Chapter 7 tuna introduction

It is fair to say that tuna represent one of the peaks of fish evolution (Bardach and Ridings, 1985). They are voracious predators with a torpedo-like body shape and an ability to elevate their body temperature above that of the surrounding water. Although strictly speaking tuna are not warm-blooded, their ability to regulate their body temperature to some extent probably explains the fact that they can swim faster than most other fish and are able to recover rapidly after rigorous exercise. Their so-called thermoconserving ability arises from adaptations of their circulatory system and musculature. These adaptations are associated with high metabolic costs, and as a result the distribution of tuna is somewhat restricted with respect to temperature and oxygen concentrations. Skipjack, the smallest of the tuna species, have the largest surface-to-volume ratio and hence the most difficult job of thermoconserving. They are confined to tropical and subtropical waters. Skipjack are also unique among the tunas in that adults have no swim bladder. While this characteristic allows them to undertake very rapid vertical swimming bursts, it also forces them to expend more energy in routine swimming than is the case for the other more buoyant tuna species, which have swim bladders. As a result skipjack have a higher oxygen demand than the other tunas, and this fact restricts their vertical distribution.

The high metabolic rate of tunas is associated with an unusually high food requirement compared to cold-blooded fish. Daily food consumption, especially for juvenile tuna, can be as high as 25-30% of their body weight. For comparison, other juvenile fish typically consume about 15% of their body weight per day, and for adult fish daily food consumption is 3-8% of body weight (Bardach and Ridings, 1985). Despite their high metabolic requirements, tuna are able to go for up to two weeks without food, an ability that serves them well as the travel across the open ocean, where the distribution of food can be very patchy.

Overall the distribution of tuna is primarily regulated by temperature and oxygen concentrations. Because the thermocline as well as the oxygen minimum zone is shallower on the eastern side of ocean basins (Chapter 1), the depth range of tuna is generally shallower on the eastern side of ocean basins. Historically this fact has facilitated the use of purse seines to catch tuna in places such as the eastern tropical Pacific (Chapter 7). Not surprisingly, the distribution of tuna is also influenced by the availability of food, which in turn is controlled by factors such as upwelling and turbulent mixing that impact the delivery of nutrients to the euphotic zone.

Certainly one very interesting characteristic of tuna is their tendency to undertake very long migrations. Knowledge concerning these migrations has come from the recapture of tagged fish. Based on such studies, we have learned that albacore and skipjack will migrate across the entire breadth of the Pacific, and bluefin tuna have been shown to undertake north-south migrations that may extend over 60 degrees of latitude (Bardach and Ridings, 1985). How tuna navigate across the vast expanse of the open ocean is not entirely clear, but there is good evidence that they have a geomagnetic sense. Skipjack, for example, have a high concentration of magnetite crystals in their ethmoid bone¹ complex, with no occurrence of magnetite in any other organ or structure, and live skipjack can be trained to respond to a change in the magnetic field surrounding them (Walker, 1982). Although tuna may also navigate by other means, e.g., simply following currents, it is therefore possible that they share the ability of animals such as homing pigeons and salmon in being able to orient their migrations relative to variations in the Earth’s magnetic field.

Some relevant information on size and age of tuna are summarized in Table 7.2. Bluefin are the largest of the tuna species, with adult weights of several hundred kilograms and lifespans of several decades. The other species attain weights of typically 10-100 kilograms and have lifespans of roughly 10 years.

The commercial catch of skipjack, yellowfin, bigeye, and albacore tuna are summarized in Fig. 7.1. All of these catches have increased more-or-less steadily since 1950. Of the four species, only the skipjack is currently considered to be underfished. The others stocks are felt to be fully exploited.

Figure 7.1. Commercial catches of skipjack, yellowfin, bigeye, and albacore tuna.

Like other tuna, skipjack are oviparous², and a typical adult female may produce 80,000 to 2 million eggs per year. In tropical waters skipjack spawn throughout the year. At higher latitudes spawning is limited to the warmer months of the year. Spawning occurs over a much narrower range of latitudes than the area otherwise occupied by the adult fish. In the western Pacific the spawning area is substantially larger than in the eastern Pacific, but this is not the case in the Atlantic and Indian oceans. The migrations of skipjack suggest that they tend to move toward the west with the equatorial current systems to their preferred spawning grounds. They then continue to drift with the flow of the subtropical gyres toward higher latitudes and then to the east, eventually reaching the productive waters of the eastern boundary current upwelling systems.

