I. The Tuna-Dolphin Association
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, to a lesser extent, common (Delphinus delphis) dolphins. The association of tuna and dolphins appears to be related to reducing the risk of predation when the habitat of the tuna is restricted to warm surface waters by the oxygen minimum zone (Scott et al., 2012). Tuna-dolphin associations occur in other oceans but are most common in the eastern tropical Pacific (ETP), where the oxygen minimum zone is especially well-developed.
The regularity and strength of the tuna-dolphin association in the ETP has permitted the development of a purse-seine fishery based on it (National Research Council, 1992). The fishermen capture tuna and dolphins together, then release the dolphins from the net. In the early years of the fishery, bycatch of dolphins was very high, leading to significant reductions in the dolphin populations. The public outcry over the large dolphin bycatch was one of the factors behind the passage of the US Marine Mammal Protection Act. The number of dolphins killed since the fishery began is estimated to be over 6 million animals. For comparison, the total number of whales of all species killed during commercial whaling in the twentieth century was about 2.9 million. However, the dolphin bycatch has now been reduced to a small fraction of its former level.
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. The twin technological developments of synthetic netting that would not rot in tropical water and a hydraulically driven power-block to haul very large nets made it possible to deploy purse-seines 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 fish-aggregating devices (“log sets” or “floating-object sets,” which catch mainly skipjack but also bigeye and small yellowfin tuna), or around unassociated schools of tuna (“school sets” or “unassociated sets,” which catch small yellowfin and skipjack tuna). The proportions of the three set types have varied over the history of the fishery, but in recent years have been about equals.
Dolphins are killed almost exclusively in dolphin sets. In dolphin-set fishing, 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 and herd them into a tight group; then the net is set 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). 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 trapped in the net and killed.
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.
From the perspective of an ecosystem approach to fisheries management, the ETP purse-seine tuna fishery poses interesting challenges. The three methods of purse-seining for tuna in the ETP, log-, school- and dolphin-set fishing, catch different mixes of tuna species and sizes, and in addition have different amounts and composition of bycatch. The effect of the fishery on the ecosystem includes both the targeted catch (tuna) and bycatch. Most of the bycatch, even in dolphin sets, is fish. Log-set fishing has about twice the ecosystem effect of dolphin- or school-set fishing when measured by the time it takes to replace the biomass removed (Gerrodette et al., 2012).
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. 3). When the US Marine Mammal Protection Act was passed in 1972, it included provisions for reducing the dolphin bycatch to “insignificant levels approaching zero” after a 2-year moratorium on regulation, during which time the US 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 US vessels were promulgated (Gosliner, 1999). By 1980 the kill had declined from about 500,000 to about 20,000 dolphins per year (Fig. 3).
As the size of the US tuna fleet decreased and the fleets of Mexico, Venezuela, Ecuador, and other Latin American countries increased, the dolphin kill began to grow again. Actions to monitor and reduce the dolphin bycatch became international. The Inter-American Tropical Tuna Commission began a dolphin conservation program in 1979 modeled on the US effort. By 1986, an international observer program with all countries participating showed that total dolphin mortality had increased to 133,000 per year (Fig. 3). Because US boats operated under restrictions that did not apply to boats of other countries, the United States began requiring that imported tuna be caught at dolphin mortality rates comparable to US boats. The concept of Dolphin-Safe tuna—tuna caught without setting on dolphins (i.e., in log and school sets)—became popular. The trade actions were important because the United States is a large market for 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 inset graph has an expanded vertical scale to show details from 2000-2015.
The dolphin kill again declined between 1986 and 1993 due to these various management actions 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 by-stock dolphin mortality limits. These features became part of the International Dolphin Conservation Program Agreement (Hedley, 2001), 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 3000 dolphins per year. In recent years, dolphin mortality has declined further to about 1000 animals per year, and for each dolphin stock, the annual bycatch is less than 0.1% of the estimated population size. The reduction of the dolphin bycatch by >99% is a conservation success story.
The Dolphin-Safe label on canned tuna in the US market is a continuing issue. Mexico, whose fleet catches tuna primarily by setting on dolphins, argues that the labeling standards discriminate unfairly against Mexico and violate US obligations under the World Trade Organization. At issue is whether the US Dolphin-Safe standards are applied in a consistent way to all tuna fisheries.
Fig. 4. Estimated population trajectories from 1959-2002 of the two dolphin stocks most affected by tuna purse-seine fishing in the eastern tropical Pacific. Estimates of abundance between 1979 and 2006 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.
IV. Status of the Dolphin Populations
Determination of the status of ETP dolphin stocks (management units) is based primarily on two time-series of data: estimates of the number of dolphins killed, based on data from observers on tuna vessels (Fig. 3), and estimates of abundance, based on line-transect data from research vessel surveys (Fig. 4). Combining these data in a population model has indicated that the stocks most affected by the tuna fishery are the northeastern stock of offshore pantropical spotted dolphins (S. attenuata attenuata) and the ETP-endemic stock of eastern spinner dolphins (S. longirostris orientalis).
Both populations declined between 1960 and 1970 during the period of high mortality on US boats, then stabilized as dolphin bycatch declined under the Marine Mammal Protection Act (Fig. 4). Since the early 1990s, under the international dolphin conservation agreements mentioned above, reported dolphin mortality has been low enough that the two dolphin populations, based on their reproductive rates and assuming that deaths in the fishery were the only factor impeding recovery, should have started to recover. By 2002, neither dolphin population appeared to be recovering at expected rates (population trajectory in Fig. 4, Wade et al., 2007), but population estimates in 2003 and 2006 suggested that the two populations have begun to increase more rapidly. Besides the observed kill in the fishery, factors that might affect dolphin recovery include ecosystem changes (affecting reproductive and survival rates, movement across stock boundaries), cryptic effects of chase and encirclement (stress, induced abortion, separation of mothers and calves), unobserved or under-reported kill, and a lag in recovery due to interactions with other species. Studies have shown support for most of these hypotheses, but it is not clear how strongly each may affect dolphin population dynamics. Further, there have been no population estimates since 2006, so the present status of the dolphin populations is uncertain. At the time of this writing (2016), a new ETP dolphin survey and comprehensive assessment modelling are being discussed.
Gerrodette, T., Olson, R., Reilly, S., G. Watters, and Perrin, W. (2012). Ecological metrics of biomass removed by three methods of purse-seine fishing for tunas in the eastern tropical Pacific Ocean. Cons. Biol. 26, 248-256.
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.
Hedley, C. (2001). The 1998 Agreement on the International Dolphin Conservation Program: Recent developments in the tuna-dolphin controversy in the eastern Pacific Ocean. Ocean Dev. Int. Law 32, 71–92.
National Research Council, 1992. Dolphins and the Tuna Industry. National Academy Press, Washington, D.C.
Scott, M. D., Chivers, S. J., Olson, R. J., Fiedler, P. C., and Holland, K. (2012). Pelagic predator associations: tuna and dolphins in the eastern tropical Pacific Ocean. Mar. Ecol. Prog. Ser. 458, 283-302.
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 Series 343, 1-14.