The Pacific bluefin tuna (Thunnus orientalis) is a predatory species of tuna found widely in the northern Pacific Ocean, but it is migratory and also recorded as a visitor to the south Pacific.^[1] In the past it was often included in T. thynnus, the 'combined' species then known as the northern bluefin tuna (when treated as separate, T. thynnus is called the Atlantic bluefin tuna).^[2] It may reach as much as 3 m (9.8 ft) in length and 450 kg (990 lb) in weight.^[3]

Like the closely related Atlantic bluefin and southern bluefin, the Pacific bluefin is a commercially valuable species and several thousand tonnes are caught each year, but unlike its relatives it does not appear to be threatened overall, despite being overfished.^[1] Monterey Bay Aquarium's Seafood Watch program have placed all bluefin tunas on the "Avoid" list,^[4] and they are also placed on Greenpeace's "Red List".^[5]

Physiology

Thermoregulation

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Most fish are cold-blooded (ectothermic). ^[6] However, tuna and mackerel sharks are warm-blooded: they can regulate their body temperature. Warm-blooded fish possess organs near their muscles called retia mirabilia that consist of a series of minute parallel veins and arteries that supply and drain the muscles. As the warmer blood in the veins returns to the gills for fresh oxygen it comes into close contact with cold, newly oxygenated blood in the arteries. The system acts as a counter-current heat exchanger and the heat from the blood in the veins is given up to the colder arterial blood rather than being lost at the gills. The net effect is an increase in temperature. Fish from warmer water elevate their temperature a few degrees whereas those from cold water may raise it as much as 20 °C (36 °F) warmer than the surrounding sea.

The tuna's ability to maintain body temperature has several definite advantages over other sea life. It need not limit its range according to water temperature, nor is it dominated by climatic changes. The additional heat supplied to the muscles is also advantageous because of the resulting extra power and speed. Bluefin tuna have been clocked in excess of 30 miles per hour (48 km/h) during 10 to 20 second sprints, enabling it to hunt squid, herring, mackerel, etc., that slower predators cannot capture.

Distribution

The Pacific bluefin tuna is primarily found in the North Pacific, ranging from the East Asian coast to the western coast of North America.^[1][3] It is mainly a pelagic species found in temperate oceans, but it also ranges into the tropics and more coastal regions.^[1] It typically occurs from the surface to 200 m (660 ft),^[3] but has been recorded as deep as 550 m (1,800 ft).^[1]

It spawns in the northwestern Philippine Sea (e.g., off Honshu, Okinawa and Taiwan) and in the Sea of Japan.^[1][7] A proportion of these migrate to the East Pacific and return to the spawning grounds after a few years.^[1] It has been recorded more locally as a visitor to the Southern Hemisphere, including off Australia, New Zealand, Gulf of Papua and French Polynesia.^[1][3]

Life history

Pacific bluefin tunas reach maturity at about 5 years of age, the generation length is estimated at 7–9 years and based on two separate sources the longevity is 15 year or 26 years.^[1] At maturity it is about 1.5 m (4 ft 11 in) long and weighs about 60 kg (130 lb).^[1] Individuals that are 2 m (6 ft 7 in) long are regularly seen, and the maximum reported is 3 m (9.8 ft) in length and 450 kg (990 lb) in weight.^[3] According to the International Game Fish Association, the all-tackle game fish record was a 325 kg (720 lb) individual caught off New Zealand in 1997.^[1]

Spawning occurs from April to August, but the exact timing depends on the region: Early in the northwest Philippine Sea (the southern part of its breeding range) and late in the Sea of Japan (the northern part of its breeding range).^[1] Large females can carry more eggs than small ones, and between 5 million and 25 million eggs have been reported.^[1]

Pacific bluefins eat various small schooling squids and fishes, but has also been recorded taking sessile animals,^[3] pelagic red crabs and krill.^[7]

Conservation status

Unlike the other bluefins (Atlantic and southern),^[8][9] the Pacific bluefin tuna is not listed as threatened by the IUCN.^[1] Overfishing is occurring in the Pacific bluefin, but overall the stock is not in an overfished condition.^[10] In 2000–2004, between 16,000 tonnes and 29,000 tonnes were caught per year.^[1] Its wide range and migratory behavior leads to some problems, since fisheries in the species are managed by several different Regional Fisheries Management Organisations that sometimes give conflicting advice. The IUCN have recommended that the responsibility is moved to a single organisation.^[1] In 2010, it was estimated that the complete spawning biomass was 40–60% of the historically observed spawning biomass.^[1]

Monterey Bay Aquarium's Seafood Watch program have placed all bluefin tunas on the "Avoid" list,^[4] and they are also placed on Greenpeace's "Red List".^[5] Pacific bluefin tuna caught both by purse seine and trolling are yellow listed (green=best, yellow=intermediate, red=worst) by the seafood guide of the Blue Ocean Institute, indicating that some problems exist with this species' status.^[11][12]

Pacific bluefin flesh may contain levels of mercury or PCBs that are harmful to humans that consume it.^[11] A similar problem exists in other tuna species.

About 80% of the Pacific and Atlantic bluefin tunas are consumed in Japan, and tunas that are particular suited for sashimi and sushi can reach very high prices: In January 2012, a Pacific bluefin weighing 269 kg (590 lb) was sold for 56.49 million yen (about US$736,000) at the Tsukiji fish market in Tokyo, equalling 210,000 yen per kilogram, or US$1,238 per pound.^[13] Both the overall price and the price per weight was a record.^[13]

Farming

Japan is both the biggest consumer and the leader in over tuna farming research.^[14][15] Kinki University of Japan first successfully farmed already-hatched bluefin tuna in 1979. In 2002, they succeeded in breeding them, and in 2007, the process was repeated for a third generation.^[16][17][18] This farm-raised tuna is now known as Kindai tuna. Kindai is a contraction of Kinki University (Kinki daigaku).^[19]

References