Rather than relieving pressure on wild fish stocks, the explosive growth of aquaculture has actually exacerbated this pressure, according to an international study led by Rosamond L. Naylor of Stanford University and published in the Proceedings of the National Academy of Sciences.

The researchers surveyed trends in the farming of several different fish species, finding that by the end of 2009, half of all fish consumed in the world will have been raised on an aquaculture farm. This has led to a concurrent increase in demand for wild-caught fish meal and fish oil to feed these fish, including for vegetarian species such as tilapia and carp.

"Our assumption about farmed tilapia and carp being environmentally friendly turns out to be wrong in aggregate, because the sheer volume is driving up the demand," Naylor said. "Even the small amounts of fishmeal used to raise vegetarian fish add up to a lot on a global scale."

Fish fed on fish meal and oil grow faster and tend to have more flavor than fish fed on a natural diet, providing a profit incentive for fish farmers to buy more wild-caught fish. The researchers found that the amount of fish meal fed to vegetarian fish increased drastically in the early 1990s. Even though this trend began to reverse in 1995, carp and tilapia farms were still responsible for consuming more than 12 million metric tons of fish meal in 2007.

Farmed salmon also place a huge burden on wild fish stocks, with five pounds of wild fish required to produce every pound of farmed salmon.

"As long as we are a health-conscious population trying to get our most healthy oils from fish, we are going to be demanding more of aquaculture and putting a lot of pressure on marine fisheries to meet that need," Naylor said.

"Our thirst for long-chain omega-3 oils will continue to put a lot of strain on marine ecosystems," she said.