Tuna are large fish (0.5–1.5 metres) which, because of their wandering habits, are said to have no native country. They form large schools and are generally more common in northern New Zealand, arriving from the north in summer.
The eyeballs are flush with the head, and the body is teardrop shaped. This bullet of muscle is driven forward by a stiff crescent-shaped tail. They need this speed to catch their prey – mackerel, herring, mullet, krill and squid.
Tuna are sought-after as big game and commercial fish. Top-quality specimens can fetch high prices at Tsukiji, the huge fish market in Tokyo.
The main species found in New Zealand waters are the albacore (Thunnus alalunga), southern bluefin (Thunnus maccoyii) and skipjack tuna (Katsuwonus pelamis). Other less common tuna found off New Zealand’s shores include the yellowfin (Thunnus albacares), bigeye (Thunnus obesus), slender (Allothunnus fallai), butterfly (Gasterochisma melampus) and frigate (Auxis thazard). The Australian bonito (Sarda australis) is rarely seen.
Tuna are superbly streamlined, and can retract their gills at high speed to reduce drag. The southern bluefin can reach speeds of 80 kilometres per hour. It does this in part by heating up its blood. As heated muscle is more efficient than cool muscle, the fish can achieve greater power.
Albacore found in New Zealand waters average 50–70 centimetres in length. An 80-centimetre fish weighs about 10 kilograms. Albacore spawn in the tropical Pacific, and are plentiful along the boundaries between currents.
Those that reach New Zealand waters are probably between two and five years old. Domestic vessels catch albacore by trolling (trailing baited lines) in coastal waters west of the country. Between 1991 and 2000, annual catches varied from 1,437 to 5,180 tonnes.
The southern bluefin lives only in the southern hemisphere, where it makes long migrations between tropical and temperate waters. In New Zealand waters a typical fish is about 1 metre long and weighs about 20 kilograms. The species is widely distributed between latitudes 10º and 50º south, and is the most southerly-ranging common tuna.
Internationally, stocks are thought to be severely depleted by overfishing. During the 1970s about 100,000 fish were taken annually by Japanese vessels. In the 2000s the species formed a small but still very valuable New Zealand fishery.
Skipjack tuna occur in all the world’s oceans, with their southern limit in New Zealand waters. They usually arrive in December, staying until May and ranging as far south as Cape Farewell. They feed opportunistically on fish and crustaceans.
Purse seiners (fishing boats with seine nets that draw up like a purse) hunted skipjack during the 1970s, taking between 5,000 and 10,000 tonnes annually. Catches in the 2000s were smaller (around 3,000–4,000 tonnes), with most fish landed in the Bay of Plenty and along the north-east coast of the North Island.