Carnivorous fish nibble at farming gain.

Fish farming overall may be supplementing the wild stocks, but for certain species, the farms cause a net loss of wild fish, warns a broad new review.

Some aquaculturists use up to 5 kilograms of wild fish as feed to grow 1 kg of carnivorous species like seabass and salmon, notes economist Rosamond L. Naylor of Stanford University. Such appetites for feed plus other environmental side effects undermine potential aquaculture benefits, she and her colleagues conclude in the June 29 NATURE.

Crops of farmed fish and shellfish have doubled in just 15 years, Naylor notes. "Many people believe that such growth relieves pressure on ocean fisheries, but the opposite is true for some types of aquaculture," the authors caution.

The 10 coauthors represent diverse expertise including aquaculture biologists and environmentalists. They produced a unique evaluation of whether seafood farms, which raise 220 species, add to world fish supplies, Naylor says.

"Aquaculture, on net, is still adding 19 million metric tons [Mt] annually to fish production," she says. Over the next few decades, that gain may shrink. "Our speculation is that we're going in the wrong direction," she says.

She and her colleagues worry about the 10 Mt of wild fish caught yearly to feed farm species unable to survive on vegetarian diets. Between 1986 and 1997, four of the top five fish species caught in the wild went mainly to feed farm fish and livestock. In contrast, Naylor says, carp, tilapia, and bivalves like clams and mussels are traditionally vegetarians or plankton feeders. She frets that the huge Asian carp farms are starting to use fish meal to speed growth.

Fish farmers need to meet demand, notes George Chamberlain, president of the industry group Global Aquaculture Alliance in St. Louis. "Why would the aquaculture industry produce carp when people really want to eat salmon?"

The new report highlights environmental hazards of some fish farming, including high effluent discharge, extensive antibiotic use, and increased disease risk to fish. It describes wasteful, unintended catch and also habitat loss. In India and Bangladesh, for example, up to 160 fish and shrimp fry collected in the wild are discarded for each shrimp fry selected for the farms. Also, Thai shrimp ponds have taken over the wild habitat of an estimated 400 grams of fish and shrimp for each kilogram produced.

For diners worldwide, about 75 percent of salmon and 25 percent of shrimp come from farms, Naylor notes. She won't eat a fish unless it came from an environmentally friendly farm or fishery. "My friends are starting to hate me; they can't order so many things off the menu," she says.

In aquaculture's defense, Chamberlain argues that methods are improving quickly (SN: 5/13/00, p. 314). "We're getting blamed for practices that aren't really occurring anymore," Chamberlain protests.

Also, the report notes that the farm boom has not diminished wild catches. Chamberlain counters that the seas haven't met the human population's burgeoning demand for seafood. "Thank God, we had aquaculture," he says.