Hites, RA, JA Foran, DO Carpenter, MC Hamilton, BA Knuth and SJ Schwager. 2004. Global assessment of organic contaminants in farmed salmon. Science 303: 226-229.

After analyzing chemical contaminants in over 2 metric tons of salmon from around the world, Hites et al. report that farmed salmon has significantly higher levels of organochlorine contaminants than wild, at least for the sources they studied. Farmed salmon from European sources carried the highest levels of contamination while Chilean was least contaminated, only slightly above levels found in wild Alaskan salmon.

Using EPA risk assessment criteria, Hites et al. calculated that consumption of the most contaminated farmed salmon--from Scotland, the Faroe Islands, and German grocery stores--should be limited to no more than one meal of salmon every two months. This assessment is based only on cancer risk and does not consider other health endpoints of organochlorine exposure, which the authors note can be affected at levels beneath those implicated for cancer.

What did they do? Hites et al. measured organochlorine contamination levels in approximately 700 salmon obtained from 8 major salmon-producing regions Europe, North and South America.

The farmed Atlantic salmon came from eastern and western North America, Europe, and Chile. Wild salmon of five species were obtained from three regions along the US and Canadian Pacific coast. No wild Atlantic salmon were measured because it is rarely available in commercial fish markets, nor were farmed Pacific salmon studied because they are not raised commercially in substantial amounts.

In addition to measuring contaminants in 594 whole salmon obtained from fish wholesalers, 144 filets of farmed salmon were obtained from supermarkets in 16 major cities in North America and Europe.

Samples of food fed to farmed salmon were also obtained for analysis from European, North and South American outlets of salmon food used by the salmon farming industry, and analyzed for contamination.

The research team measured the concentrations of 14 organochlorine contaminants: mirex, total DDT (DDT plus DDD plus DDE); endrin, dieldrin, cis-nonachlor, trans-nonachlor, a-chlordane, ?-chlordane, heptachlor epoxide, lindane, hexachlorobenzene, toxaphene, PCBs and dioxins (including dibenzofurans and dioxin-like PCBs). They then compared farmed vs. wild salmon, overall as well as between regions, both in terms of their contaminant loads as well as what these measurements mean with respect to how much salmon people should eat.

What did they find?

• Concentrations of all measured contaminants were higher in farmed salmon from Europe than wild salmon.
• Concentrations in farmed salmon from Europe were significantly higher than farmed salmon from North America or Chile
• Concentration in farmed salmon from Chile were higher than wild salmon in 6 of the contaminants measured, but lower than wild salmon for two of the contaminants (lindane and hexachlorobenzene).
• Pooling all farmed salmon and comparing them to all wild salmon (figure below) , contamination was higher in farmed salmon than wild salmon for all but one contaminant (lindane).

In the figure to the left, all concentrations are in nanograms/gram except for dioxins, which are in picograms per gram The figure to the left displays concentrations on a log scale. The 3 vertical lines for each contaminant represent, respectively, the 10th, 50th and 90th percentiles, while the boxes show the 25th and 75th percentiles.

In a more detailed analysis of dioxins, PCBs, dieldrin and toxaphene, Hites et al. determined additionally that these contaminants were:

• highest by far in farmed salmon from Scotland and the Faroe Islands.
• lowest in farmed salmon from Chile and Washington state.
• Higher in farmed salmon, even the least contaminated from Chile and Washington state, than from wild salmon.
• highest in farmed salmon fillets bought in supermarkets in Frankfurt, Edinburgh, Paris, London and Oslo, although farmed salmon from Boston and San Francisco approached the levels found in European-purchased farmed salmon.

Hites et al. found concentrations of contaminants in the salmon food similar to those levels found in farmed salmon. They conclude that the large differences between farmed and wild salmon are most likely caused by farmed salmon being fed contaminated food. Salmon food is manufactured mostly from small pelagic fishes obtained within the region where the food is used. European salmon food was significantly more contaminated than North American and South American salmon food, paralleling differences in contaminant loads in the farmed salmon.

What does it mean?

These calculations indicate that to keep cancer risks below the EPA recommended threshold, people should eat Scottish and Faroe Island less often than once every other month. In contrast, some stocks of Alaska wild salmon can be eaten as often as 8 meals a month.

These calculations assume the risks of the contaminants are additive. They only take into account cancer risks, not possible neurocognitive, immune or endocrine disrupting effects, which Hites et al. note "occur at lower concentrations than those implicated in cancer."

The calculations also do not incorporate the beneficial effects of eating fish such as salmon, rich in nutrients like the omega-3 polyunsaturated fatty acids. Hites et al. acknowledge a dietary trade-off in their discussion of the study's results. "Although the risk/benefit computation is complicated, consumption of farmed Atlantic salmon may pose risks that detract from the beneficial effects of fish consumption.

The results presented by Hites et al. suggest several responses:

• while fish consumption is important for a healthy diet, people wishing to minimize their intact of organochlorine contaminants should favor wild salmon over farmed salmon, and within farmed salmon should seek Chilean-grown over European.
• labeling should allow consumers to identify the origin of salmon sold in markets
• salmon farmers should place a high premium on finding uncontaminated sources of salmon food.