TUNA

Thunnus thynnus

Big, fast, and valuable, a large northern bluefin tuna measures three metres long, weighs more than 600kg, and can accelerate to speeds of up to 80 kph. The bluefin's red oily flesh is much prized in Japan for gourmet sushi and sashimi, commanding enormous prices in the Tokyo fish markets. A large bluefin which can fetch over US$30,000 on the dockside, will sell at a Tokyo auction for US$60,000, and cost the customer US$200 a plate at a sushi bar. Eighty-five percent of Japan's bluefin tuna imports come from the Pacific, Indian, and eastern Atlantic Oceans, but Atlantic bluefins reach a greater size than those in the Pacific and are the most sought-after.

Of the 13 species of tunas, those belonging to the genus Thunnus are subjected to the most intensive commercial fishing. The bluefin tunas, T. thynnus (northern bluefin), and T. maccoyi (southern bluefin) have suffered dramatic declines in the past few decades.

Northern bluefin are pelagic but often frequent shallow coastal waters. Highly mobile, they make seasonal migrations which appear to be tied to spawning behaviour, water temperature, and prey concentrations. Tagged northern bluefin have travelled 7,770km across the Atlantic in 119 days - an average distance of 65km per day. Bluefin are social animals, forming large `peer' schools in which individuals of similar size are found together.

Current data suggests that although there are two populations of northern bluefin in the Atlantic, there is only a single population in the Pacific, spawning in the western part of the ocean around Japan, and between Japan and the Philippines. At different stages of life, parts of this population migrate to the eastern Pacific but the extent and significance of these movements is not known. In the eastern Pacific, bluefin migrate northward from Baja California to the Gulf of Alaska. In the western Pacific , around Japan, they migrate north in summer and south in winter, and are found as far south as the Philippines.

In the western Atlantic, giant northern bluefin arrive in the warm waters of the Gulf of Mexico to spawn in January. Here they remain until June, migrating north for another feeding season off New England and maritime Canada after spawning is completed. Only one other spawning ground - in the Mediterranean - is known for Atlantic bluefin.