WASHINGTON – Farm-raised salmon contain significantly more dioxins and other potentially cancer-causing pollutants than salmon caught in the wild, says a study that could confuse consumers long told the fish is heart-healthy.

The study tested contaminants in 700 salmon bought around the world and found those farmed in Northern Europe contained the most pollutants, followed by North America and then Chile.

Eating more than a meal of farm-raised salmon per month, depending on its country of origin, could slightly increase the risk of getting cancer later in life, according to the study published Thursday in the journal Science.

Researchers blamed the feed used on fish farms for concentrating ocean pollutants, advised farmers to switch feed and recommended that consumers in the meantime eat more wild salmon.

But the study’s conclusions are controversial. The Food and Drug Administration insisted that the levels of pollutants in farm-raised salmon are too low for serious concern and urged Americans not to let the new research frighten them into a diet change.

The study “will likely over-alarm people in this country,” said Eric Rimm of the Harvard School of Public Health, a specialist on nutrition and chronic disease. “To alarm people away from fish because of some potential, at this point undocumented risk of long-term cancer, that does worry me.”

The American Heart Association advises eating fish at least twice a week because it helps prevent heart disease. Salmon is usually listed as a top choice because it is particularly high in heart-healthy omega-3 fatty acids and low in a completely different seafood contaminant, brain-harming mercury.

Most farm-raised salmon sold in the United States come from Chile and the pollutant level in this salmon was not too much higher than that found in some wild-caught salmon.

In addition, the study tested salmon raw, with the skin on. Removing the skin and grilling it removes a significant amount of PCBs, dioxins and other pollutants stored in fish fat, the FDA noted.

“We are certainly not telling people not to eat fish. … We’re telling them to eat less farmed salmon,” said David Carpenter of the University at Albany, N.Y., who led the new research.

Farm-raised salmon contained significantly higher concentrations of 13 organochlorine pollutants, he found. Among the most important are dioxins, which are released when industrial waste is burned, and PCBs, once widely used as insulating material.

The average dioxin level for farm-raised salmon was 11 times higher than 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.

Animals absorb those pollutants through the environment, storing them in fat that people then eat. High levels are believed to increase the risk of certain cancers and, in pregnant or breast-feeding women, harm the developing brains of fetuses and infants.

The salmon farming industry points out that all the pollutant levels are well within the FDA’s legal limits and says other foods eaten far more often, such as beef, are more important sources of exposure.

The government has no one set level of dioxins and PCBs considered safe in foods. In setting his consumption advice, Carpenter cited Environmental Protection Agency guidelines that are far stricter than the FDA’s legal limits.

Farm-raised salmon eat lots of fish oil and meal made from just a few species of ocean fish, concentrating the contaminants they’re exposed to – while wild salmon eat a greater variety, he explained.

Raising salmon in floating pens is an industry that began just two decades ago but has helped the fish’s popularity to soar. More than half the world’s salmon now is farmed, available year-round while wild salmon is generally available June through October. Farm-raised salmon sells for about $4 or $5 a pound compared with $15 for wild salmon, said Alex Trent of the trade group Salmon of the Americas.

Many farmers in the United States, Canada and Chile are slowly replacing some of the fish oil in salmon feed with soybean and canola oil to address the pollutants, Trent said.

“PCB levels are coming down 10 to 20 percent a year. Every year we take more steps,” he said.

One in two Americans will die of cardiovascular disease, a far bigger risk than the cancer concern, said nutritionist Alice Lichtenstein of the Agriculture Department’s Human Nutrition Research Center at Tufts University.