Are fast-swimming predacious fish of the open ocean belonging to the family Scombridae. They are important both for commercial fisheries and as sports fish. They are caught by purse seine nets, long-lining, and trolling, and like many commercial species are overfished. The thirteen species of true tunas include tunny (Thunnus maccoyii), albacore (T. alalunga), big-eye (T. obesus), yellow-fin (T. alacares), and black-fin tunas (T. atlanticus) and there are numerous other smaller tuna-like bonitos (Katsuwonus sp. and Sarda sp.) and Spanish mackerels (Scoberomorus sp.). The largest is the blue-fin (Thunnus thynnus), which in twenty years grows to a length of over 3 metres (10 ft) and a weight of 680 kilograms (1,500 lbs). Tracking devices have shown that a large blue-fin spends most of its time in the upper 30 metres (100 ft), but makes frequent excursions to depths of 300 metres (1,000 ft). Its average swimming speed is around 6 kph (4 mph), but it can achieve bursts of 20–30 kph (12–19 mph). The fastest recorded burst was 70 kph (43.4 mph). A tuna can achieve such fast speeds because its body is highly streamlined but also because, like marlin, it has red muscle that keeps its body 10 °C (50 °F) warmer than the surrounding sea water. Blue-fin undertake long migrations. Fish tagged off the east coast of the USA have been recaptured off Africa and in the Mediterranean. Commercially large tunas demand high prices, because of the Japanese passion for eating the flesh raw in sushi and sashimi. However, eat them in moderation because, being top predators, they accumulate high concentrations of pollutants such as mercury and organohalogens in their bodies. See also environmental issues; pollution.
M. V. Angel
Subjects: Arts and Humanities — Medicine and Health.