Pacific bluefin tuna

Thermoregulation

Almost all fish are cold-blooded (ectothermic). 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. It is mainly a pelagic species found in temperate oceans, but it also ranges into the tropics and more coastal regions. It typically occurs from the surface to 200 m (660 ft), but has been recorded as deep as 550 m (1,800 ft).

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

The species is considered to consist of only one stock.

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. At maturity it is about 1.5 m (4 ft 11 in) long and weighs about 60 kg (130 lb). 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. Elsewhere, a mass of up to 550 kg (1,210 lb) has been reported for the species. According to the International Game Fish Association, the all-tackle game fish record was a 411.6 kg (907 lb) individual (Donna Pascoe) caught on 19 February 2014 onboard charter boat Gladiator during the National Tournament.

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). Large females can carry more eggs than small ones, and between 5 million and 25 million eggs have been reported.

Pacific bluefins eat various small schooling squids and fishes, but have also been recorded taking sessile animals, pelagic red crabs and krill.

Conservation status

Unlike the other bluefins (Atlantic and southern), the Pacific bluefin tuna was not considered threatened initially, resulting in a Least Concern rating in 2011. In 2014, it was found to be threatened and the status was upgraded to Vulnerable. Overfishing is occurring in the Pacific bluefin, but overall the stock was not yet believed to be in an overfished condition in 2011. In 2010, it was estimated that the complete spawning biomass was 40–60% of the historically observed spawning biomass. In 2013–2014, scientists found a drastic population decline, estimated at 96%, due to overfishing. In 2000–2004, between 16,000 tonnes and 29,000 tonnes were caught per year. 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. Other recommendations include a substantial reduction of fishing of this species, especially juveniles. As much as 90% of the caught Pacific bluefins are juveniles.

Monterey Bay Aquarium's Seafood Watch program have placed all bluefin tunas on the "Avoid" list, and they are also placed on the "Red List" by both Greenpeace and the Blue Ocean Institute.

Pacific bluefin flesh may contain levels of mercury or PCBs that are harmful to humans that consume it. 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 Japan, some foods made available for the first time of the year are considered good luck, especially bluefin tuna. Winning these new year auctions is often used as a way to get publicity, which raises the prices considerably higher than their market value: on January 5, 2013, a 489-pound (222 kg) Pacific bluefin tuna caught off northeastern Japan was sold in the first auction of the year at the Tsukiji fish market in Tokyo for a record 155.4 million yen (US$1.76 million) – leading to record unit prices of $3,603 per pound, or ¥ 703,167 per kilogram.

Farming

Japan is both the biggest consumer and the leader in tuna farming research. 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. This farm-raised tuna is now known as Kindai tuna. Kindai is a contraction of Kinki University (Kinki daigaku).