In spite of possible environmental problems, specialists say aquaculture can help spur the recovery of natural populations of fish and other aquatic species - and provide much-needed food and income, especially for small-scale farmers in developing countries.

In intensive marine aquaculture, fish are raised in the ocean in cages, like these on Hainan Island in China

Aquaculture has been practiced for thousands of years. Jim Diana, a professor of natural resources at the University of Michigan, says the first written textbook on aquaculture was published in China in about 400 B.C.

Asia continues to dominate the farming of fish and other aquatic species, accounting for approximately 92 percent of the world harvest.

"But," says Diana, "there is aquaculture in the United States and virtually every continent other than Antarctica."

According to the United Nations Food and Agriculture Organization, annual production from commercial fishing has stabilized at about 95 million metric tons, while aquaculture has increased by almost 9 percent each year since 1970. In 2005, fish farms produced 48 million metric tons - one-third of the total harvest.

Aquaculture producers use paddlewheel aerators to improve water quality at this shrimp farm in Thailand

"And if you looked at it in terms of how much people eat," says Diana, "aquaculture's probably producing about 50 percent of the fish that are eaten, because a lot of capture fisheries don't go into human consumption."

But, Diana notes, there are environmental impacts associated with some kinds of aquaculture. He describes them in a review in the January issue of the journal BioScience.

Probably the most problematic is the spread of non-native or specially bred species, when farmed fish escape from ponds or cages. Diana says the best way to avoid the spread of invasive species is to raise only fish that are native to that area.

Tilapia, originally native to Africa, is one of the top 10 species groups produced in aquaculture, worldwide

Other potential problems include nutrient pollution from excessive feeding and waste products, and the clearing of environmentally sensitive land to create ponds for aquaculture. Also, when the ponds are filled with salt water - for shrimp, for example - the salt can contaminate the soil.

Diana points to the use of already declining wild stocks for farming, or as food for farmed fish, as yet another downside.

Diana emphasizes that when practiced sustainably, aquaculture can benefit the environment by reducing pressure from commercial fishing and even helping to rebuild wild populations.

Aquaculture specialist Don Webster shows a bag of oyster shells on which new young oysters are growing

Don Webster, an extension specialist at the University of Maryland, agrees. Since the late 1970s, he has worked to support aquaculture production in the mid-Atlantic region of the United States - specifically, in the Chesapeake Bay, the nation's largest estuary. His specialty is oysters.

Webster says that when he first started working in the Maryland, there were probably several thousand watermen harvesting oysters in the Chesapeake Bay.

"Right now, our industry probably has about 150 people who are left oystering. It's not many."

Disease, poor water quality and decades of overharvest have drastically reduced the bay's natural oyster population.

Webster sees aquaculture as a way to bring the oysters back. Hatchery "seed" - farm-raised juvenile oysters - are being used to create oyster sanctuaries, where harvesting is prohibited.

The Chesapeake Bay once teemed with oysters, but overharvest, disease and pollution have reduced populations to near collapse

In addition to these protected sanctuaries, says Webster, there are also managed reserves, where diseased oysters are removed and replaced with disease-free hatchery seed. The oysters are allowed to grow until 60 percent of them are at least 10 centimeters long. Once the oysters have reached adult size, the reserves are opened up for harvest, supporting the commercial fishery.

Diana also sees important benefits to aquaculture, particularly for people in developing countries. Unlike commercial fishing, most of which is done by large fleets of trawlers, Diana says aquaculture in developing countries commonly is practiced by many small-scale producers.

"So it does a lot more for local employment and quality of life, I think, than commercial fishing does," he says.

Aquaculture also contributes to local food security and generates income.

Global demand for seafood is expected to increase, and Diana predicts that aquaculture will continue to be the most rapidly growing food production system.