sharply increased difficulties in responding to each other and may provide even less assistance to those trying to release them than they do during daytime sets. If sundown sets come to be practiced again, underwater illumination of the net in the region of the apron should be tested.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×
Dolphin Behavior on Release

Observers at the lip of the backdown channel during the backdown operation report seeing almost passive masses of dolphins being sluiced toward them as the ship pulls the net (Norris et al., 1978). The backdown channel may be pulled 1–2 meters below the surface at this time, and dolphins may slide out forward or backward. The instant a dolphin perceives that it is free of the net, it is almost immediately galvanized into action and streaks away from the seiner and its net. This almost instantaneous change in behavior—from total passivity to high-level activity—is remarkable (Pryor and Kang, 1980) and makes it clear that the dolphins in the net are not in a catatonic state in which they cannot protect themselves at all. Given an opportunity (such as having the channel kept open or preventing canopies), they will rise to the surface to breathe. It is possible that research into further exploitation of this readiness to escape could lead to further reduction of dolphin mortality.

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.

  • What is the nature of the tuna-dolphin bond?

  • What regulates the persistence of the bond?

  • What underlies the different responses of tuna and dolphins to seining?

  • Why is it that tuna are escape artists once they are encircled in the seine, and dolphins refuse to help themselves and must be assisted out of the net? This difference lies at the heart of the problem of dolphin release and may be due to the different senses used by dolphins and by fish to regulate the structure of their schools— vision and hearing are used by dolphins, and vision and mechanoreception (through lateral lines and head canals) are used by the fish.

  • What underlies the different responses of various species to each other and to floating objects? Clearly, the various tuna species differ greatly in size and other properties, which affects whether they associate with dolphin species. If we understood the underlying causes, the nature of the bond itself might become clearer. The association is probably food-based, and tuna probably follow dolphins, although some believe that dolphins follow tuna. Fish-aggregating devices usually gather smaller tuna than those that collect with spotted dolphins. One center of aggregation is drifting in the water and the other is swimming. Does motion or lack of it regulate associations?

  • What oceanographic variables underlie the associations between tuna

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

and dolphins? Analysis of oceanographic and seasonal catch data, especially current, trophic, and thermal structural data, should allow assessment of the correlates of aggregation, provided such catch data can be freely obtained.

  • What are the effects of seining upon dolphin societies and population structures?

  • What are the specifications of a net opening that will allow dolphins to swim from the net on their own volition? A research group has begun to define the spatial relations that operate within dolphin herds, and these ideas in turn may help define the opening in a tuna seine that will allow dolphins to escape on their own. The theory says that a basic protective school unit exists that dolphins will not abandon and that it must be released as a unit. This unit plus undefined space on either side of a dolphin in a herd may define minimum openings needed for quick release of dolphins.

ALTERNATIVE METHODS OF LOCATING YELLOWFIN TUNA

Because tuna fishermen find tuna by looking for birds and dolphins, an obvious way to prevent dolphin mortality is to find ways to locate and catch tuna when they are not associated with dolphins. 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. The question of how to catch the large yellowfin that usually are caught with dolphins comes up in every meeting and workshop on the tuna-dolphin problem (Ralston, 1977; Hofman, 1981; DeMaster, 1989). Variants of all the methods for locating large tuna proposed here have been discussed at those workshops. Researchers propose ideas that usually languish because of lack of funding for the research that needs to be done to test them.

Acoustical Methods

Large schools of any fish moving in the water must make noise. If we knew the characteristics of the sound produced by tuna, then listening devices could tell fishermen where the fish were. One such device is a towed array, a streamer of hydrophones that is towed behind a vessel. It is sensitive only to sounds that come from the sides, which eliminates noise made by the towing vessel and facilitates locating and tracking sound sources. Towed arrays are used by the geophysics industry for petroleum exploration and by the Navy for finding and tracking submarines. Thomas and Evans (1982) used a towed array aboard a tuna seiner to evaluate its usefulness for detecting dolphins and tuna. Sound spectra taken when large numbers of yellowfin tuna were present showed a consistent increase in broadband acoustical energy between 1 and 6 kilohertz. Detecting the signal in their tests required a trained technician to operate a spectrum analyzer and to maintain the array and its electronics.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

Because the fishing effort concentrated mostly on dolphins, the array's potential for locating tuna apart from dolphins remains untested. However, the subtlety of the tuna's sound implies that towed arrays probably would be unable to detect tuna more than a mile or two away from the vessel unless they become more effective than those available in the early 1980s. Towed arrays are likely to be impractical for use in the tuna fishery, at least as a primary tool.

