Thunnus orientalis

Pacific Bluefin Tuna

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Pacific Bluefin Tuna

 

Description

The bluefin tunas are among the largest and fastest open ocean fishes and are important economically and culturally in many parts of the world. There are three species of bluefin tuna- the prized and endangered Atlantic bluefin (Thunnus thynnus), the widespread but similarly overfished Pacific bluefin (Thunnus orientalis), and the smaller but also tasty Southern bluefin tuna (Thunnus mccoyi). Bluefin tunas are spectacular swimming machines with torpedo-shaped, streamlined bodies built for speed and high-powered muscle and tendon systems that have evolved for high endurance. Bluefin tunas are warm-blooded, a rare trait among fish, and are thus able to adjust their body temperature, keeping their body temperatures higher than the surrounding water, which is why they are so well adapted to cooler ocean waters. Bluefin tunas are considered exceptionally good to eat, particularly by those who enjoy various forms of raw fish such as sushi and sashimi, and all species of bluefin tuna are pursued constantly by the fishing industry and by sport fishermen. As a result, overfishing throughout their range has driven their numbers to critically low levels. Some populations of bluefin tuna are thought be extinct and others are critically endangered.

Geography

North Pacific: Gulf of Alaska to southern California and Baja California and from Sakhalin Island in the southern Sea of Okhotsk south to northern Philippines. There are four substantiated records of this subspecies in the southern hemisphere: off Western Australia, southeast Pacific (37°11'S, 114°41'W) and Gulf of Papua (Ref. 10997). The species occurs mainly in the northern Pacific but ventures into New Zealand waters for at least three months during spring and early summer (Ref. 83312).

Ecosystem

This is an epipelagic and usually oceanic species, but seasonally comes close to the shore. It tolerates wide temperature ranges and forms schools by size, sometimes with other Scombrids. It migrates between June and September in a northward direction along the coast of Baja California, Mexico and California. A model of migration is presented by Bayliff (1994). It is found to 550 m depth. It is a voracious predator that feeds on a wide variety of small schooling fishes or squid, and also eats crabs and less sessile organisms (Collette and Nauen 1983).

Longevity may be as long as 15 years (Hsu 2000) or 26 years (Shimose 2009). Spawning occurs between Japan and the Philippines in April, May, and June, off southern Honshu in July, and in the Sea of Japan in August. The sex ratio is about 1:1. Size at first maturity is 150 cm FL and 60 kg at an age of approximately five years. Batch fecundity increases with length, from about five million eggs at 190 cm FL to about 25 million eggs at 240 cm FL (Collette 2010, Schaefer 2001, Sawada et al. 2005, Chen et al. 2006).
 

Conservation

This a is highly commercial and valuable species. The stock is considered fully exploited, with catches being relatively stable in the past five to 10 years. Estimates of spawning stock biomass (SSB) have shown an increasing trend over the past 21–27 years (three generation lengths). It is listed as Least Concern.

 

Threats

Major Threats
In the Northern Pacific, this species is fished with set net, trolling, and purse seines. Most of the catch in the Eastern Pacific is taken by purse seines. A considerable portion of the purse seine catch is transported to holding pens for fattening and later sale as sashimi grade fish (IATTC 2008).

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