Yellowfin tuna tend to associate with dolphins more than any other tuna species. As noted in Chapter 7, the reason for this association is poorly understood. One possibility is that the association helps both species find prey. Both tuna and dolphins have excellent vision. Tuna also have a keen sense of smell, whereas dolphins have no olefactory sense at all. On the other hand, dolphins are able to track prey using ultrasonic echolocation. Whether a school of prey is smelled by the tuna or “heard” by the dolphins, each species could take advantage of the sensory strengths of the other to home in on a meal.

About 67% of the commercial catch comes from the Pacific and 22% from the Indian Ocean. The central Pacific accounts for 57% of the catch. Most of the yellowfin catch is accounted for by purse seining, although yellowfin are also taken by longlining and to a much lesser extent by trolling and hand lining. The primary market form is canned tuna. Like the skipjack, yellowfin tuna is classified as “light meat” tuna. There are also markets for fresh fillets (sashimi), fresh whole fish, frozen loins, and smoked fish.

Stock assessments indicate that there is little or no prospect for sustainable expansion of the yellowfin tuna catch. The principal nations involved in the fishery at the present time are listed in Table 7.4. Unlike the skipjack fishery, the western and eastern central Pacific make roughly equal contributions to the overall catch, 32% and 25%, respectively. Mexico and Venezuela are the principal yellowfin fishing nations in the east central Pacific. Indonesia, the Philippines, Japan, and Taiwan account for most of the catch in the west central Pacific. Indonesia, Japan, Taiwan, Spain, and France share most of the catch in the Indian Ocean. The principal markets for yellowfin tuna are Japan, western Europe, and the United States.

Not a great deal is known about the ecology of bigeye tuna, and scientists do not agree as to whether the bigeye resource is being fully exploited. The bycatch of small bigeye associated with the skipjack and yellowfin purse seining fisheries is a concern, since it could adversely affect reproduction.

Due to its white appearance, albacore tuna meat is sometimes referred to as “chicken of the sea”. It is the only canned tuna marketed as “white tuna”. Due to its unusual bone structure and the soft consistency of its meat, albacore cannot be filleted. Fresh or frozen albacore is available only as loins/steaks. There is no purse seine fishery for albacore. Virtually all albacore are caught either with pole-and-line, surface trolling, or long lines. The former methods target younger albacore. Long lines take the older, deep-swimming fish. Albacore are marketed primarily as canned tuna. The major markets are the United States, Canada, Japan, and Spain.

Atlantic bluefin are excellent swimmers, and tagging studies have revealed that they make rapid trans-Atlantic migrations and dive to depths of greater than 1,000 meters when foraging (Block, et al., 2001). Their diet includes mackerel, herring whiting, and squid. For many years the International Commission for the Conservation of Atlantic Tunas (ICCAT) has taken the position that the western and eastern Atlantic bluefin stocks were separate, but the most recent tagging studies have indicated that mixing between the two stocks occurs at a more rapid rate than presumed by ICCAT (Block, et al., 2001). This fact has important implications for management of the resource.

For many years fishing for Atlantic bluefin was done strictly for recreational purposes, as the commercial value of giant Atlantic bluefin³ prior to 1970 was only about $0.10 per kilogram (Buck, 1995). A commercial purse seine fishery that targeted small Atlantic bluefin for the canned market began in the western Atlantic in the late 1950s. This fishery expanded rapidly for several years, but the focus of the fishery soon changed in response to increasing demand for giant bluefin to supply the Japanese sushi and sashimi markets. Fresh bluefin meat is the fattiest of tuna species and is considered a supreme delicacy by sushi and sashimi connoisseurs. With the development of air freight in the early 1970s, giant Atlantic bluefin were being caught and immediately shipped to Japan, where they were sold for more than $2 per kilogram.⁴

