Nearly half of the fish eaten around the world now comes from farms instead of the wild, with more foresight needed in China and other producers to limit the ecological impact, a study said Tuesday.

With rising demand for fish and limited scope to step up the wild catch, aquaculture -- the raising of seafood in confined conditions -- is bound to maintain strong growth, said the report released in Washington and Bangkok.

The WorldFish Center, a non-governmental group that advocates reducing hunger through sustainable fishing, and environmental organization Conservation International found that 47 percent of food fish came from aquaculture in 2008.

The study said that China alone accounted for 61 percent of the world's aquaculture -- a significant part of it carp, which is highly demanding in resources -- and Asia as a whole for some 90 percent.

Aquaculture has long been controversial, with some environmentalists concerned about the pollution to coastal areas.

But the study argued that aquaculture was not as destructive as raising livestock such as cattle and pigs, which places severe strains on land and water use and is a major source of climate change.

A vegetarian diet would be the healthiest for the environment, but the study said it was a simple fact that more people in the developing world were eating meat as they moved to cities.

"I think the likelihood for the demand for aquaculture products diminishing is very unlikely at this point," said Sebastian Troeng, vice president for marine conservation at Conservation International.

"So what we need to figure out is, if this growth is continuing, how can we make sure that it is met in a way that doesn't put an undue burden on the environment, so that best practices are used and species groups are cultured that don't have excessive impact," he said.

The study looked at the impact of aquaculture in areas including energy use, acidification and climate change.

Along with carp, the species with the greatest environmental impact include eel, salmon, shrimp and prawn as they are carnivorous, meaning that farms need fish feed -- and more energy -- from the outside.

On the other end of the spectrum, the farming of mussels and oysters -- along with seaweed -- has a lesser impact.

The study found wide variations between countries, giving hope that the sharing of best practices could limit impact on the environment.

In one striking comparison, the study said the environmental impact of shrimp and prawn farms in China would decline by 50 to 60 percent if they used the same energy levels as those in Thailand.

Aquaculture production has been growing by 8.4 percent since 1970 and is spreading to new areas such as Africa, the study said, which pointed to increasing demand for fish in Egypt and Nigeria since the bird flu crisis in the mid-2000s.

The study called for further study on how supermarket chains, particularly in emerging Asian nations, can improve environmental performance in the farmed fish they bring to consumers.

The study was released days after the United States -- a comparatively small player in aquaculture -- authorized guidelines that would open up some federal waters to fish farms.

Commerce Secretary Gary Locke said the United States had a $9 billion trade deficit in seafood and that a boost in aquaculture would both meet local demand and create jobs, including on the struggling Gulf Coast.

The plan was attacked by some environmentalists, who said it would bring waste perilously close to people and may depress market prices.

"The last thing we need is enormous ocean fish farms that can and do spread disease, allow for millions of fish to escape, kill off wild populations, jeopardize the tourism industry and further destroy the livelihood of local fishermen," advocacy group Food & Water Watch said.

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