Farm-raised salmon contain significantly more dioxins and other potentially cancer-causing pollutants than do wild salmon. That's the finding of a new study recommending consumers eat a lot less of the farmed fish.

The study, published in the journal Science Thursday, found that salmon farmed in northern Europe had the most contaminants, followed by those farmed in North America and Chile.

The researchers conclude that eating more than one serving of farm-raised salmon per month could slightly increase the risk of getting cancer later in life. The pollutants are also believed to harm the developing brains of infants in pregnant or breast-feeding women.

Researchers from universities in Indiana, Michigan and New York tested 700 salmon from around the world. They found the average dioxin level in farm-raised salmon was up to 11 times higher than that in wild salmon -- 1.88 parts per billion compared with 0.17 ppb. For PCBs, the average was 36.6 ppb in farm-raised salmon and 4.75 in wild salmon.

Farmed salmon from Chile and Washington state rated safest. According to the study, consumers can safely enjoy them for one meal, or eight ounces, each month.

Farm-raised salmon from Norway, Canada and Maine ranked second, safe for a half meal each month.

Salmon from the Faroe Islands and Scotland fared worst, warranting a monthly recommendation of a quarter serving.

Regardless of where the salmon is raised, the study found samples of the fish on supermarket shelves in 16 cities worldwide varied widely in their contaminant content.

According to the study, applying U.S. EPA standards would mean eating no more than:

• two meals a month in Denver and New Orleans.
• one meal a month in  Vancouver, Los Angeles, Washington, Seattle, Chicago and New York.
• half a meal each month in Toronto, Boston, Edinburgh, London, Oslo, Paris and San Francisco.
• only a quarter of a serving in Frankfurt, Germany.

In contrast, some species of wild Pacific salmon could be safely served for dinner eight times a month, the study said.

Meal, pollutants, drugs and dye blamed

The researchers blame the feed used on fish farms. They say farmed salmon eat lots of fish meal made from just a few species of ocean fish, which concentrates the contaminants they are exposed to. Wild salmon eat a greater variety, they say.

David Carpenter, director of Institute for Health and the Environment at the University at Albany, N.Y., who took part in the study, says farmed salmon are also routinely given high doses of antibiotics and are even fed dye.

"Most people don’t recognize that farmed salmon have their pink or red color totally by adding dye to their food," he says. "Wild salmon have that colour naturally from the food they eat, which are crustaceans and shrimp and small kinds of seafood that have pigment in their shell."

For all these reasons, he urges consumers to buy only wild salmon.

More than half the world's salmon now is farmed. Farmers say the farms allow fish to be available to consumers all year round and at lower prices. The farms also have brought economic benefits to many coastal regions.

Dave Rideout of the Canadian Aquaculture Industry Alliance disputes the study and says the levels of pollutants in salmon are too low for serious concern.

"I honestly don't agree that the levels that they're talking about are as serious as what they say. The reality is that we know that dioxins and PCBs are ubiquitous in the environment," he says.

"When we look at it from a point of view of risk and benefits, the risks are infinitesimally small compared to the benefits of eating fish, particularly fatty fish like salmon," Rideout adds, pointing out that fatty fish are rich in omega-3 fatty acids, which have been shown help to reduce cholesterol levels and heart disease.

He adds that the industry is looking to improve itself.

"We're looking at finding different sources of oil -- canola, soy oil, for example."

Carpenter says he doesn't want his study to convince consumers to stop eating fish.

"We are certainly not telling people not to eat fish. ... We're telling them to eat less farmed salmon," he said.

But Rideout says he worries this latest study will only confuse consumers further.

"The greatest fear I have, quite frankly, is that it's going to put confusion in the minds of consumers and that they'll leave the fish counter and go to other protein sources."

A spokesman with the Canadian Food Inspection Agency told Canadian Press the study requires further examination, but its findings indicate pollutant levels in farm salmon are currently below Health Canada standards.