[updated from W. F. Perrin, B. Wursig and J. G. M. Thewissen, eds. Encyclopedia of Marine Mammals. Academic Press, San Diego, California., p.1269-1273 (2002)]
I. The Problem
In the tropical waters of the Pacific Ocean west of Mexico and Central America, large yellowfin tuna (Thunnus albacares) swim together with several species of dolphins: pantropical spotted, Stenella attenuata, spinner, S. longirostris, and common, Delphinus delphis, dolphins. This ecological association of tuna and dolphins is not clearly understood, but it has had two important practical consequences: it has formed the basis of a successful tuna fishery, and it has resulted in the deaths of a large number of dolphins. This is the heart of the tuna-dolphin issue.
The bycatch of dolphins in the eastern tropical Pacific (ETP) purse-seine tuna fishery stands apart from marine mammal bycatch in other fisheries, not only in scale but in the way the dolphins interact with the fishery. Marine mammals interact with most fishing gear only incidentally, but in the ETP tuna fishery the dolphins are an intrinsic part of the fishing operation. The fishermen intentionally capture both tuna and dolphins together, then release the dolphins from the net. Further, unlike most other fisheries, the vast majority of dolphins captured by the ETP tuna fishery are released alive; thus, an individual dolphin may be chased, captured and released many times during its lifetime.
The number of dolphins killed since the fishery began in the late 1950s is estimated to be over 6 million animals, the highest known for any fishery. For comparison, the total number of whales of all species killed during commercial whaling in the 20th century was about 2 million. The bycatch of dolphins in the ETP tuna fishery has now been successfully reduced by more than 99%, but even at the present level of about 1,000 dolphins/year, it remains among the largest documented cetacean bycatch in the world.
Fig. 1. Purse seine being set on tuna and dolphins. The net is not yet closed, and four speedboats are driving in tight circles near the opening to prevent the dolphins (and tuna) from escaping.
II. Purse-Seining for Tuna
Prior to the development of modern purse seines, tropical tuna were caught one at a time using pole-and-line methods. In the late 1950s the twin technological developments of synthetic netting that would not rot in tropical water and a hydraulically driven power-block to haul the net made it possible to deploy very large purse-seine nets around entire schools of tuna, and thus to catch many tons of fish at a time. Purse-seining for tuna in the ETP can be conducted in one of three ways: the net may be set around schools of tuna associated with dolphins (“dolphin sets,” which catch large yellowfin tuna), around schools of tuna associated with logs or other floating objects (“log sets,” which catch mainly skipjack but also bigeye and small yellowfin tuna), or around unassociated schools of tuna (“school sets,” which catch small yellowfin and skipjack tuna). The proportions of different set types have varied over the history of the fishery, but in recent years about half have been dolphin sets, one quarter log sets and one quarter school sets.
Dolphins are killed almost exclusively in dolphin sets. During “porpoise fishing” (the fishermen’s term), schools of tuna are located by first spotting the dolphins or the seabird flocks which are also associated with the fish. Speedboats are used to chase down the dolphins, herd them into a tight group, and set the net around them (Fig. 1). The tuna-dolphin bond is so strong that the tuna stay with the dolphins during this process, and tuna and dolphins are captured together in the net. Dolphins are released from the net during the backdown procedure (Fig. 2).
Fig. 2. Backdown procedure in progress. As the tuna vessel moves backwards to the right in this photo, the net is drawn into a long channel. The corkline at the far end is pulled under water slightly, and the dolphins escape. Speedboats are positioned along the corkline to help keep the net open.
If all goes well, the dolphins are released alive, but the process requires skill by the captain and crew, proper operation of gear, and conducive wind and sea conditions. As with any complicated procedure at sea, things can go wrong, and when they do, dolphins may be killed.
