Tuna-Dolphin Issue (marine mammals)

 

I. The Problem

In the tropical waters of the Pacific Ocean west of Mexico and Central America, large yellowfin tuna (Thunnus albacares) swim togedier with several species of dolphins (pantropical spotted, Stenella attenuata; spinner, S. longirostris; and short-beaked common, Delphinus delphis). 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 deadis of a large number of dolphins. This is die 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 and then release the dolphins from the net. The vast majority (more than 99%) of dolphins captured by the ETP tuna fishery are released alive; thus, an individual dolphin may be chased, captured, and released manv times during its lifetime.

The number of dolphins killed since the fishery began some four decades ago is estimated to be over 6 million, the highest known for any fishery. For comparison, the total number of whales of all species killed during commercial whaling in the 20th century is about 2 million. In recent years, the killing of dolphins in the ETP tuna fishery has declined by two orders of magnitude, but even at this level it remains one of the largest documented cetacean kills in the world.

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 hy-draulically 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 vellowfin 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 proportion of each set type has varied considerably, but during the past decade, dolphin and school sets have been roughly equal in number (about 45% of the 15,000-20,000 sets each year), and the remainder (about 10%) have been log 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 that 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 thus tuna and dolphins are captured together in the net (Fig. 2). Dolphins are released from the net during the backdown procedure (Fig. 3). 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.

The bycatch in log and school sets is larger than in dolphin sets, but consists primarily of other fish, not dolphins. 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 unknown.

Purse seine being set on tuna and dolphins in the eastern tropical Pacific Ocean. The net is not yet closed, and four speedboats are driving in tight circles near the opening to keep the dolphins from escaping.

Figure 1 Purse seine being set on tuna and dolphins in the eastern tropical Pacific Ocean. The net is not yet closed, and four speedboats are driving in tight circles near the opening to keep the dolphins from escaping.

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 moratorium on regulation during which the tuna industry was expected to solve the problem through the 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. 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 modeled 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 United States began requiring that imported tuna be caught at dolphin mortality rates comparable to U.S. boats. 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 United States. These trade actions were important because the United States is the largest market of the canned tuna product of the fishery.

Spotted dolphins in a purse seine net waiting to be released.

Figure 2 Spotted dolphins in a purse seine net waiting to be released.

The dolphin kill declined between 1986 and 1993 due to these various political and economic pressures (Fig. 4). Starting in 1993, the ETP fishing countries voluntarily agreed to increased observer coverage, skipper review panels, and 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 International Dolphin Conservation Program Agreement, 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/year (Fig. 4).

Backdown procedure in progress. As the tuna vessel moves backward, the net is drawn into a long channel. The corkline at the far end is pulled underwater slightly, and the dolphins escape. Speedboats are positioned along the corkline to help keep the net open.

Figure 3 Backdown procedure in progress. As the tuna vessel moves backward, the net is drawn into a long channel. The corkline at the far end is pulled underwater slightly, and the dolphins escape. Speedboats are positioned along the corkline to help keep the net open.

Estimated animal number of dolphins killed (all dolphins and two dolphin stocks with the highest number killed) in the eastern tropical Pacific purse seine tuna fishery.

Figure 4 Estimated animal number of dolphins killed (all dolphins and two dolphin stocks with the highest number killed) in the eastern tropical Pacific purse seine tuna fishery.

The Declaration of Panama also called for the United States 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 on that set. Before changing the dolphin-safe label, however, the United States undertook studies to determine if the process of chasing and encircling dolphins was having a significant adverse impact on depleted dolphin populations. At the time of this writing these studies were underway.

IV. Status of Dolphin Populations

The status of ETP dolphin stocks (management units) is based on three time series of data: estimates of the number of dolphins killed, estimates of abundance from dedicated research vessel surveys, and estimates of an index of abundance from sightings on tuna vessels. 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) and the ETP endemic subspecies, the eastern spinner dolphin (S. longirostris orientalis). Both populations declined between 1960 and 1975 (Fig. 5), during the period of high mortality on U.S. boats (Fig. 4), but have remained approximately constant since then. As of 1998, northeastern spotted and eastern spinner dolphins were estimated to be at 15 and 34%, respectively, of population sizes when the fishery began. Other stocks have apparently been less affected, although little is known of the small populations of coastal forms of spotted and spinner dolphins.

Estimated population trajectories of northeastern offshore spotted dolphins and eastern spinner dolphins in the eastern tropical Pacific Ocean. The populations declined due to the high number of dolphins killed in the tuna fishery from 1960 to 1975.

Figure 5 Estimated population trajectories of northeastern offshore spotted dolphins and eastern spinner dolphins in the eastern tropical Pacific Ocean. The populations declined due to the high number of dolphins killed in the tuna fishery from 1960 to 1975.

Since 1993, reported dolphin mortality has been a very small fraction of population size so that recovery of the dolphin populations was expected. By 1999, however, there was no clear indication of a recovery for either northeastern offshore spotted or eastern spinner dolphins (Fig. 5). Several hypotheses could explain these apparent failures to recover: cryptic effects of repeated chase and encirclement on survival and/or reproduction (internal injuries, stress, hyperthermia), separation of nursing calves from their mothers during the fishing process, unobserved or observed but unreported mortality, effects due to breakup of dolphin schools (increased predation, social disruption), ecological effects due to removing tuna from the tuna-dolphin association, and ecosystem or environmental changes. Until there are clear recoveries of the affected dolphin stocks, the tuna-dolphin issue is likely to remain highly controversial.

Next post:

Previous post: