PRINCIPAL MARKET TUNAS
The following classification of the status of stocks is used throughout this document.

• U Non-fully exploited.
• F Fully exploited.
• O Overexploited.

For Further clarifications on the criteria for the classification of fish stock status, see the Source Report (Appendix - Assessment methodology, Table 1)
A summary on the status of various stocks of tuna and tuna-like species is given in Table D19. It was obtained by interpreting results of stock assessments according to the classification procedure adopted by FAO in this review. Those assessments available at the time of preparation of this review (end of March 2011) were taken mostly from Web pages of:

• Commission for the Conservation of Southern Bluefin Tuna (CCSBT );
• Inter-American Tropical Tuna Commission (IATTC ) for the eastern Pacific;
• International Commission for the Conservation of Atlantic Tunas (ICCAT );
• Indian Ocean Tuna Commission (IOTC );
• Western Central Pacific Fishery Commission (WCPFC ).

The knowledge and data on the principal market tunas are generally much better than those for other species of tuna and tuna-like species. They have been studied for many years and more research effort is devoted to them. However, even for these species, significant uncertainties exist in the basic biological knowledge and data. For example, relatively recent research indicates that the life span of southern bluefin tuna, one of the best studied tuna, may be considerably longer than previously believed. Moreover, for this species, as compared with trade statistics, the catches were substantially under reported for a number of years. For Atlantic bluefin tuna, another well-studied species, officially reported catches might be significantly smaller in the past than those actually taken. This conclusion is based on information from a trade-based statistical programme introduced by ICCAT (Miyake, 1998) as well as from capacity estimates (ICCAT, 2009). When considering the information on the stock status, uncertainties in stock assessment need to be taken into account.

Most tuna stocks are fully exploited, some are overexploited. Generally, some temperate tuna species (i.e. Atlantic and southern bluefins [most desired for sashimi]) are much more overexploited (depleted) than any of the tropical tuna species. For the Pacific bluefin (also used for sushimi), the yield-per-recruit could be increased if the number of small bluefin caught by trolling and purse seining can be reduced.

The stocks of albacore (temperate species) used mostly for canning are not fully exploited in the South Pacific but they are fully exploited in the Indian Ocean and the South Atlantic and overexploited in the North Atlantic and the North Pacific. The status of albacore in the Mediterranean Sea is unknown.

Generally, most tropical principal market tunas have reacted well to exploitation owing to their very high reproductive potential, wide geographical distribution, opportunistic behaviour and other population dynamics characteristics that make them highly productive. With proper management, they are capable of sustaining high yields.

There may still be potential for increasing catches of skipjack in the western and central Pacific with lower potential in the other oceans. However, skipjack are caught together with tuna species that are fully exploited or overexploited. Therefore, until more selective fishing methods are developed, it is not desirable to increase the catches of skipjack.

Most other stocks of tropical tunas have become fully exploited and a few are overexploited. Generally, a possibility of further deterioration in the status of tropical tunas should not be underestimated. Concerns are increasing over the exploitation of bigeye in all oceans. This is another species that is highly desired for sashimi and has a shorter life span than bluefin. In addition to possibly causing overfishing in the future, the increasing purse seine catches of small bigeye may negatively affect the yield per recruit.

The status of many tuna and tuna-like species other than the principal tunas is highly uncertain or simply unknown. Therefore, the intensification of their exploitation raises concerns. Significant uncertainties in the status of many billfishes represent a serious conservation problem. Some stocks are overexploited in the Atlantic and the Pacific, while their status is mainly unknown in the Indian Ocean. Because of commercial exploitation, there is more known about swordfish than other billfishes. In the Mediterranean Sea, the swordfish stock seems to be overexploited, but the overall situation in the remainder of the Atlantic and Pacific is more optimistic. However, in the Indian Ocean, there are concerns about the intensification of swordfish fishing owing to the risk of potential local overexploitation.

Table D19: State of exploitation and annual nominal catches of tuna and tuna-like species in all Oceans, 1950–2009.

Information on the habitat and biology of tunas and tuna like species is further described in the factsheet on the Biological characteristics of tunas and tuna-like species and in Goujon and Majkowski (2010). Global aspects of tuna resources, fishing, fisheries management, processing and trade can be found in Allen (2010), Joseph (1998, 2000, 2003), Miyake, Miyabe and Nakano (2004) and Miyake et. al. (2010). Information references at regional scale are given in the respective sections on resource status.

