The tunny or northern bluefin tuna, at up to 3.5m (lVAft) long, is the largest member of the tuna family and one of the world’s most sought-after game fish. It doesn’t have a swim bladder, so must swim constantly from birth to death — or sink to the bottom and die of suffocation. Great travellers, bluefin are known to have covered several thousand miles in a year in search of food and spawning grounds.
A powerful, streamlined fish, it is well suited to a life spent roaming the oceans in search of food. Its muscular body is blue-black on the back and fades to a silvery-white belly. Most of its scales are small, but there is a band of larger scales that start behind the head and continue to the rear of the first dorsal fin – this is called the corse- let and it is common to the tuna family.
Despite its name, the fins are not really blue. The pelvic and anal fins and the small fmlets are all yellow, and the others are purple-black. There are two dorsal fins, the first with 13-15 spines, the second with 13-15 rays. The pectoral fins are short – not extending beyond the first dorsal fin – and the large crescent-shaped tail has distinct keels on either side.
The rover’s range
Bluefin tuna are one of the sea’s great travellers. In June each year they gather in large shoals and spawn in the western Mediterranean and the Atlantic off North Africa. During the entire spawning period they stop feeding, but as soon as spawning is over some migrate south to the coast of southern Africa, while others head northward as far as Norway and Denmark in search of food.
On the other side of the Atlantic there is a similar pattern of shoaling, spawning and spreading out. Tagging has shown that a considerable number of these ‘American’ fish cross the Atlantic to northern Europe -some complete the journey in less than two months. As northern seas cool in the autumn the bluefin move south and spend the winter months in the warmer waters off the Azores in the mid-Atlantic.
As a result of their diet of small fish and crustaceans, young bluefin grow very quickly. They become sexually mature at three or four years old and at a weight of 35-60 lb (16-27kg). As they grow older they feed on the herring and mackerel shoals whose movements dictate their own migration patterns. Bluefin live for 20 years at most and have a maximum weight of 1,500 lb (680kg). They are not strictly speaking coldblooded fish, since their body temperature can be as much as 10°C higher than that of the water around them.