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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.

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You read about some concerns that have been raised about farming tuna. However, researchers are finding promising solutions to these problems.

First, it’s true that tuna farms need to be stocked with wild and young, but this may soon no longer be the case thanks to a breakthrough. Scientists found they can get captive females to lay their eggs by injecting them with certain hormones. These injected hormones get the females to lay eggs reliably without posing health risks to the tuna or to consumers later on. With this advancement, tuna farming can be done without further reducing the wild populations.

Second, yes, tuna must eat a high protein diet, and they usually get protein from fish. But the protein tuna need doesn’t have to come from fish. It can also be gotten from plants. Certain plants that high in protein can be processed to make an inexpensive food for tuna. This food made from plants could supply captive tuna with all the protein and nutrients they need. And since the plant-derived food is inexpensive, the cost of tuna for tuna farms can be kept low.

Third, parasites attack may ocean farms, but steps can be taken to treat infestations. For example, scientists studying parasite problems in tuna farms in southern Australia discovered how to greatly reduce the blood flukes infestations there. They found that when tuna cages were moved farther offshore into the deeper waters, far fewer tuna had blood flukes in their bodies. It turns out that blood flukes need to stay close to shore to do well. That’s because certain resources that blood flukes require for their life cycle are not available farther offshore. Because the tuna cages were moved into deeper waters with blood flukes cannot survive, blood fluke infestations among the cage to tuna were basically stopped. 

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