Listening devices may be valuable with fish-aggregating devices (FADs). A hydrophone under each FAD would pick up the sounds in its vicinity. Digital circuitry could then check the characteristics of the sound and store information about the presence of tuna for retrieval by a seiner. One attempt was made to equip logs with sonobuoys, but no tuna were located (F. Awbrey, San Diego State University, personal commun., 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. Fishermen in the eastern Pacific avoid using sonar because dolphins detect sonar at long range and move away from it, taking the tuna with them. A solution may be to use only frequencies above the hearing range of dolphins, about 150 kilohertz for one species and unknown for others. The major disadvantage of high frequencies is limited range—the higher the frequency, the shorter the range. However, high frequencies give better resolution and thus would enable surer identification of species and their numbers. Properly used, a high-frequency scanning sonar may let fishermen follow tuna and dolphins and set their nets whenever the fish move away from the mammals. If the fish return to the dolphins as soon as a disturbance starts, this technique would not work, but it ought to be tried. By not using sonar, fishermen also may be passing tuna schools that are below the surface but within reach of their nets.

The use of such equipment, in addition to or instead of helicopters, to determine the number of tuna in a school also might help eliminate sets on dolphins when only a few fish are present.

Other Methods

Remote sensing satellites now send back information about oceanic conditions all over the world. Timely information on sea-surface temperature and water color is available to everyone at a reasonable cost. Southern California albacore fishermen use water-color data to find the edges of clear water and green water where they catch albacore.

Satellite imagery may be of use in locating tuna not associated with dolphins. Given surface color and temperature, current boundaries and areas of high and low mixing can be identified. With this information, any areas with

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

non-dolphin-associated tuna could be discovered. Such information could hold an important key to developing a dolphin-safe fishery for tuna.

Shipboard receivers can be purchased that receive images directly from satellites or from shore-based transmitters, much like a weather facsimile, and some are being tested at sea by tuna fishermen. Interpretation of the information is required, and its usefulness will depend on the expertise of the captain.

Human observers on ships or in aircraft need tools to aid their limited ability to see what is under the surface. A relatively new technology involving the use of LIDAR (light-induced detecting and ranging) has been proposed for use in tuna fishing to detect subsurface schools of fish (Summers, 1990). If this laser-based technique is successful, it would help to detect tuna schools without reliance on associated dolphins. LIDAR can detect profiles of oceanographic conditions to at least 30 meters depth. These devices provide detailed information about such things as phytoplankton biomass and scattering layers (Hoge et al., 1988) and are said to be useful in the menhaden fishery (B.D. Treadwell, Remote Sensing Industries, personal commun., 1990). The manufacturer is working with Honor Marine 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 commun., 1990). LIDAR and high-resolution sonar may be useful for studying the conditions where the tuna-dolphin bond breaks up, such as studying tuna at night to determine whether they separate from dolphins and become vulnerable to seiners. They also may be a good tool to use in deciding where to place FADs to maximize the chances of aggregating large tuna.

Another recent development that would be of value in searching for school fish is the synthetic aperture radar (SAR). Of particular interest to the military, this technology has proved capable of detecting the wave variations in the path of a ship long after it has passed. The wave-detecting ability of SAR could be used to detect “breezers ” (tuna feeding on the surface causing local waves to form). SAR could locate the higher-frequency breezer waves from within ocean swells or even superimposed on ambient wind waves. The question is would high-resolution SAR in a helicopter detect surface activity, such as breezers, that human eyes miss because of the sun's angle or wave action?

Height is important for SAR to be most effective. Optimal angles are 30° to 40° to the surface. Therefore, ship-borne SAR would be much less effective than helicopter-borne SAR. An intriguing approach would be the use of a balloon to support the unit, increasing the search radius far beyond present visual range.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

ALTERNATIVES TO DOLPHIN-ASSOCIATED FISHING

Fish-Aggregating Devices

Tuna fishermen call flotsam, such as trees and other debris, logs. Anchored or drifting artificial objects deliberately placed to attract tuna are known as FADs. Discussions about alternatives to dolphin-associated fishing for yellowfin tuna usually include using such floating objects to aggregate fish. In all the world's tuna fisheries, logs and FADs are important aids for fishermen (fishermen generally refer to all of them as “logs”). 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. The successors of these FADs, along with natural flotsam, are now important fishing tools. In the western Pacific Ocean and the Indian Ocean, where tuna and dolphins appear to associate less frequently than in the ETP, purse seiners concentrate on free-swimming schools and flotsam-associated fish, and they rely heavily on logs and FADs for finding and catching tuna. In the right circumstances, flotsam and free FADs have some very advantageous features. Flotsam-associated fish are much less likely to leave the net during pursing than free-swimming schools are. Catches can be large but are on average smaller than the catches in dolphin sets. Sometimes a seiner returns to a log periodically for several days, taking substantial amounts of fish nearly every day and saving fuel in the bargain. FADs with transmitters are easy to find. If equipped with proper sensors, FADs can provide information about fishing conditions through radio signal, saving time and fuel and increasing catch per unit effort.

