Half of all the fish eaten in the world now is raised on fish farms rather than caught in the wild, according to new research by an international team of scientists.

But while the aquaculture industry is more efficient than ever, it is putting a strain on marine resources by consuming large amounts of feed made from wild fish harvested from the sea, the study shows.

"Aquaculture is set to reach a landmark in 2009, supplying half of the total fish and shellfish for human consumption," the authors write in their paper, published in Monday's online edition of the Proceedings of the National Academy of Sciences.

While the fish that end up on dinner plates are produced in farms, they are fed fish meal made from wild-caught fish, putting pressure on marine fisheries - a problem warned about by ocean explorer Jacques Cousteau back in 1992 at the United Nations Earth Summit in Rio de Janeiro.

Between 1995 and 2007, global production of farmed fish nearly tripled in volume, in part because of rising consumer demand for long-chain omega-3 fatty acids, the authors point out. Oily fish, such as salmon, are a major source of omega-3s, which reduce the risk of cardiovascular disease, according to the National Institutes of Health.

"The huge expansion is being driven by demand," said the Stanford study's lead author Rosamond Naylor, a professor of environmental Earth system science at Stanford University and director of the Stanford Program on Food Security and the Environment. "Our thirst for long-chain omega-3 oils will continue to put a lot of strain on marine ecosystems, unless we develop commercially viable alternatives soon."

To maximize growth and enhance flavor, fish farms use tons of fishmeal and fish oil made from less valuable wild-caught species, including anchoveta and sardines.

"Aquaculture's share of global fishmeal and fish oil consumption more than doubled over the past decade to 68 percent and 88 percent, respectively," the authors write in the study.

In 2006, aquaculture production was 51.7 million metric tons of farmed fish, and about 20 million metric tons of wild fish were harvested for the production of fishmeal to feed the farmed fish.

"It can take up to five pounds of wild fish to produce one pound of salmon, and we eat a lot of salmon," said Naylor, the William Wrigley Senior Fellow at Stanford's Woods Institute for the Environment and Freeman Spogli Institute for International Studies.

One way to make salmon farming more environmentally sustainable is to lower the amount of fish oil in the salmon's diet. The authors say a four percent reduction in fish oil would reduce the amount of wild fish needed to produce one pound of salmon from five pounds to 3.9 pounds.

But reducing fishmeal use by four percent would have very little environmental impact, they said. "Reducing the amount of fish oil in the salmon's diet definitely gets you a lot more bang for the buck than reducing the amount of fishmeal," Naylor said. 

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