Northern bluefin tuna  (Thunnus thynnus)

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).