PORT McNEILL, British Columbia - 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 waters around British Columbia. 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," says 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 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 says. "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," says Mary Ellen Walling, executive director of the British Columbia Salmon Farmers Association.

"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."

Nearly 30 farms are preparing to move to less ecologically fragile areas, under orders from Canadian authorities.

Some farms have installed underwater video cameras to detect when fish quit feeding, so workers can stop scattering food pellets. Many farms are switching to sturdier nets to stop fish from escaping and keep out marauding sea lions, which are shot if they penetrate the perimeter.

The industry now recognizes that it will soon be pushing the limits of the ocean.

"There will come a time when our industry will use more of the fish oil and fish meal than is available," says Odd Grydeland, an executive at Heritage Salmon in British Columbia. "Our biggest challenge is to find substitute grains for fish meal and fish oil."

Farm-raised salmon now dominates West Coast markets, arriving daily from Canada and Chile. About 80 percent of the salmon grown in British Columbia goes to markets from Seattle to Los Angeles.

Today, farms typically put 50,000 to 90,000 fish in a pen 100 feet by 100 feet. A single farm can grow 400,000 fish. Others raise a million or more.

Five international companies - three of them based in Norway - control most of the existing farms. Nearly all are situated around Vancouver Island.

It's a lightly populated place of stunning beauty. Cedar, hemlock and Douglas fir grow right down to the high-water mark.

Each farm is a cluster of pens, often interconnected by metal walkways and tethered offshore by a lattice of steel cables, floats and weights. In the midst of this idyllic setting, signs of strain on the marine environment are bubbling to the surface.

Farms are typically required to bury the dead in landfills to protect wild marine life and the environment. But Grieg Seafood recently got an emergency permit from the Canadian government to dump in the Pacific 900 tons of salmon killed by a toxic algae bloom. The emergency? The weight of the dead fish threatened to sink the entire farm.

About 1 million live Atlantic salmon - favored by farmers because they grow fast and can be packed in tight quarters - have escaped through holes in nets and storm-wrecked farms in the Pacific Northwest.

Biologists fear these invaders will out-compete Pacific salmon and trout for food and territory, hastening the demise of the native fish. An Atlantic salmon takeover could knock nature's balance out of whack and turn a healthy, diverse marine habitat into one dominated by a single invasive species.

European health officials have debated whether there is any human health risk from synthetic pigment added to the feed to give farmed salmon their pink hue.

In the wild, salmon absorb carotenoid from eating pink krill. On the farm, they get canthaxanthin manufactured by Hoffman-La Roche. The pharmaceutical company distributes its trademarked SalmoFan, similar to paint store swatches, so fish farmers can choose among various shades.

The industry complains that environmental activists have misinterpreted contaminant studies, needlessly frightening consumers.

"The concern is that people will stop eating fish," says Walling, of the British Columbia Salmon Farmers Association. "Salmon is a healthy food choice. Our Canadian government says this is a safe food."

Kenneth R. Weiss is a reporter for the Los Angeles Times, a Tribune Publishing newspaper.