The 1982 UNCLOS classifies the principal market tunas, billfishes, blackfin tuna, bullet and frigate tuna, little tunny and kawakawa as highly migratory. This is despite little tunny and kawakawa being mostly confined to the continental shelf and upper slope. Black skipjack is not classified as highly migratory, but it is probably more oceanic than little tunny and kawakawa.

The suborder Scombroidei is usually referred to as tuna and tuna-like species (Klawe, 1977; Collette and Nauen, 1983; Nakamura, 1985). It is composed of tunas (sometimes referred to as true tunas), billfishes and other tuna-like species. They include some of the largest and fastest fishes in the sea. The tunas are classified into 5 genera (Thunnus, Euthynnus, Katsuwonus, Auxis and Allothunnus) with 15 species all together.

The most economically important tuna species on the global scale are referred to as principal market tunas. From the genus Thunnus, they include albacore (T. alalunga), Atlantic bluefin tuna (T. thynnus), bigeye tuna (T. obesus), Pacific bluefin tuna (T. orientalis), southern bluefin tuna (T. maccoyii) and yellowfin tuna (T. albacares). Skipjack tuna (Katsuwonus pelamis) is the seventh principal market tuna species. They are subject to intensive international trade for canning and sashimi (raw fish regarded as a delicacy in Japan and, increasingly, in many other countries).

The efficient physiology of principal market tunas allows them to retain or dissipate heat as required for peak biological performance and efficiency. They are all oceanic (Figure C1.1), capable of long migrations or movements, but not necessarily all species re-distribute or mix well within the areas of their stocks’ distribution. Most species constitute one or two stocks in each ocean, although the albacore in the Atlantic consists of three stocks (including that in the Mediterranean Sea). The exceptions are Atlantic and Pacific bluefins, which occur only in their eponymous oceans. Southern bluefin constitute a single stock extending in the Atlantic, Indian and Pacific Oceans.

The tunas other than the principal market species are more neritic (living in water masses over the continental shelf). They include longtail tuna, blackfin tuna (T. atlanticus), black skipjack (E. lineatus), kawakawa (E. affinis), little tunny (E. alleteratus), bullet tuna (A. rochei) and frigate tuna (A. thazard).

The billfishes (Istiophoridae) are composed of marlins (Makaira spp.), sailfish (Istiophorus spp.), spearfish (Tetrapturus spp.) and swordfish (Xiphias gladius, only species in the genus). With the exception of two species (Mediterranean and roundscale spearfish), all billfishes have very wide geographical distributions, but not all species occur in all oceans. Billfishes are mostly caught by longlines as bycatch. The exceptions are swordfish, which are targeted in certain regions with longlines and harpoons. Billfishes are also taken in sport fisheries, where they are greatly valued. They are all considered excellent eating.

Other important tuna-like species include slender tuna (Allothunnus fallai), butterfly kingfish (Gasterochisma melampus), wahoo (Acanthocybium solandri), bonitos (Cybiosarda, Orcynopsis and Sarda), Spanish and king mackerels, seerfish and sierra (Scomberomorus spp.). They and other tuna-like species are all the object of fishing. They have a significant fishery potential, especially for developing countries where mostly artisanal and recreational fisheries are now catching them. Slender tuna and butterfly kingfish (with a circumpolar distribution in the Southern Ocean) are now caught mostly as bycatch of the Japanese longline fishery targeting southern bluefin tuna.

Figure C1.1 Distribution of principal market tunas and fishing areas

Information on fishery are available at the following link: World oceans Global Tuna fisheries

The bibliographic references are available through the hyperlink displayed in "Source of Information".

ACKNOWLEDGEMENTS
The authors are grateful to Dr Peter Miyake and his colleagues for their help with the provision of information for the preparation of the section ob Fisheries and Table C1.1. They also appreciate the assistance of the Secretariats and tuna scientists of FFA, t-RFMOs and SPC with obtaining information for this review and their cooperation and collaboration with FAO. Drs Robin Allen and Victor Restrepo have kindly provided helpful suggestions for improving an earlier version of this review.