The success of FADs elsewhere in the world and the need to find alternatives to dolphins for aggregating tuna should make FADs an attractive option for fishermen in the eastern Pacific. Typically, however, in the Indian Ocean, school fish and flotsam-associated fish tend to be mixtures of skipjack and small yellowfin. Free-swimming schools are mostly yellowfin, and flotsam-associated schools are mostly skipjack ( Table 7-1 ). FADs are expensive to place and maintain and they are subject to piracy. Boat owners, understandably, do not want to see their investment appropriated by someone else. There is also the question of who owns the fish under a FAD. Technological and legal steps may reduce those problems.

The experience with flotsam in the ETP has been that, in most areas, it does not attract the large yellowfin that associate with dolphins. Figure 7–8 shows

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

TABLE 7-1 Species Composition of the Tuna Catch in the Indian Ocean, January 1982 to December 1984 a

 

Flotsam

Schools

Species

Metric Tons

Percent

Metric Tons

Percent

Yellowfin

10,356

24.4

39,855

78.2

Skipjack

30,031

70.7

10,132

19.9

Bigeye

2,075

4.9

725

1.4

Albacore

6

<0.1

292

0.6

Total

42,468

100.0

50,994

100.0

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). In contrast, yellowfin caught in sets on logs are usually small (<80 cm, <23 lb) and they are mixed with larger amounts of skipjack (Greenblatt, 1979; IATTC, 1989b). From the point of view of tuna conservation and production, shifting the fishery toward logs could have several undesirable outcomes and is probably not ecologically sound. The fundamental question, therefore, is whether it is possible to develop FADs capable of attracting large yellowfin. Although tuna seiners in the ETP take large amounts of school fish and flotsam-associated fish, including skipjack and other species, yellowfin are the most important component of the catch in the ETP purse-seine fishery, comprising up to 57% of the catch between 1980 and 1987 (IATTC, 1989a). Of all yellowfin caught in the ETP, about 65% are caught on dolphins (IATTC, unpublished data). Overall, in the major oceanic fisheries, yellowfin comprised about 32% of the catch between 1980 and 1987 (IATTC, 1989a).

a Data from Hallier, 1985.

Status

As part of the effort by IATTC to evaluate alternative fishing methods, Guillen and Bratten (1981) anchored five wooden FADs in the ETP in water 1,700–2,100 fathoms (3,108–3,840 meters) deep, 420–600 miles offshore, between 9° N and 15° N. As usual for that area, the tuna they attracted were skipjack and small yellowfin, not the large yellowfin that are important in the dolphin-based fishery. Because of the small size of the tuna caught, and the high costs of deployment and difficulty of maintenance, IATTC concluded that this approach was unlikely to succeed. However, IATTC suggested that free-drifting FADs might be better, especially if they were instrumented so that seiners could monitor them for position and information about the fish associated with them, but IATTC still emphasized the small size of the fish (IATTC, 1989e) and the possible consequences of shifting to that resource. IATTC argued that as the fishery is constituted now, switching from dolphins

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

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, IATTC, 1989b.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

to logs or drifting FADs could reduce effective recruitment as well as yield per recruit. Heavier exploitation of flotsam-associated fish in the usual places would catch a larger proportion of yellowfin while they were sexually immature and still below optimal size for maximizing yield per recruit (IATTC, 1989c).

The conventional wisdom that logs and FADs will catch only small yellowfin in the ETP may not be as well founded as it seems. The geographic distribution of log fishing in the ETP strongly reflects the sources of flotsam in south and central America. The areas of most intense log fishing are close to the coast, where logs are most plentiful but where the yellowfin tuna are young and small. As the logs drift westward in the currents on each side of the equator, they tend to become waterlogged and sink. Few natural logs survive in the ETP west of 120° longitude, yet the sparse data about the logs that are there suggest that FADs placed there would accumulate a larger yellowfin on the average than those found with logs in the usual places nearer to land (M. Hall, personal commun., 1990). In its program on alternative fishing methods, IATTC has preliminary data for the years 1970–1988 (Figure 7-9) showing that in some 10-degree squares, some yellowfin caught under logs are large (>100 cm). Although sample sizes are small, these observations suggest that experiments with FADs should not be done in the traditional log-fishing areas, but rather in locations farther from land where FADs have greater potential for aggregating large yellowfin. Support for this suggestion comes from the western Pacific, where large yellowfin tuna (>80 cm) often associate with flotsam. Fishermen say they catch more small yellowfin near shore and find larger fish by searching farther out. Placing FADs where larger fish are more likely to associate with them makes more sense than putting them elsewhere. 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. So far, however, we can do little more than speculate. To generate hypotheses, we need fundamental environmental and biological information that is still missing. Conjectures about the tuna-dolphin bond are that its basis may be location of food, protection from predators, active movement, environmental conditions, or a host of other factors. The correct hypothesis must explain why the bond between the same species of dolphins and tuna is more frequent or longer-lasting in the ETP than in other ocean areas where these species also are found together. It also must explain why large yellowfin aggregate with logs in some places and not in others.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

