Atlantic Bluefin Tuna

The northern bluefin tuna spawns at just a few locations. In the Pacific Ocean, spawning takes place off the Philippines, while in the Atlantic, this tuna spawns only in the Mediterranean between June and August, and in the Gulf of Mexico, between April and June (2) (4). During these periods, females release up to ten million eggs into the ocean (4), which are carried substantial distances by currents. From these eggs hatch tiny larvae; initially measuring just three millimetres long, the larvae grow at a rate of one millimetre per day (2). The northern bluefin tuna is a slow growing and long-lived species, maturing between the ages of four and five years in the Mediterranean and at eight years in the Gulf of Mexico (5) and living for up to 30 years (3). In schools of similar-sized individuals, the northern bluefin tuna cruises the oceans in search of food, often joining schools with other tuna, such as albacore, yellowfin and bigeye (4). The northern bluefin tuna has two types of muscle; one suited to long-distance, continuous swimming (a bluefin tuna can cross the Atlantic Ocean in less than 60 days (2)), the other providing short, fast bursts of speed (3). Reaching speeds of 45 miles per hour (2), the northern bluefin tuna employs this explosive swimming power when in pursuit of small schooling fish, such as anchovies, while it swims slowly with its mouth open to catch small slow-moving prey, such as red crab (4). When wandering the expansive oceans, the northern bluefin tuna tends to stay fairly close to the surface, but it is capable of diving to depths of 1,000 metres when in pursuit of prey (3). A fascinating system of blood vessels prevents any heat created through exertion being lost to the surroundings, thus allowing this tuna to swim in water too cold for other fish (3). The northern bluefin tuna has been observed undertaking seasonal migrations in some areas of its range. During the summer months, bluefin tuna migrate northwards along the coast of Japan and the Pacific coast of North America (4), while migrations across the oceans have also been observed (2). Small deposits of magnetite in the heads of tuna are believed to act like a built-in compass, enabling the tuna to orientate itself in its vast habitat by picking up the earth's magnetic field (3).

In 1966, the International Commission for the Conservation of Atlantic Tunas (ICCAT) was formed, taking responsibility for the conservation of tunas in the Atlantic Ocean and its adjacent seas (5). Since 1998, catch limits have been in place for the northern bluefin tuna in the eastern Atlantic and Mediterranean, but until 2006, it was believed that these limits were not respected and were largely ineffective (5). In 2006, ICCAT adopted a 15 year recovery plan for the highly threatened stocks of the eastern Atlantic and Mediterranean. The plan includes stricter catch limits and more extensive closures of fisheries in certain areas and at certain times (5). As the northern bluefin tuna is such a slow-growing and long-lived species, it will take years, possibly over ten, before any benefits of these measures are observed (5). Hopefully this recovery plan will have some success, and prevent the eastern Atlantic stock falling into the same dire situation as the western Atlantic stock. Unless catches of this stock are reduced to near zero, the extinction of this large, economically valuable fish in the western Atlantic seems sadly inevitable (6).

One of the largest of the tunas (2), the northern bluefin tuna has been called a 'pinnacle of fish evolution', referring to its remarkable swimming ability (3). Its body, shimmering deep metallic blue above and silvery white on the underside (2), is well adapted for a life wandering the vast oceans. It is deep bodied, but flattened from side to side, and tapers to a point before the sickle-shaped tail fin, which can power the tuna with ease through the water (3). Colourless lines and rows of dots may be visible on the lower sides and belly (4). The northern bluefin tuna has two dorsal fins; the first may be yellow or blue, while the second, taller dorsal fin, is reddish-brown (2) (4). The second dorsal fin is followed by seven to ten yellow finlets, edged with black, which run down the back toward the tail (2).

This fast-swimming predator is an oceanic species, inhabiting the upper waters that are reached by sunlight, but it may also sometimes come near to shore. It can tolerate a wide range of temperatures (4).

The northern bluefin tuna occurs in subtropical and temperate waters throughout the Atlantic and Pacific Oceans (4).

Classified as Data Deficient (DD) on the IUCN Red List (1). The Western Atlantic stock is classified as Critically Endangered (CR) and the Eastern Atlantic stock is classified as Endangered (EN) on the IUCN Red List (1).

With its large size and high quality flesh, the northern bluefin tuna has long been a favourite of fishermen and a highly valued delicacy in Japan (3). Its popularity has led to severe exploitation in several areas, particularly the North Atlantic Ocean (2). Despite catch quotas being in place, limits are often not respected and catches are frequently under-reported (5). Unless adequate management measures are quickly implemented and, most importantly, enforced, the collapse of some northern bluefin tuna stocks is a real possibility (5). It is thought that the Critically Endangered western Atlantic stock may have already collapsed, the result of overfishing and poor management. This stock is now in grave danger of extinction (6). In the Mediterranean, where adult northern bluefin tuna decreased by 80 percent between around 1979 and 1999, tuna ranching now poses the greatest threat to the survival of this species. Tuna are captured alive and taken to one of the many ranches that have spread along the Mediterranean coast, where they are fed and fattened for months; the enormous amounts of fish needed to feed the tuna during this time is itself a matter of great concern. The tuna is then sold, primarily to Japan, creating a lucrative business that is considered to be the main force driving illegal and unreported fishing (7).