Pacific bluefin tuna

Pacific bluefin tuna
At Kasai Rinkai Park, Japan
Conservation status
Least Concern  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Perciformes
Family:	Scombridae
Tribe:	Thunnini
Genus:	Thunnus
Subgenus:	Thunnus
Species:	T. orientalis
Binomial name
Thunnus orientalis (Temminck and Schlegel, 1844)
Synonyms[2]
Thunnus saliens Jordan and Evermann, 1926 Thynnus orientalis Temminck and Schlegel, 1844 Thunnus thynnus orientalis (Temminck and Schlegel, 1844)

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).^[3] It may reach as much as 3 m (9.8 ft) in length and 450 kg (990 lb) in weight.^[4]

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,^[5] and they are also placed on Greenpeace's "Red List".^[6]

Physiology

Thermoregulation

Most fish are cold-blooded (ectothermic). ^[7] 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 less heat loss through the gills. 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][4] 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),^[4] 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][8] 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][4]

Life history

At Osaka Aquarium Kaiyukan, Japan

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.^[4] Elsewhere, a mass of up to 550 kg (1,210 lb) has been reported for the species.^[9] According to the International Game Fish Association, the all-tackle game fish record was a 325 kg (717 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 have also been recorded taking sessile animals,^[4] pelagic red crabs and krill.^[8]

Conservation status

Unlike the other bluefins (Atlantic and southern),^[10][11] 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.^[12] 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] In 2013, scientists estimated that Pacific bluefin tuna are suffering at least a 96% decline due to overfishing. ^[13]

Monterey Bay Aquarium's Seafood Watch program have placed all bluefin tunas on the "Avoid" list,^[5] and they are also placed on Greenpeace's "Red List".^[6] Pacific bluefin tuna caught by longline, purse seine and trolling are all 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.^[14][15][16]

Pacific bluefin flesh may contain levels of mercury or PCBs that are harmful to humans that consume it.^[14] 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: On December 30, 2012, a 489-pound (222 kg) Pacific bluefin tuna caught off northeastern Japan was sold at the Tsukiji fish market in Tokyo for a record 155.4 million yen (1.76 million US$). The price works out to $3,603 per pound.^[17]

Farming

Japan is both the biggest consumer and the leader in tuna farming research.^[18][19] 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.^[20][21][22] This farm-raised tuna is now known as Kindai tuna. Kindai is a contraction of Kinki University (Kinki daigaku).^[23]

References

External links

Wikispecies has information related to: Thunnus orientalis

Wikimedia Commons has media related to Thunnus orientalis.

• Encyclopedia of Life
• TOPP, Tagging of Pacific Predators, a research group that tags and tracks the amazing Pacific bluefin tuna to learn more about it. The site features information, photos, blog posts and multimedia stories about the bluefin tuna.
• PEW, Charitable Trust, new data (1/2013) shows that the population of Pacific bluefin is a small fraction of what it used to be, and is in danger of all but disappearing, - actually the Pacific Bluefin Tuna Population is down over 96%

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