On the one hand, farmed salmon has more heart-healthy omega-3 fatty acids than wild salmon. On the other hand, it also tends to have much higher levels of chemical contaminants that are known to cause cancer, memory impairment and neurobehavioral changes in children. What's a consumer to do?

In general, a new study shows that the net benefits of eating wild Pacific salmon outweigh those of eating farmed Atlantic salmon, when the risks of chemical contaminants are considered, although there are important regional differences.

Those are the conclusions of Barbara Knuth, Cornell professor of natural resources who specializes in risk management associated with chemical contaminants in fish, and Steven Schwager, Cornell associate professor of biological statistics and computational biology and an expert in sampling design and statistical analysis of comparative data. The two have co-authored a benefit-risk analysis of eating farmed versus wild salmon in the Journal of Nutrition (November, Vol. 135).

"None of us [study authors] argues that the benefits of salmon are not real. But the dirty little secret is that there are risks," said Schwager, noting that even taking into account the risks, the benefits of salmon may be particularly worthwhile for some groups.

"For a middle-aged guy who has had a coronary and doesn't want to have another one, the risks from pollutants are minor ones, and the omega-3 benefits him in a way that far outstrips the relatively minor risks of the pollutants," he said. "But for people who are young -- and they're at risk of lifetime accumulation of pollutants that are carcinogenic -- or pregnant women -- with the risks of birth defects and IQ diminution and other kinds of damage to the fetus -- those risks are great enough that they outweigh the benefits."

Knuth added: "Because we found regional differences in contaminants in farmed salmon, with Chilean salmon showing the lowest levels and European (particularly Scottish) farmed salmon showing the highest levels, careful consumers with a history of heart disease could choose farmed salmon from Chile for their high omega-3 content and relatively lower level of contaminants." She noted that farmed salmon from North America would be a better second choice than European farmed salmon.

The researchers' benefit-risk analysis showed that consumers should not eat farmed fish from Scotland, Norway and eastern Canada more than three times a year; farmed fish from Maine, western Canada and Washington state no more than three to six times a year; and farmed fish from Chile no more than about six times a year. Wild chum salmon can be consumed safely as often as once a week, pink salmon, Sockeye and Coho about twice a month and Chinook just under once a month.

In a study published last spring (Environmental Health Perspectives, May 2005), the research team reported that the levels of chlorinated pesticides, dioxins, PCBs and other contaminants are up to 10 times greater in farm-raised salmon than in wild Pacific salmon, and that salmon farmed in Europe are more contaminated than salmon from South and North American farms.

The team also published a study this fall (Environmental Science and Technology, Vol. 39:8622) that found that farmed salmon, on average, contain roughly two to three times more beneficial fatty acids than wild salmon, presumably because of the differences in the diet on which the fish are raised.

"Our results also support the need for policy and regulatory efforts to limit pollution of our waters and clean up pollution that has occurred, and thus ultimately reduce the risk side of this equation by reducing the potential for human exposure to these contaminants," said Knuth, adding that the country of origin of fish sold should be clearly labeled so consumers can make informed decisions.

Other co-authors of the risk-benefit study include lead author Jeffrey Foran, University of Illinois-Chicago; David Carpenter, University at Albany; David Good, Indiana University; and Coreen Hamilton, AXYS Analytical Services Ltd., British Columbia, Canada. The study was funded by the Environmental Division of the Pew Charitable Trusts.