Tunas, bonitos and mackerels all belong to the family Scombridae, with approximately fifty species around the world. Tunas and bonitas are very well adapted to high speed long distance swimming. They have bullet shaped bodies, with scales that are either absent or smaller to reduce drag when swimming. Most tunas belong to the genus Thunnus.
The internal structure of these fish is unique. The outer body layers consist of longitudinal blocks of white muscle. This can generate short bursts of speed. However, in the centre of the body, there is block of red muscle, similar to that found in mammals. This muscle is surrounded by a lattice of blood vessels regulating the temperature. The muscle is maintained at a temperature several degrees above the ambient. This is to ensure that the tuna can swim for extended periods.
Unlike most fish species, which have white flesh, the flesh of tuna is pink to dark red. This is because tuna muscle tissue contains greater quantities of myoglobin, an oxygen-binding molecule, than the muscle tissue of most other fish species. Some of the larger tuna species such as the bluefin tuna can raise their blood temperature above the water temperature with muscular activity. This enables them to live in cooler waters and survive a wider range of circumstances.
Due to their high position in the food chain and the accumulation of heavy metals from their diet, mercury levels can be very high in some of the larger species of tuna such as bluefin and albacore.
Albacore Tuna (Thunnus alalunga)
Albacore Tuna is an important food fish. It is found in the open waters of all tropical and temperate oceans, and the Mediterranean Sea. The pectoral fins of the albacore are very long, as much as 30% of the total length. Lengths range up to 140 cm and weights up to 60.3 kg.
Albacore is a prized food, and the albacore fishery is economically significant. Methods of fishing include pole and line, long-line fishing, and some purse seining. The best quality canned tuna is made from albacore. They are the only tuna species that may be classified as white meat tuna.
The Bigeye Tuna (Thunnus obesus)
Bigeye tuna are found in the open waters of all tropical and temperate oceans, but not the Mediterranean Sea. Its length is between 60 and 250 centimetres. Bigeye Tuna are large deep-bodied streamlined fish with large heads and eyes. The pectoral fins are very long, reaching back as far as the second dorsal fin. The dorsal and anal fins are yellow.
The Northern Bluefin Tuna (Thunnus thynnus)
This is a species of tuna fish, living in both the Western and the Eastern Atlantic Ocean and extending into the Mediterranean Sea and the Black Sea. Although not native to the Pacific Ocean, it is cultivated off Japan. Northern Bluefin Tuna can live up to 30 years old. The typical size is 6.6 ft (2 m) at about 1100 lb (500 kg).
The body of the Northern Bluefin Tuna is robust. These tuna can easily be distinguished from other members of the tuna family by the relatively short length of their pectoral fins.
The Northern Bluefin Tuna is an important source of seafood, providing most of the tuna used in sushi. It is a particular delicacy in Japan where the price of a single giant tuna can exceed $100,000 on the Tokyo fish market. Some fisheries of bluefin are considered overfished, and this problem is compounded by the bluefin's slow growth rate and late maturity.
Tuna is an important commercial fish. Some varieties of tuna, such as the bluefin and bigeye tuna are threatened by overfishing, dramatically affecting tuna populations in the Atlantic and north western Pacific Oceans. Increasing quantities of high grade tuna are entering the market from operations that rear tuna in aquaculture set-ups. In Australia the Southern Bluefin tuna is one of two species of bluefin tunas that is kept in tuna farms by former fishermen. Its close relative, the Northern Bluefin Tuna, is being used to develop tuna farming industries in the Mediterranean, North America and Japan.
Long lining is a method using baited hooks suspended on traces hanging from a line supported by a series of floating buoys. An alternative method of catching is poling. Poling is carried out by using a baited hook attached to a pole by a short line. The fish are landed with a gaff. Under suitable conditions, poling has been a cost-effective method of catching. This method led to renewed interest in tuna fishing and in 1979, when there was an abundance of yellowfin tuna, there were one hundred and fifteen vessels, excluding sports craft hunting tuna. In that year more than six thousand tonnes were landed. There was a heavy investment in boats and equipment, but in 1980 the yellowfin tuna failed to appear and the industry virtually collapsed. The remaining tuna fishery was directed at the longfin tuna, with smaller catches of yellowfin, bigeye and skipjack since providing the bulk of the tuna catch in South Africa. The average annual tuna catch is three thousand tonnes.