FIGURE 7-9 Size distribution of flotsam-associated pure schools of yellowfin tuna in 10-degree squares in the eastern Pacific. Squares marked with arrows have size distribution of flotsam-associated catch very similar to that of dolphin-associated catch in the same square. Source: Adapted from IATTC, 1991a.

Conclusions

The possibility of substituting FADs for dolphins as a means of aggregating large yellowfin tuna in the ETP has yet to receive the attention it deserves. Research has been going on for almost 4 years but has been underfunded. Essential information for establishing hypotheses has been lacking for far too long. Well-planned and funded research on why tuna associate with floating objects and dolphins deserves a high priority. Parallel programs also are essential to determine the properties of the best FADs, what technological devices can make them more efficient to use, and where they can be placed to attract the class of yellowfin tuna that are now caught mostly with dolphins. An institutional workshop has been organized by IATTC with industry support to study the association of tuna with floating objects in all oceans of the world that may provide answers to some of these questions.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×
Non-Purse-Seine Techniques

Alternatives to using purse seines to fish for tuna in the ETP exist. Government and industry workshops have explored this approach (DeMaster, 1989). A variety of techniques are used worldwide to exploit tuna and other midwater schooling fish, but few techniques offer the production rates now enjoyed by the modern tuna purse seiner and thus cannot compete economically with purse seiners at present. In addition, other fishing techniques are known to kill some dolphins (Northridge, 1984, 1991).

Bait Boats

The use of bait boats is clearly feasible but unlikely to be economically viable. As explained in Chapter 1, this was the method of choice before the introduction of purse seining. The use of hand-pole fishing is largely incompatible with the vessels currently employed in the ETP. Tuna-seiner designers quickly abandoned the low stern freeboard essential to pole fishing. Typical production rates of bait boats compare poorly with the rates averaged by today's purse-seine fleet. In addition, because of the labor-intensive nature of such a fishery, it is unlikely that the price offered for tuna by canners could support a U.S.-based bait-boat operation.

Longline Fishing

Longline fishing is a common method of fish capture and is the typical method used for tuna species, including yellowfin, in parts of the Atlantic, Pacific, and Indian oceans (see Chapter 3). Hook size and type of bait are specified according to the size and species of the targeted fish and, as a result, longline fishing can be quite selective.

An important consideration in comparing longline fishing with present techniques of purse seining used in the ETP is the differences in production rates and markets. The actual hauling process is time consuming, not to mention the time spent setting the longline, returning to the beginning of the line, and possibly waiting for bait to be taken. 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.

Two variations of longlining may deserve further consideration. The first is the conversion of a seiner into a mother vessel for a fleet of suitably sized but launchable catcher boats. The present ability of seiners to launch and retrieve the large net skiff would need some modification to handle several smaller longline boats. In such an operation, the catcher boats would return frequently to the mother vessel for off-loading. All catcher boats would be taken on board during bad weather and during return to port.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

The details of such an operation would need serious thought including economic assessment and optimization. The advantage is that the catching efficiency of a compact longliner can be combined with the endurance, searching ability, and capacity of the present seiner.

Another even more innovative approach incorporates the use of the seiner's helicopter to place a series of longlines in the path of a sighted school of tuna or mixed school of dolphins and tuna. According to the helicopter pilot who devised this technique, the longline could be placed without scattering the school, and the dolphins would not take a hook (K.R. Thomas, Thomas Atmospherics, personal commun., 1990). Because this operation would be directed at specific schools of tuna, catch rates per hook might be extremely high. Conversion costs for such an operation would be low. Experiments are needed to determine whether the tuna will stop to feed if their prime intent is keeping pace with the dolphins and to be sure that the dolphins will not take the bait.

