Allison's Tuna

Gillrakers 26 to 34 on first arch. A large species, deepest near middle of first dorsal fin base. Some large specimens have very long second dorsal and anal fins, which can become well over 20% of fork length; pectoral fins moderately long, usually reaching beyond second dorsal fin origin but not beyond end of its base, usually 22 to 31% of fork length. Swimbladder present. No striations on ventral surface of liver. Vertebrae 18 precaudal plus 21 caudal. Colour: back metallic dark blue changing through yellow to silver on belly; belly frequently crossed by about 20 broken, nearly vertical lines; dorsal and anal fins, and dorsal and anal finlets, bright yellow, the finlets with a narrow black border.

Worldwide in tropical and subtropical seas, but absent from the Mediterranean Sea.

Maximum fork length is over 200 cm. The all-tackle angling record was a 176.4 kg fish of 208 cm fork length taken off the west coast of Mexico in 1977. Common to 150 cm fork length. Off the Philippines and Central America, the smallest mature fish were found within the size group from 50 to 60 cm fork length at an age of roughly 12 to 15 months (Davidoff, 1963), but between 70 and 100 cm fork length the percentage of mature individuals is much higher. All fish over 120 cm attain sexual maturity.

Epipelagic, oceanic, above and below the thermocline. The thermal boundaries of occurrence are roughly 18° and 31°C. Vertical distribution appears to be influenced by the thermal structure of the water column, as is shown by the close correlation between the vulnerability of the fish to purse seine capture, the depth of the mixed layer, and the strength of the temperature gradient within the thermocline. Yellowfin tuna are essentially confined to the upper 100 m of the water column in areas with marked oxyclines, since oxygen concentrations less than 2 ml/l encountered below the thermocline and strong thermocline gradients tend to exclude their presence in waters below the discontinuity layer. Larval distribution in equatorial waters is transoceanic the year round, but there are seasonal changes in larval density in subtropical waters. It is believed that the larvae occur exclusively in the warm water sphere, that is, above the thermocline. Schooling occurs more commonly in near-surface waters, primarily by size, either in monospecific or multispecies groups. In some areas, i.e. eastern Pacific, larger fish (greater than 85 cm fork length) frequently school with porpoises. Association with floating debris and other objects is also observed. Although the distribution of yellowfin tuna in the Pacific is nearly continuous, lack of evidence for long-ranging east-west or north-south migrations of adults suggests that there may not be much exchange between the yellowfin tuna from the eastern and the central Pacific, nor between those from the western and the central Pacific. This hints at the existence of subpopulations. Spawning occurs throughout the year in the core areas of distribution, but peaks are always observed in the northern and southern summer months respectively. Joseph (1968) gives a relationship between size and fecundity of yellowfin tuna in the eastern Pacific.

There are important yellowfin tuna fisheries throughout tropical and subtropical seas. The most important catches (well over 100 000 t) are recorded from Fishing Areas 71 (321,458 t in 1995), 51 (250,353 t) and 77 (198,696 t). Landings have been steadily increasing since 1970 to 1990 when exceeded 1,000,000 t. In the recent years the catches seem to be stabilized around this quantity. Near-surface schooling yellowfin tuna are captured primarily with purse seines and by pole-and-line fishing, while trolling and gillnetting are of much lesser importance. The 1979 eastern Pacific surface fleet numbered 259 purse seiners, 45 bait boats, and 17 other vessels flying 16 flags. The carrying capacity of this fleet amounted to 169 149 t. Purse seining is increasing in the western Pacific, initially taking mainly skipjack and bluefin tuna. In 1982, the yellowfin tuna catch by US purse seiners in this area probably exceeded that of skipjack tuna, and the total purse seine catch of yellowfin by all vessels may have been higher than that of bluefin tuna. Pole-and-line fishing is still one of the major surface fishing techniques for yellowfin tuna in the Pacific, even though this method is declining in overall importance throughout the world. The most important fishing method for deep swimming yellowfin tuna is longlining, primarily by vessels from Japan, the Republic of Korea and Taiwan (Province of China). Although these fisheries operate virtually throughout the geographical range of the species, the largest catches are made in the equatorial waters of the Pacific.The total catch reported for this species to FAO for 1999 was 1 258 386 t. The countries with the largest catches were Indonesia (176 320 t) and Mexico (121 884 t).