Chapter 7 tuna introduction

There is apparently only one stock of Pacific bluefin tuna. Adults are known to spawn between the Philippines and southern Japan and in the Sea of Japan during the spring and summer. Tagging studies indicate that some Pacific bluefin remain in the western Pacific for their entire lives. Others migrate to the eastern Pacific, mostly during the first and second years of life, and remain there for 1-2 years. They begin their return migration to the western Pacific at roughly age 3 and apparently require about two years to make the return trip (Bayliff, 2001a). Some Pacific bluefin may migrate more than once from the western to the eastern Pacific and back.

Pacific bluefin are caught using a wide variety of gear. In the eastern Pacific most of the fish are taken using purse seines within about 150 km of the coast at latitudes between 23^o and 33^oN, i.e., off the coast of California and Baja California. Most of the catch occurs between May and October in waters with temperatures between 17^o and 23^oC. In the western Pacific juvenile Pacific bluefin are caught by trolling south of the island of Shikoku, south of Shizuoka Prefecture, and south and west of the island of Kyushu. Fish one year of age and older are caught by purse seining in the open Pacific to the south and east of Japan. Roughly 15-20% of the western Pacific catch is taken on long lines, not infrequently as bycatch on gear targeting tropical tuna and billfish. Small numbers of Pacific bluefin are also taken by Japanese pole-and-line vessels in the high seas.

Certainly one of the disturbing aspects of the Pacific bluefin fishery is the fact that the fishery clearly involves the capture of sexually immature fish. In most years a majority of the catch in the eastern Pacific consists of two-year-old fish. In the western Pacific the Japanese catch Pacific bluefin less than one year old at about 15-30 centimeters in length by trolling south of Shikoku Island and Shizuoka Prefecture in waters where the sea surface temperature is between 24^o and 29^oC. Somewhat larger fish, 35-60 centimeters in length, but still less than one year old, are taken south and west of Kyushu. Pacific bluefin do not become sexually mature until roughly age 6. Why direct a fishery at sexually immature fish, particularly when the fish are no more than 15-20% of the size they will attain as adults? The trend in the commercial catch is hardly reassuring. Catches peaked in 1961 at over 31,000 tonnes and, despite some good years, have been declining ever since. The official catch in 2002 was about 9,100 tonnes. About 85% of the catch comes from the western Pacific, and the fishery is very much dominated by Japan. This does not appear to be a case of the tragedy of the commons. It does, however, appear to be a case of less-than-enlightened fishery management. Bayliff (2001a), for example, has concluded that yield per recruit would be maximized if Pacific bluefin were not recruited to the fishery until age 3-5. Eliminating the take of fish less than three years old would in the long run almost certainly increase the sustainable yield from this fishery.

Southern bluefin tuna are found throughout the southern hemisphere mainly at latitudes of 30^o-45^o south. There is only one known breeding ground, in the Indian Ocean southeast of Java, Indonesia. Breeding occurs from September to April. A mature female can produce several million eggs in a single spawning. Juveniles migrate down the west coast of Australia. They may spend the first few summers of their life in surface waters off the south coast of Australia, but during the winter they move into deeper, temperate oceanic waters. Beyond age five, they are seldom found in nearshore surface waters. Southern bluefin grow slowly, roughly 1.5 cm per month at age three. Their slow growth rate, the fact that they do not become sexually mature until age 8-12 (Table 7.2), and their high market value have all made them vulnerable to overexploitation.

The principal nations involved in the southern bluefin fishery are shown in Table 7.8. The fishery is dominated by Japan and Australia, who together account for 73% of the world catch. Like the Pacific bluefin, southern bluefin are taken by the fishery long before they become sexually mature. The surface fishery, which involves both pole-and-line with live bait and trolling, historically has taken juveniles age one to five, but the fishery now targets fish aged 3-4 with very few younger fish. The longline fishery takes fish from about age three through adults. In recent years 60-70% of the global catch of southern bluefin tuna has been longlined. In the case of Indonesia, the southern bluefin are a bycatch associated with longlining for tropical tunas in the spawning grounds of the southern bluefin. Since about 1990 a fishery has developed off the coast of South Australia for juvenile fish that are transferred to floating enclosures and fattened to increase their market value. Initially these farmed fish were caught with pole-and-line, but they are now taken with purse seines. The fish are transferred to floating pens, which are towed to Port Lincoln (South Australia). From there the tuna are transferred to moored farm pens. At the current time about 98% of the southern bluefins caught by Australia are transferred to these farms, where they are fattened for up to six months prior to export to Japan.