The airborne aspects of this technique are the subject of a patent application. According to the inventor, conversion costs for such an operation would be low and negotiations are under way with hook, line, and baiting machine manufacturers with the intent of rapidly commercializing the method. An economic assessment of this technique would also be essential.

Midwater Trawls

Few alternative methods of fishing offer the productivity rates enjoyed by tuna purse seining in the ETP. A significant exception is trawling and a notable example is the U.S. pollock fishery off Alaska where 100-ton tows are common.

Tremendous progress has been made over the last decade in the design of gear for such high-volume trawling. High horsepower vessels, new materials, and the exploitation of the herding characteristics of pollock have allowed the development of midwater trawls in which the area of the mouth of the trawl is measured in acres. The applicability of trawling to tuna fishing in the ETP is unclear. Factors such as the herding characteristics of tuna, the towing speed required, the effect of vessel noise, and the presence of dolphins all prevent the direct extrapolation of most trawling experience.

Pair Trawls Several of the committee members believe that pair trawling is not a viable option for catching yellowfin in the ETP, but a few members felt it to be worth considering. A brief description of the French experience is thus provided.

In France, midwater trawls have been somewhat successful in capturing albacore in the Bay of Biscay (Prado, 1988). This technique was developed to increase the productivity of small (17–20 meters) longline vessels that normally have low catch rates. Pairs of these vessels were fitted with a midwater trawl normally used for bass. These 400–500 horsepower vessels were able to

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

tow the net at 3.5 to 4.0 knots while achieving a vertical gap of 30–40 meters and a horizontal opening of 60-80 meters. Trawling was done at night on the surface of the water or close to it. The thermocline in this location is 10–15 meters, which is shallow.

The sound of the pair trawlers, the tow cables, and the 18-meter meshes in the forepart of the trawl were found to be effective in herding the tuna toward capture. Catch rates of 3–8 metric tons per day have been reported. Catch rates of purse seiners are considerably higher.

Incidental Catch of Dolphins in Trawls In evaluating the utility of trawling as a substitute for purse seining, the incidental catch of dolphins in established trawl fisheries is relevant. As discussed earlier, little useful information is available to aid in the design of gear that would select only tuna from a mixed school of tuna and dolphins.

Anecdotal reports of bottlenose dolphins being caught in large midwater trawls used along the U.S. east coast exist. Reports compiled by NMFS on marine mammal mortality state a trawl-related incidental catch of 20 and 42 dolphins of all species for years 1987 and 1988, respectively (Waring et al., 1990). Clearly, trawls are not designed to spare dolphins. Specific techniques may be applied, however, to avoid their capture.

A Hypothetical Trawling Technique for Tuna in the ETP The committee did not agree on the promise of this option; several members believe that it is not worth pursuing. Perhaps the greatest disadvantage is that if any dolphins were caught in such a net, they would quickly drown and would not be discovered until long afterwards. However, a few members felt that this option might be developed effectively, and therefore it is briefly described.

Conventional single-boat midwater trawling probably would not be useful in tuna fishing because of vessel noise and especially because of the high swimming speeds of tuna. By comparison with single-boat trawling, pair trawling offers the following potential advantages:

  • Vessel noise may herd the tuna into the path of the net.

  • Trawl doors are not required.

  • Extreme spreads are possible.

  • The trawl could be larger than a single-boat trawl because twice the horsepower would be available.

Pair trawling would not necessarily require purpose-built vessels. Adaptation of purse seiners currently in use in the ETP is feasible. Their seining capabilities may be retained, yielding a combination trawler-seiner. In addition, although present methods of tuna location could be retained, dolphins would not be encircled by a net and their herds would remain intact.

The combined horsepower of two 3,800-horsepower seiners would allow

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

the use of a net with approximately 266,000 pounds of drag. Though the drag would vary considerably with speed, 7,600 horsepower would be sufficient to pull a very large net. Depending on the extent to which large mesh or ropes could be used, vertical openings of 550 feet and horizontal openings of 3,000 feet might be possible.

Based on the French experience, the towing speed does not have to equal or surpass the swimming speed of tuna, at least not at night. 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. Several of the small craft could be used to direct a school of dolphins into the gap between two approaching pair trawlers. Eventually, optimal values of pair-trawl speed and vessel separation would be determined.

Data on the vertical separation of tuna and dolphins during normal swimming and during a chase would be essential to allow the selective catch of tuna; indeed, if tuna and dolphins are not adequately separated, this technique might kill more dolphins than purse seining does. The depth of the headrope could be set either by varying the speed or the length of tow cable (typical of most midwater trawling) or by the use of surface buoys on pendants of specific length. The intention would be to place the headrope of the trawl between the two species or slightly deeper. Experimentation would be required to learn the best depth for minimizing tuna loss without risk of capturing dolphins.