From an ecosystem perspective, management of the ETP purse-seine tuna fishery poses interesting challenges. The three methods of purse-seining for tuna, log-, school- and dolphin-fishing, catch different mixes of tuna species and sizes, and in addition have different amounts and composition of bycatch. Dolphin sets result in dolphin mortality, but dolphin sets have the least bycatch overall. Log sets have about 30 times the bycatch of school sets by weight per set, which in turn have about 3 times the bycatch of dolphin sets. Most of the bycatch, though, even on dolphin sets, is fish, primarily tuna, marlin and dorado. These fish have much higher reproductive rates than dolphins, sea turtles, sharks and rays, so the effect of the bycatch is smaller. While the effects of the fishery on dolphin populations have been strong and are relatively well known, the effects on other marine populations of concern, such as sharks and sea turtles, are mostly unknown.
III. Actions to Reduce Dolphin Bycatch
The magnitude of dolphin mortality in the ETP tuna fishery first came to widespread attention in the mid-1960s. The dolphin kill at that time is not known with precision, but without question was very high (Fig. 4). When the U.S. Marine Mammal Protection Act was passed in 1972, it included provisions for reducing the bycatch to “insignificant levels approaching zero” after a 2-year moratorium on regulation during which the tuna industry was expected to solve the problem through development of improved fishing methods. Under this law, scientific studies were initiated, observers were placed on fishing boats, fishing gear was inspected, and boat captains with high dolphin mortality rates were reviewed. Modifications of fishing gear and procedures were developed to reduce dolphin kill. After much litigation, the first regulations to reduce the dolphin kill on U.S. vessels were promulgated (Gosliner 1999). By the end of the 1970s, the kill had declined from about 500,000 to about 20,000 dolphins per year (Fig. 4).
As the U.S. tuna fleet decreased in size and the fleets of Mexico, Venezuela, Ecuador, and other Latin American countries increased, the dolphin kill began to grow again, and actions to monitor and reduce the dolphin bycatch became international. The Inter-American Tropical Tuna Commission began a dolphin conservation program in 1979 modelled on the U.S. effort. By 1986, an international observer program with all countries participating showed that total dolphin mortality had increased to 133,000/year (Fig. 4). Because U.S. boats operated under restrictions that did not apply to boats of other countries, the U.S. began requiring that imported tuna be caught at dolphin mortality rates comparable to U.S. boats (Gosliner 1999). The concept of Dolphin-Safe tuna – tuna caught without setting on dolphins (i.e., log and school sets) – became popular, and by 1994 only Dolphin-Safe tuna could be sold in the U.S. These trade actions were important because the U.S. is the largest market of the canned tuna product of the fishery.
Fig. 3. Estimated annual number of dolphins killed in the eastern tropical Pacific purse-seine tuna fishery, total for all dolphins and separately for the two dolphin stocks with the highest number killed.
The dolphin kill declined between 1986 and 1993 due to these various political and economic pressures (Fig. 3). Starting in 1993, the ETP fishing countries decided to increase observer coverage, institute skipper review panels, and meet a schedule of decreasing dolphin quotas on an individual boat basis (the La Jolla Agreement). The Declaration of Panama of 1995 carried these ideas further, proposing observers on every boat over 400 tons and strict per-stock dolphin mortality limits. These features became part of the Agreement on the International Dolphin Conservation Program (AIDCP), a binding document among the major fishing countries that went into force in 1999. By this time total reported dolphin mortality had fallen to fewer than 3,000 dolphins/year.
The Declaration of Panama also called for the U.S. to change its definition of Dolphin-Safe tuna to include tuna caught by setting on dolphins as long as no dolphins were observed killed or seriously injured onthat set. Before changing the Dolphin-Safe label, however, the U.S. undertook studies under the International Dolphin Conservation Program Act (IDCPA) to determine if the process of chasing and encircling dolphins was having a significant adverse impact on depleted dolphin populations. A series of reports were issued at the conclusion of the studies in 2002. The head of the National Marine Fisheries Service issued a finding that the fishery was not significantly affecting the dolphin populations. This decision would have allowed the new, less-restrictive definition of Dolphin-Safe to take effect, but the decision was overturned in a 2004 legal decision. Therefore, the original definition of Dolphin-Safe (ie, tuna caught on a trip on which no dolphins were chased, encircled or killed) applies to all canned tuna sold in the U.S.