As the fishery for Atlantic bluefin evolved, it became apparent that some form of management was needed. In 1966 four nations, Brazil, Japan, South Korea, and the United States met in Rio de Janeiro and signed the International Convention for the Conservation of Atlantic Tunas. The principal objective of the Convention was, “To co-operate in maintaining the population of tunas and tuna-like species found in the Atlantic Ocean and the adjacent seas at levels that will permit the maximum sustainable catch for food and other purposes.” At the present time a total of 38 nations have ratified the Convention, including most major fishing nations on the North Atlantic rim. In 1969 parties to the Convention established the International Commission for the Conservation of Atlantic Tunas (ICCAT) to develop conservation and management recommendations. Concern over the status of the bluefin tuna stocks, particularly in the western Atlantic, led to a 1974 ICCAT recommendation for an Atlantic-wide minimum size limit of 6.4 kg and an agreement among member nations to cap entry into the fishery and limit fishing mortality. The following year the U.S. Congress enacted the Atlantic Tunas Convention Act (ATCA), which implemented ICCAT for the United States. During the next few years the stock of bluefin tuna continued to decline in the western Atlantic, and in 1981 ICCAT set a quota of 1,160 tonnes for the western Atlantic. Perhaps equally important, ICCAT accepted the concept that the Atlantic bluefin resource could be managed as two separate stocks, one in the eastern Atlantic and the other in the western Atlantic. This was a controversial decision, because tagging studies had shown that bluefin sometimes migrated across the width of the Atlantic. At issue was the frequency of these migrations and their impact on the fishery resources.

Since 1982 the annual catches of bluefin tuna from the western Atlantic have fluctuated around an average of about 2,200 tonnes. During the same time catches of bluefin in the eastern Atlantic and Mediterranean increased to roughly 40,000 tonnes, with most of the catch coming from the Mediterranean Sea (Fig. 8.2). By the early 1990s the western Atlantic bluefin stock had declined by about a factor of 10 from its level in 1975, and by 1993 the eastern Atlantic population was estimated to be only 13% of its size in 1970. This discovery seriously questioned ICCAT’s ability to manage the fishery. U.S. fishermen complained bitterly that there was effectively only one stock and that the heavy fishing in the eastern Atlantic was adversely impacting the adult stock in the western Atlantic. The nations involved in the eastern Atlantic fishery were accused of violating the recommendations they had agreed to in 1974.

In 1991 Sweden announced that it intended to propose listing of the western Atlantic bluefin population in Appendix I of the Convention on International Trade in Endangered Species (CITES). Such a listing would have prohibited international trade in western Atlantic bluefin. At the same time, the U.S. National Marine Fisheries Service and the U.S. Fish and Wildlife Service expressed an interest in listing western Atlantic bluefin in CITES Appendix II, a less restrictive status that would have increased monitoring but not prohibited international trade. At the 1992 CITES meeting in Kyoto, intense lobbying by Japan with support from the United States, Canada, and Morocco caused Sweden to withdraw its proposal for a CITES vote. In 1994 Kenya proposed listing both north and south Atlantic bluefin on CITES Appendix II, but Japanese pressure again resulted in withdrawal of the proposal.

Figure 7.2. Commercial catches of Atlantic, Pacific, and Southern bluefin tuna, and the total catch of all bluefin tuna since 1950.

The good news is that restrictions on fishing have apparently stopped the decline of the western Atlantic bluefin stock in recent years. The bad news is that the stock is either not recovering or is recovering very slowly. At its 1998 meeting, ICCAT adopted a rebuilding program for the western stock with a goal of reaching the maximum sustainable yield in 20 years. The total allowable catch (TAC) in the western Atlantic is currently 2,700 tonnes, with about 55% of that quota being allocated to the United States. In the Mediterranean, about 30% of the catches in 1993 were below the minimum size limit, an indication that the nations involved in the fishery were not adhering to ICCAT recommendations. The catch of Atlantic bluefin peaked in 1996 at a little over 52,000 tonnes, with more than 95% of the total catch coming from the eastern Atlantic/Mediterranean. At its 1998 meeting, ICCAT for the first time adopted firm quotas for the catch of bluefin tuna in the eastern Atlantic and Mediterranean Sea. The quotas for 1999-2001 were about 30,000 tonnes. At its 2002 meeting ICCAT was successful in adopting quotas of 32,000 tonnes y^-1 for the years 2003 through 2006. However, scientific evidence indicates that the catch in the eastern Atlantic/Mediterranean must be reduced to no more than 25,000 tonnes before the stock can begin rebuilding. The reported catch in the eastern Atlantic/Mediterranean in 2002 was a little less than 33,000 tonnes.