Such a pair-trawling operation can be undertaken only if the vertical separation of the two species is predictable or if they do not associate at night. Certainly the depth of the headrope should be well below the deepest dolphin, even though the upper portions of the tuna school may be lost.

In addition to the risk of killing dolphins, a disadvantage of this method is the requirement of two seiners for one fishing operation. This problem is lessened, however, because most of the trip time is spent searching, and two vessels would offer increased search coverage.

Gillnets

Gillnets can be a selective method of capture in some fisheries. In other fisheries, or when used irresponsibly, they can be indiscriminate in their catch. Productivity with gillnets is similar to longline rates and falls far short of purse-seine rates.

Marine mammals often are entangled in gillnets, and it is extremely doubtful that a pure catch of tuna could be pulled from an aggregation of mixed species. A dolphin mortality of three animals per net has been observed in the Japanese driftnet fishery for albacore (Anon., 1990). The committee recommends against the use of gillnets as an alternative to purse seining in the ETP.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

REGULATORY ALTERNATIVES

The behavior of the tuna fishermen can be changed voluntarily or through regulation. The tuna fishermen themselves have instituted or suggested changes in their tuna-seining processes over the past two decades. Some of these changes have been voluntary, and some have been stimulated by laws and regulations. Unless additional voluntary change is anticipated, further development of law and policy must be considered if behavior is to change and if dolphin mortality is to be reduced.

Any of the changes referred to in the above sections on alternative fishing methods, if not voluntarily adopted, would have to be the subject of regulatory action. Education and training, discussed in the next section, may stimulate voluntary changes in methods of tuna seining. The purpose of this section is to enumerate the major alternatives for regulatory action, the assumption being that voluntary action may not be sufficient to accomplish various public-policy goals with respect to the tuna-dolphin issue. The following discussion of the alternatives should not imply that any particular alternative should or should not be implemented. It is intended simply to show the range of possible public-policy options. The success of such options would depend on the degree to which they were implemented by all nations with boats fishing for tuna in association with dolphins.

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. The IATTC staff (J. Joseph, personal commun., 1990) is quite confident that a prohibition of dolphin fishing in the ETP would result in a substantial reduction in the overall production of yellowfin. In addition, if some of the fishing capacity were redirected toward smaller yellowfin, the overall biomass of yellowfin might be significantly reduced as well. Some of this loss would be made up by increased catches of skipjack, but the level of that catch cannot be predicted, because the availability of skipjack in the ETP varies greatly from year to year.

The IATTC staff is not able to make accurate predictions of the long-term effects of catching only small fish on the spawning biomass and subsequent recruitment of yellowfin because no apparent relationship has been observed between spawning biomass, as currently estimated (see Figure 7-10), and recruitment. This may be due to the lack of a relationship over the range of observed stock sizes or due to deficiencies in the data. IATTC is continuing to work on this problem. The lowest spawning biomass was estimated at about 90,700 metric tons (Figure 7-10). Females do not reach sexual maturity, on the

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

FIGURE 7-10 Estimates (in short tons) of the biomass of large yellowfin (upper panel) and of all yellowfin (lower panel) in the eastern Pacific. (1 short ton = 0.907 metric ton.) Source: Adapted from IATTC, 1990a.

average, until they exceed 40 pounds, which is about the average size of the yellowfin taken in the fishery for fish associated with dolphins ( Figure 7–11).

The most recent IATTC estimate (IATTC, 1991b) of the long-term effect of a prohibition of dolphin fishing in the ETP is that it would result in a

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

FIGURE 7-11 Percent maturity of female yellowfin tuna as a function of average weight and age. Data from IATTC, 1990b.

30–60% reduction in ETP yellowfin catch, as well as a significant reduction in yellowfin stocks, if no alternative method were developed to capture large yellowfin.

Dolphin Mortality Certificates

Under this alternative, certificates would be issued to the captain of each vessel in the tuna fleet for a certain share of the total allowable dolphin mortality. The certificates would be broken down into units of, say, 1–10 dolphins each. The permitted level of dolphin mortality for each captain would be equal to his number of certificates. These certificates could be nontransferable, in which case a captain would be limited to the mortality on his original certificates, or transferable among captains, perhaps on a market basis, in which case a captain could buy certificates from other captains. In either case, when a captain reached the mortality totaling the number of certificates he possessed, he would have to stop fishing on dolphin-associated tuna.