Tuna caught by setting on dolphins may also be sold but may not be labeled Dolphin-Safe. Other countries either follow the US definition or the competing AIDCP definition (ie, tuna caught in sets on which no dolphins were observed to be killed or seriously injured).
Fig. 4. Estimated population size, with shaded 90% probability interval, of the two dolphin stocks most affected by tuna purse-seine fishing in the eastern tropical Pacific. Estimates of abundance between 1979 and 2000 are shown as points with 95% confidence intervals. The populations declined due to high numbers of dolphins killed in the tuna fishery from 1960-75, as shown in Fig. 3. (from Wade et al., 2007).
IV. Status of the Dolphin Populations
The status of ETP dolphin stocks (management units) is based on two time-series of data: estimates of the number of dolphins killed, based on data from observers on tuna vessels (Fig. 4), and estimates of abundance, based on data from research vessel surveys (Gerrodette and Forcada 2005; Fig. 5). Combining these data in a population model has indicated that the stocks most affected by the tuna fishery are the northeastern stock of the offshore pantropical spotted dolphin (S. attenuata attenuata) and the ETP endemic subspecies, the eastern spinner dolphin (S. longirostris orientalis). Both populations declined between 1960 and 1975 during the period of high mortality on U.S. boats, but have remained approximately constant since then (Fig. 5). As of 2002, northeastern spotted and eastern spinner dolphins were estimated to be at 19% and 29%, respectively, of population sizes when the fishery began (Wade et al. in press). Other stocks have apparently been less affected, although little is known of the small populations of coastal forms of spotted and spinner dolphins.
Since the early 1990s, reported dolphin mortality has been low enough that the dolphin populations should have started to recover. As of 2002, however, neither dolphin population was recovering at expected rates (Wade et al. 2007). Hypotheses to explain the lack of recovery (Gerrodette and Forcada 2005) have included underreporting of kill by observers, cryptic effects of the fishery not detectable by observers, such as stress, induced abortion or separation of mothers and calves (Archer et al. 2004, Noren and Edwards 2007), long-term ecosystem changes, and a lag in recovery due to interactions with other species. In years with a high number of dolphin sets, there are fewer calves in the spotted dolphin population. Reproduction in both dolphin populations declined between 1993 and 2003, which is at least one reason why recovery has been at a lower-than-expected rate (Cramer et al in prep). On the other hand, perhaps pelagic dolphins inherently have low reproductive rates, and our expectation for rate of recovery needs to be revised. Research is continuing, but until there are clear recoveries of the affected dolphin stocks, the tuna-dolphin issue is likely to remain highly controversial.
Gerrodette, T., G. Watters, W. Perryman, and L. Ballance. 2008. Estimates of 2006 Dolphin Abundance in the Eastern Tropical Pacific, with Revised Estimates from 1986-2003. NOAA Tech. Memo. NMFS-SWFSC-422.
Gerrodette, T., G. M. Watters, and J. Forcada. 2005. Preliminary estimates of 2003 dolphin abundance in the eastern tropical Pacific. Southwest Fisheries Science Center, Administrative Report LJ-05-05, 26 p.
Gosliner, Michael L., 1999. The Tuna-Dolphin Controversy. Pages 120-155 in Conservation and Management of Marine Mammals, John R. Twiss, Jr. and Randall R. Reeves (eds.), Smithsonian Institution Press, Washington, D.C.
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National Research Council, 1992. Dolphins and the Tuna Industry. National Academy Press, Washington, D.C.
Perrin, William F. 1969. Using porpoise to catch tuna. World Fishing 18: 42-45.
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Wade, P. R., Watters, G. M., Gerrodette, T., and Reilly, S. B. (2007). Depletion of spotted and spinner dolphins in the eastern tropical Pacific: modeling hypotheses for their lack of recovery. Marine Ecology Progress Series343, 1-14.