Wild tuna, a species of large ocean fish, have decreased in number because of overfishing Recently, attempts have been made to farm tuna by feeding the fish in ocean cages until they become large enough for sale. However, tuna farming has faced several problems and criticisms.   First, female tuna do not lay eggs in captivity, so tuna farmers must capture large quantities of young wild tuna to stock their farms, further reducing wild tuna populations. These young tuna are caged and fed until they are large enough to be sold. Since the captured females in tuna farms cannot lay eggs to replace the tuna that are sold, tuna farmers continue to catch young wild tuna to keep their farms going and so worsen the decline in wild tuna populations.   Second, tuna raised on farms are very expensive because they depend on a costly, high-protein food derived from other fish. Tuna feed almost exclusively on small fish, which supply them with the proteins and nutrients they require. A single tuna can grow more than four meters long, weigh close to 700 kilograms, and eat 70 kilograms of food a day! It is unlikely that tuna farms can remain profitable while supplying so much expensive food for the tuna.   Third, tuna confined to ocean cages are likely to become infested with parasites, organisms that feed on and weaken the animal they attach to. Tuna farmers off the coast of southern Australia have had problems with infestations of blood flukes, a type of parasite that lives within the blood vessels and heart of infected fish. Parasite infestations weaken tuna, slow their growth, and can even cause death, usually by making them susceptible to other diseases. Tuna farms in southern Australia have had 10 percent of their tuna die before they could be sold.