Farm-raised salmon contain substantially higher levels of PCBs and other potentially cancer-causing industrial pollutants than their wild counterparts, a new study has says.
Researchers at Indiana University measured the levels of 14 toxic compounds, called organochlorines, in about 700 North American, South American and European salmon and discovered that farm-raised Atlantic salmon had "significantly higher levels of 13 toxins compared with wild Pacific salmon."
The researchers, whose findings are published in Friday's journal Science, did not study farmed Pacific salmon or wild Atlantic salmon as fish from these groups are rare.
The average dioxin level in farm-raised salmon was 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, compared with 4.75 in wild salmon.
Overall, salmon farmed in Europe had significantly higher levels of toxicity than salmon farmed in North America or South America, the study said.
Farmed salmon from Scotland and Denmark's Faroe Islands registered the highest levels of polychlorinated biphenyls (PCBs), dioxins, toxaphene and dieldrin -- the four toxins thought to have the greatest impact on human health. Farmed salmon from Chile and Washington state registered the lowest levels of these four toxins.
The researchers point to "salmon chow" - a mix of ground up fish and oil fed to farm-raised salmon - as a likely cause of toxicity.
Farmed salmon eat lots of fish oil and meal made from just a few species of ocean fish, which concentrates the contaminants they are exposed to, while wild salmon eat a greater variety, David Carpenter, one of the researchers told the Associated Press.
But several farmers in the United States, Canada and Chile are beginning to replace the fish oil in the feed with soybean and canola oil.
Based on their findings, the researchers recommend that farmed salmon bought in Toronto supermarkets (as well as those in San Francisco, Boston, London, Oslo London, Paris, Edinburgh and Frankfurt) not be consumed in quantities of more than one half to one meal a month. Eight ounces of uncooked fish constitutes one meal.
Farmed salmon bought in Vancouver supermarkets (along with those in London, Washington D.C., Seattle, Chicago and New York) should not be eaten in more than two meals per month.
People who can consume more than the recommended amounts, which are based on strict guidelines established by the U.S. Environmental Protection Agency, could slightly increase their risk of developing cancer later in life. The same guidelines allow wild salmon to be consumed in quantities of up to eight meals per month.
Purdue University researcher and nutritionist Dr. Charles Santerre says he agrees with the overall findings of the study, but disagrees with its conclusion that consumers should limit their intake of farmed salmon because of increased cancer risk, noting the heart benefits of the fish, which is rich in omega-3 fatty acids.
"The study shows that the cancer risk from eating large amounts of salmon is significantly lower than the risk of developing heart disease from not eating generous amounts of the fish," Dr. Santerre said.
He also recommends farm-raised or wild salmon for pregnant and nursing mothers as an ideal source of nutrients for a developing fetus and infant and says salmon it is one of the safest fish on the market.
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