This alternative would make sense only if the total dolphin mortality would be reduced steadily over time. To achieve reduction in total mortality, each captain would have to surrender a percentage of his certificates each year. Under a transferable system, a captain could purchase more certificates from another captain. The price of a certificate could be determined by the market

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

for the certificates among the captains, not by the government. Under this plan, the government would simply record certificate transfers and monitor compliance. Under a nontransferable system, all captains would reduce their dolphin mortality by an equal percentage each year.

The second set of options discussed below centers on alternatives that would create incentives for behavior that reduces dolphin mortality, as opposed to direct or indirect prohibition on dolphin mortality itself.

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.

Price Incentives for Fishing on Non-Dolphin-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. This alternative would involve altering the price structure for tuna to eliminate the perceived or real advantages of purchasing larger yellowfin. This change would probably have to be made through the mechanism of a tax or tariff on yellowfin over a certain size or through a price incentive for other species such as skipjack.

The major canning firms in the U.S. market recently (1990) created such a disincentive by announcing that they would no longer purchase tuna taken in association with dolphins in the ETP. Although the mechanisms of this proposal are unclear, this move would appear to place a significant constraint on fisheries involving dolphin mortality, at least those delivering tuna to canneries owned by these firms. The important questions concerning this proposal from the point of view of this committee are the following:

  • How will the proposed program be monitored? The initial understanding is that the existing U.S. and international observer programs will be continued, although they would have to be expanded considerably to serve a certification function for all countries and vessels involved in dolphin-related tuna fisheries. Will the monitoring and enforcement of this program be done through government programs, such as the NMFS observer program, through international or regional programs involving such organizations as IATTC, or through the mechanisms described in the Dolphin Protection Consumer Act of 1990 (P.L. 101 –267, Title IX)?

  • How will any potential embargoes on tuna products coming into the United States under the 1988 amendments to the Marine Mammal Protection

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

Act of 1972 (MMPA) affect international or foreign nation observer programs? What, in turn, will be the effect of changes in foreign-nation observer programs on the ability of canning companies or the U.S. government to monitor catches of tuna associated with dolphin mortality?

  • What will be the effect of the canning-company decisions on the U.S. high-seas fleet operating in the ETP? Five general alternatives present themselves:

  1. Fishing on school fish not associated with dolphins outside the 200-mile Exclusive Economic Zones of the Resource Adjacent Nations in the ETP.

  2. Making arrangements with Resource Adjacent Nations to fish on school and log fish within their Exclusive Economic Zones.

  3. Moving from the ETP to other fishing areas and fishing on school and log fish. (This option does not consider the fate of coastal canneries left behind.)

  4. Selling dolphin-associated tuna through other outlets.

  5. Perhaps sale of U.S. vessels to nations willing to continue dolphin fishing in the ETP.

The recent decline in the number of U.S. vessels that fish on dolphins in the ETP might be an early indication of the economic consequences of the canneries' decision.

Captain Performance Standards

The 1988 amendments to the MMPA mandate that the Secretary of Commerce develop and implement “a system of performance standards to maintain the diligence and proficiency of certificate holders” among the U.S. captains, the implication being that a similar system of performance standards would be expected of foreign fleets as well. The implementation of such an international system is now being developed through IATTC. NMFS also is developing a system whereby captains in the U.S. fleet would be penalized when their performance was below a given standard.

Training and Evaluation of Fishing Captains

Initial Training Most captains of tuna vessels learn their craft from another captain aboard a boat and serve as navigators for several years before being put in charge of a vessel. If they serve under a captain with a record of low dolphin mortality, they are likely to be well trained and motivated in this regard, although the degree of motivation also tends to depend on the individual.

In addition, some governments require that captains undergo more formal

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

and specialized instruction on fishing laws and regulations, as well as in the use of fishing gear and techniques required for fishing for tuna in association with dolphins. Unless they comply with this requirement, captains are not issued the license that permits them to fish on dolphins. NMFS conducts such workshops for captains of U.S. vessels, with the following agenda:

  • Introduction.

  • Training requirements.

  • Responsibilities of the Tuna-Dolphin Management Branch.

  • The MMPA.

  • Certificates of inclusion.

  • International aspects of the tuna-dolphin problem.

  • The observer program.

  • Reporting requirements.

  • Mortality reduction technology.

  • Suggestions from the participants for improving dolphin safety.

  • Marine mammal identification and geographical distribution.

  • Quotas.

  • Dolphin safety gear requirements.

  • General procedural requirements.

  • Dolphin safety and release procedures.

  • Observers.

IATTC organizes workshops and seminars for captains, crews, and managers of tuna vessels in the international fleet. These workshops typically include:

  • Activities of IATTC's international tuna-dolphin program.

  • Historical review of the purse-seine fishery in the ETP.

  • Fleet sampling and mortality estimation.

