One of the largest bony fish in the world, the northern bluefin tuna has a very streamlined and powerful body. It can reach up to 300 cm in length and weigh over 450 kg. The body is deepest near the middle of the first dorsal fin and tapers to a pointed snout. The upper body is dark blue in colour while the lower half is silvery white. Thunnus thynnus has a large and crescent-shaped tail and its dorsal and anal fins are yellowish in colour. Bluefin tuna are known for the finlets that run down the dorsal and ventral sides towards the anal fin.Northern bluefin tuna school by size, sometimes together with other species. It preys on small schooling fishes or on squids and red crabs. They are commercially cultured although not in the UK or Ireland (Frimodt, 1995), and have become rare elsewhere because of substantial overfishing (Muus & Nielsen, 1999).