If you bought a salmon filet in the supermarket recently or ordered one in a restaurant, chances are it was born in a plastic tray here, or a place just like it.

Instead of streaking through the ocean or leaping up rocky streams, it spent three years like a marine couch potato, circling lazily in pens, fattening up on pellets of salmon chow.

It was vaccinated as a small fry to survive the diseases that race through these oceanic feedlots, acres of net-covered pens tethered offshore. It was likely dosed with antibiotics to ward off infection or fed pesticides to shed a beard of bloodsucking sea lice.

For that rich, pink hue, the fish was given a steady diet of synthetic pigment. Without it, the flesh of these caged salmon would be an unappetizing, pale gray.

While many chefs and seafood lovers snub the feedlot variety as inferior to wild salmon, fish farming is booming. What was once a seasonal delicacy now is sometimes as cheap as chicken and available year-round. Now, the hidden costs of mass-producing these once-wild fish are coming into focus.

Begun in Norway in the late 1960s, salmon farming has spread rapidly to cold-water inlets around the globe. Ninety-one salmon farms now operate in British Columbian waters. The number is expected to reach 200 or more in the next decade.

Industrial fish farming raises many of the same concerns about chemicals and pollutants that are associated with feedlot cattle and factory chicken farms. So far, however, government scientists worry less about the effects of antibiotics, pesticides and artificial dyes on human health than they do about damage to the marine environment.

"They're like floating pig farms," said Daniel Pauly, professor of fisheries at the University of British Columbia in Vancouver. "They consume a tremendous amount of highly concentrated protein pellets and they make a terrific mess."

Fish wastes and uneaten feed smother the sea floor beneath these farms, generating bacteria that consume oxygen vital to shellfish and other bottom-dwelling sea creatures.

Disease and parasites, which would normally exist in relatively low levels in fish scattered around the oceans, can run rampant in densely packed fish farms.

Pesticides fed to the fish and toxic copper sulfate used to keep nets free of algae are building up in sea-floor sediments. Antibiotics have created resistant strains of disease that infect both wild and domesticated fish.

Clouds of sea lice, incubated by captive fish on farms, swarm wild salmon as they swim past on their migration to the ocean.

Of all the concerns, the biggest turns out to be a problem fish farms were supposed to help alleviate: the depletion of marine life from overfishing.

These fish farms contribute to the problem because the captive salmon must be fed. Salmon are carnivores and, unlike vegetarian catfish that are fed grain on farms, they need to eat fish to bulk up fast and remain healthy.

It takes about 2.4 pounds of wild fish to produce one pound of farmed salmon, according to Rosamond L. Naylor, an agricultural economist at Stanford's Center for Environmental Science and Policy.

That means grinding up a lot of sardines, anchovies, mackerel, herring and other fish to produce the oil and meal compressed into pellets of salmon chow.

"We are not taking strain off wild fisheries. We are adding to it," Naylor said. "This cannot be sustained forever."

In British Columbia, the industry, under pressure from environmentalists, marine scientists and local newspapers, is taking steps to mitigate some of the ecological problems.

"We have made some mistakes in the past and we acknowledge them," said Mary Ellen Walling, executive director of the British Columbia Salmon Farmers Assn. "We feel the industry is sustainable, if well-managed, and we have a code of practices that is followed by all of our member companies."