  • Historical review of incidental dolphin mortality.

  • International controversy over the tuna-dolphin program.

  • Factors affecting dolphin mortality; responsibilities of captains and companies.

  • Laws and regulations.

  • Comparison of mortality rates, such as those between captains and between fleets.

  • IATTC gear program and services; recent gear innovations.

  • Review of dolphin safety gear.

  • 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. However, the crew's skill and motivation play a major role in determining a vessel's dolphin mortality

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

record. The best captains train their crews in the procedures for reducing dolphin mortality and instill in them the desire to do their best in every set to avoid killing dolphins. Even the best captains occasionally experience a set with high mortality, but in the long run their records show the difference (see Figure 6-3). From the point of view of management, captains who are responsible for the most mortality must be identified and measures must be taken, which may range from requiring additional training to revoking their fishing licenses.

To assess a captain's performance fairly, the effects of biases arising from fishing in different areas or on different stocks should be removed. This approach is used by IATTC. Alternatively, a simpler measure of performance may be used with the assumption that the effects of these factors will even themselves out over a long period of time for all captains. In 1989, NMFS proposed a system of the latter type, in which the average mortality per set from one trip would be compared with the average value for all captains. If the figure for a trip exceeded the average by 50% or more, certain actions would be taken. This system is being revised; new guidelines were not available for this report.

Remedial Actions Procedures have been developed for dealing with captains with records of high dolphin mortality. When such a captain has been identified, an expert-captain panel is convened. A group of experienced captains from a national fleet, both active and retired and with low dolphin mortality records, meets with the captain in question, reviews data from his trips, and discusses problems and solutions. This approach has proved to be effective, but it has some drawbacks. Small fleets may not have enough captains in port at any one time to convene such a panel, and new fleets may not have enough captains, active or retired, with sufficient experience to form an effective panel. The rapid expansion of foreign fleets in the 1980s created a need for vessel captains, and many were promoted from the ranks or brought in from fisheries in the Atlantic and Indian oceans. 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, IATTC is providing the technical expertise for the assessment and correction of captain performance for the international fleet.

In the United States, the Porpoise Rescue Foundation tracks the performance of all U.S. fleet captains and intervenes when necessary. Other countries have their own national programs, usually coordinated through industry associations or specialized organizations, which monitor the performance of the captains of the national fleet. The procedures vary for dealing with captains with records of high dolphin mortality. The captain 's record and the magnitude of the problems experienced are considered before action is

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
×

taken. IATTC cooperates with all these national initiatives and provides data and technical support for their interpretation.

Both NMFS and IATTC 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. The procedure is especially recommended when a captain is experiencing chronic problems that may be due to gear that is in poor condition or is used incorrectly. IATTC also offers set-by-set analyses of observed trips, detailing the use of dolphin safety equipment.

Suggested Citation:"7 Techniques for Reducing Dolphin Mortality." National Research Council. 1992. Dolphins and the Tuna Industry. Washington, DC: The National Academies Press. doi: 10.17226/1983.
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This book presents key conclusions about the controversial killing of thousands of dolphins each year during tuna fishing in the eastern tropical Pacific.

Dolphins drown in nets that are set to catch yellowfin tuna, which tend to swim beneath dolphin herds. After 20 years of intense debate among environmentalists, the tuna industry, and policymakers, this fatal by-product of tuna fishing remains a high-profile public issue.

Dolphins and the Tuna Industry provides a neutral examination of the scientific and technical questions at the core of the problem. Recommendations for solutions are offered in two areas: developing new techniques that promise to reduce dolphin mortality with the existing purse-seine method of tuna fishing, and developing entirely new methods of finding tuna that are not swimming with dolphins.

Dolphins and the Tuna Industry provides a comprehensive, highly readable overview of the dolphin-tuna controversy, useful to experts and newcomers to the issue. It explores the processes of tuna fishing and dolphin mortality, the status of the tuna industry, and the significant progress made in reducing dolphin mortality through modifications in fishing practice.

The volume includes:

  • An overview of U.S. laws and policies relating to tuna and dolphins.
  • An illustrated look at how tuna fishing crews use their equipment, focusing on the purse seine, which is the method most economical to the industry but most deadly to the dolphins.
  • An overview of what is known about tuna and dolphin populations and the remarkable bond between them.
  • A step-by-step description of the fishing process and efforts to let dolphins escape from the nets.
  • An analysis of possible approaches to reducing dolphin kill, including more stringent regulatory approaches and incentives for the tuna industry.

This book will be indispensible to environmental and animal protection groups, tuna fishing crews and processors, companies that market tuna products, policymakers, regulators, and concerned individuals.

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