A new study published in Environmental Toxicology and Chemistry finds that although mercury levels in both wild and farmed salmon from British Columbia are substantially below human health consumption guidelines, the levels found in wild salmon were three times higher than in farmed salmon.

A large proportion of the farmed salmon consumed in the United States originates in British Columbia, Canada. Over the years, there have been health concerns because high levels of methylmercury have been found in long-lived fish species nearer to the top of the food chain – such as tuna and salmon. High mercury levels have been associated with an increase in the risk of cancer, and this has led many people to avoid consuming certain fishes.

This most recent study has determined that levels of mercury and other trace metals measured in both farmed and wild salmon were significantly below Health Canada’s consumption guidelines. Compared to wild salmon, the researchers found that farmed salmon did not have significantly higher concentrations of metals such as arsenic, cobalt, copper, or cadmium. The threefold higher mercury concentration observed in the flesh of wild salmon than in farmed salmon is potentially explained by farmed salmon’s low gastrointestinal absorption efficiency, its negligible transfer of metals to muscle tissue, and its rapid growth cycles (growth dilution). In farmed fish, there were no differences in metal levels found between pre- and post-processing.

For comparison to other parts of the human diet, the researchers indicate that total mercury levels were slightly higher in wild or farmed salmon than in chicken, beef, or pork and about the same as in fruit, vegetables, honey, and eggs. Compared to other foods, salmon contains lower levels of other trace elements. The average dietary intake of mercury and trace metals from salmon still remains a paltry 0.05% to 32% compared to the 68% to 99% that is absorbed from meat, poultry, fruit, and vegetables. Salmon also contains its own protection against mercury in the form of the element selenium. The moderate surplus of this metal can counteract mercury’s toxicity.

“Estimates of human dietary exposure indicate that human health risks associated with trace metal exposure via consumption of farmed and wild British Columbia salmon are negligible,” conclude the authors. “The current scientific evidence therefore supports the weekly consumption of oily fish species (including all British Columbia salmon sources) as recommended by the American Heart Association.”

Mercury and Other Trace Elements in Farmed and Wild Salmon from British Columbia, Canada
Barry C. Kelly, Michael G. Ikonomou, David A. Higgs, Janice Oakes, and Cory Dubetz
Environmental Toxicology and Chemistry (2008). Vol. 27(6):1361-1370.
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Written by: Peter M Crosta