Table 7.8. Principal nations contributing to the catch of Southern bluefin tuna in 2002.
Country	Principal fishing area(s)	% of catch
Japan	Indian Ocean, Pacific southwest, Atlantic southeast	38.2
Australia	Indian Ocean eastern	34.8
Indonesia	Indian Ocean eastern	9.7
Taiwan	Indian Ocean	7.2
South Korea	Indian Ocean	4.3
New Zealand	Pacific southwest	3.0

The commercial catch of southern bluefin peaked during the 1960s, when most of the catch was taken by Japan. The combination of heavy fishing pressure and the capture of sexually immature fish caused the spawning stock to decline more-or-less continuously from the 1950s until roughly 1995. The recruitment of young fish declined substantially after 1980, and fisheries biologists agree that the stock has been the victim of recruitment overfishing since that time. Current estimates indicate that the spawning stock biomass is roughly 7-15% of the level in 1960 (when substantial reductions had already occurred) and 25-53% of the level in 1980 (Anonymous, 2004).

Recognizing that the southern bluefin stock was in trouble, Japan, Australia, and New Zealand set informal catch quotas from 1989 to 1993, with Australia allocated 5,265 tonnes, Japan 6,065 tonnes, and New Zealand 420 tonnes. The three countries signed an international convention in 1993 called the Commission for the Conservation of Southern Bluefin Tuna (CCSBT)⁵. After 1994, the catch quotas were allocated by the CCSBT.

In 1998 Japan began an “experimental fishing program” (EFP) without approval of the CCSBT and unilaterally increased its catch quota to 7,052 tonnes. Australia and New Zealand appealed to the International Tribunal for the Law of the Sea (ITLOS), but the ITLOS ruled that it did not have jurisdiction over the dispute. Subsequently Japan voluntarily abandoned its EFP and restricted its catch to 6,065 tonnes.

By the end of the 1990s there was much concern over the catch of southern bluefins by countries that were not members of the CCSBT, in particular Indonesia, South Korea, and Taiwan. This issue has been largely resolved in the last few years. South Korea and Taiwan joined the CCSBT in 2001 and 2002, respectively, and Indonesia has become a cooperating non-member. The formal CCSBT catch limits for 2003-2004 are shown in Table 7.9.

Table 7.9. CCSBT catch limits for 2003-2004.
Country	Catch limit (tonnes)
Japan	6,065
Australia	5,265
South Korea	1,140
Taiwan	1,140
New Zealand	420
Cooperating non-members	900

The good news is that the stock is being managed. The bad news is that under current management there is no evidence that the spawning stock is recovering. The initial goal of the CCSBT was to rebuild the spawning stock to the 1980 level by 2010, but that target has now been pushed back to 2020. However, Australian and New Zealand scientists give the CCSBT little chance of achieving that goal and feel that further reductions in the catch quotas will be needed to give the stock a chance to recover. The time required for southern bluefin to reach sexual maturity makes them particularly slow to recover from overfishing, a situation that contrasts sharply with tropical tunas such as skipjack and yellowfin, which reach sexual maturity in 2-3 years. Interestingly, Japanese projections have been much more optimistic, mainly because the Japanese have assumed that southern bluefin reach sexual maturity at age 8. This is almost certainly a case where a larger catch could be sustained if the nations involved in the fishery could agree to substantially reduce their catches for 10-15 years to allow the spawning stock time to recover. The demand and value of southern bluefin in Japan make it unlikely that such restraint will be exercised.

The role of the IATTC in regulating tuna catches has been focused almost exclusively on yellowfin tuna. Yellowfin have received the most attention, in part because yellowfin catches have exceeded those of any other species in most years and in part because the need for management has been evident since the 1960s. With the exception of yellowfin tuna management and some indirect controls on the catches of bigeye tuna⁶, the activities of the IATTC with respect to tuna have been in the area of research rather than management.

T
Figure 7.3. Geographical extent of the CYRA.

Figure 7.4. IATTC quotas and commercial catches of yellowfin tuna in the CYRA.

The record of CPUE and spawning stock biomass is summarized in Fig. 7.5. IATTC policy has been to keep the CPUE above 2.7 tonnes per day fishing and to base a given year’s quota largely on whether the preceding year’s quota resulted in an increase or decrease in the CPUE. Comparing Fig. 7.3 and 7.4, it is apparent that during the time from 1972 through 1983 when CPUE was clearly declining, the quotas remained more-or-less constant. CPUE rose dramatically from 1983 to 1985 and has been declining since then, while catch quotas continue to rise. Comparison of the CPUE data with the Beverton-Holt model calculations of Maunder and Harley (2004) indicate that there was indeed about a threefold rise in spawning stock biomass between 1983 and 1985. This regime shift in productivity (Maunder and Harley, 2004) appears to be the factor primarily responsible for the ability of the yellowfin tuna to withstand the recent level of fishing effort. Strong year-classes entered the adult stock from 1998 to 2000, and those good recruitment years pushed the spawning stock biomass to its highest level in a quarter century. The most recent recruitment years have not been so impressive, and the spawning stock biomass has begun to decrease. The lack of responsiveness of the IATTC quotas to changes in CPUE suggest that the ability of the yellowfin tuna stock to withstand the current level of fishing effort has been more a result of biological resiliency and good luck than wise management.

Figure 7.5. (A) Yellowfin CPUE for unregulated class-6 purse seiners in the eastern tropical Pacific and (B) spawning biomass estimated by Maunder and Harley (2004).