Tuna

Tuna support large commercial fisheries wherever they are abundant, and are thus economically important fish. However, as with any fishery, stocks of tuna can be easily exhausted through excessive harvesting. Indeed, most if not all stocks of tuna have been significantly degraded by overharvesting. This problem can be illustrated by the case of the fishery for bluefin tuna in the western Atlantic Ocean.

The bluefin tuna is a very large, fast-swimming fish which undertakes regular trans-oceanic migrations. Bluefin tuna are extremely valuable since they are eagerly sought for sale as a delicacy in Japanese sushi restaurants. A tuna. © Lanceau Y. Jacana, National Audubon Society Collection/Photo Reasearchers, Inc. Reproduced by permission. During the early 1990s, a prime bluefin tuna caught in North America could be sold for $30,000 (U.S. currency) at the wharf, and then for at least $60,000 at an auction in Tokyo. The tuna meat might then be sold for about $350 per pound, as prepared sushi in restaurants. This is equivalent to about $230,000-$385,000 per fish, depending on its weight.

Because bluefin tuna are so enormously valuable, they have been exploited intensively, and their populations are declining rapidly. For example, in 1975 there were an estimated 150,000 bluefin tuna in the western Atlantic Ocean, but by the early 1990s this number had decreased by 90% through excessive harvesting, to only 22,000 animals. This resource collapse occurred even though the fishery was regulated by an international agency, the Atlantic Commission for the Conservation of Atlantic Tunas. The problem was that the managers of the commission consistently ignored the advice of their resource scientists, and set the allowable catches higher than was recommended, or was prudent. In addition, there was substantial unregulated pirate fishing by ships flying the flags of nations that are not members of the commission. As of 1999, this situation has not significantly improved.

The regulated and non-regulated overfishing were both ultimately caused by greed, and a desire to reap large, short-term profits. This was done without significant regard for the sustainability of the enterprise, or of the natural resource of bluefin tuna.

Some of the smaller species of tuna are caught using a type of net called a purse seine, set around a school of fish. The net initially floats vertically, with one side buoyed at the surface. Once the purse seine is set around a group of fish, the deeper side of the net is closed using a drawstring-like apparatus, trapping the fish inside the "purse." Unfortunately, purse seines also trap other species, including dolphins and porpoises that often associate with schools of tuna in some regions. In fact, in some fishing sectors boat captains deliberately set their nets around groups of these marine mammals, because they know that schools of tuna are generally found beneath them. From the 1960s to the late 1980s, huge numbers of some species of dolphins and porpoises were killed in purse seines set for tuna, perhaps 200,000 of these marine mammals each year. More recently, the dolphin kill rate has declined to about 100,000 per year. This non-target fishing mortality has significantly depleted the populations of dolphins and porpoises in some regions.

Large tuna often have a significant contamination of their flesh with mercury. This commonly occurs to a degree that exceeds the maximum acceptable concentration of mercury in fish intended for human consumption, that is, 0.5 ppm (parts per million, on a fresh weight basis). The contamination of tuna and other large, oceanic fish by mercury is apparently a natural phenomenon. One study found no difference in the mercury concentrations of modern tuna, and animals collected between 1878 and 1909 and stored in a museum.

The World Conservation Union (IUCN) considers the southern bluefin tuna (Thunnus maccoyii), bigeye tuna (Thunnus obesus), and Monterrey Spanish mackerel (Scomberomorus concolor) to be threatened species. The southern bluefin tuna is listed as critically endangered.