A just-released study commissioned by the Pew Foundation measured levels of organic contaminants in 700 fish (about two metric tons), purchased from wholesalers and retailers in large cities in North America and Europe. Whole, raw, farmed salmon, farmed salmon fillets, and whole wild fish (representing five species of Pacific salmon) were tested. Fourteen chemicals were studied, including toxins such as PCBs, dioxins, dieldrin and toxaphene - all fat-soluble compounds that tend to accumulate in the fat of ocean fish.
The general findings are shocking: the expert commission found that the total organic contaminants were consistently and significantly more concentrated in the farmed salmon as a group than in wild salmon. This includes dioxins and PCBs - both believed to increase the risk of certain cancers and to be harmful to the developing brains of fetuses and infants of women who are pregnant or breastfeeding. The average dioxin level in farm-raised salmon was 11 times higher than in wild salmon, and the average PCB levels were 36.6 parts per billion (ppb) in farm-raised salmon, versus 4.75 ppb in wild salmon. The study also connected the levels of contaminants to location: farmed salmon from Europe were more contaminated than fish from North or South America. The most contaminated farmed samples came from Scotland and the Faroe Islands in the North Atlantic, and the least contaminated farmed samples came from Chile and Washington State in the U.S. However even the least-contaminated farmed salmon still had significantly higher contaminant levels than wild salmon.
The reason for the higher contaminant levels in farmed salmon is due to the feed given to them - a fish meal composed of ground fish and fish oil from only a few species of ocean fish. These species accumulate the toxins in run-offs from agriculture and industry that wind up in oceans. With a limited variety of fish to eat, farmed salmon are exposed to higher concentrations of contaminants than wild salmon, which eat a more varied diet. Researchers say it would be possible to develop oil-rich feed for farmed salmon that was free of toxic contaminants.
Based on this information, the researchers involved in this study recommend limiting consumption of farmed salmon to one-half to one meal total per month. Bottom line: I agree. Until salmon farmers clean up their acts, stick to wild Alaskan salmon (which I still strongly recommend), sardines or distilled fish oil supplements for your omega-3 fatty acids.
Andrew Weil, M.D.