Tuna are fast-swimming, torpedo-shaped fish found in all warm tropical and subtropical seas. Each species ranges widely, its distribution controlled by the water temperature, ocean currents, and available food (small fishes). Tuna tend to be most abundant in areas of upwelling and along current boundaries, particularly where there are local eddies. There are four such areas of upwelling off the New Zealand coast, which may prove to be productive fishing grounds: the subtropical convergence, which extends eastwards from the coast between Banks Peninsula and East Cape, according to the season; the Southland – sub-Antarctic convergence, about or south of Stewart Island; the Westland – west Auckland convergence, north of Cape Egmont; and some areas located off East Cape, Fishing Industry). Various methods are used in other countries to catch tuna commercially: trolling, with a feathered lure; pole fishing, using barbless hooks on short lines and attracting the tuna to the boat with small live fish thrown into the sea; long lining, with drifting mid-water setlines; purse seining, with large encircling nets; or arranging enormous traps (madragues) to intercept the tuna on their seasonal migrations.
At least four species of tuna are known from New Zealand waters: the southern tunny (or southern bluefin tuna), Thunnus maccoyi; the yellowfin tuna, Neothunnus macropterus; the long-finned albacore, Thunnus alalunga; and the striped bonito, Katsuwonus pelamis. Others which may occur are the northern tunny (northern bluefin tuna), Kishinoella tonggol; and the frigate mackerel, Auxis thazard. In view of the importance of tuna fishing in some overseas countries, attempts are now being made to develop the industry in New Zealand.
Closely related to the tunas are the mackerels (represented in New Zealand by the southern mackerel, Pneumatophorus japonicus), the marlins, and the broadbill swordfish.
by Lawrence James Paul, B.SC., Fisheries Division, Marine Department, Wellington.