"It appears that the feed used for the farm-raised salmon, concentrated fish products, is the source of the chemical contaminants, while the wild salmon get their food from disparate sources," says Barbara A. Knuth, Cornell professor of natural resources and a co-author on the study. Ronald A. Hites, professor of public and environmental affairs at Indiana University, is the study's lead author.
The scientists analyzed more than 2 metric tons of farmed and wild salmon, gathered from North America, South America, and Europe, to test for carcinogens.
About 600 individual whole salmon were purchased from wholesale distributors and filleted, while nearly 150 more fillets were purchased from retail grocers in major American, Canadian and European cities. All samples, from the farmed and the wild, were analyzed by gas chromatographic, high-resolution mass spectrometry.
Concentrations of all 14 contaminants tested were significantly higher in the farmed salmon raised in Europe than in salmon raised in North and South America, researchers found.
In the comparison of farmed salmon to wild fish, 13 of 14 organochlorine contaminants such as PCBs, dioxins, toxaphene and dieldrin, were higher. Toxaphene and dieldrin are pesticides that have long been banned in Europe and North America but persist in the environment for an extended period.
Steven J. Schwager, Cornell associate professor of biological statistics, says that in designing the research, he knew it would be controversial and sought to maintain the integrity of the results by gathering the data with scrupulous care. "In this kind of research, it is easy to collect too few observations," he says.
The scientists gathered information that is representative of the salmon available to consumers. "We wanted to make the research was scientifically neutral and above reproach," says Schwager. He and his colleagues ensured the soundness of the data by obtaining salmon from a diverse group of wholesalers and retailers, while also purchasing fish from the Pacific Northwest, Europe, South American and eastern North American waters. "This was a bigger challenge than it sounds," he says.
In addition to Knuth, Schwager, and Hites, the research titled "Global Assessment of Organic Contaminants in Farmed Salmon" was authored by Jeffery Foran, University of Michigan; David O. Carpenter, University at Albany; and Coreen Hamilton, AXYS Analytical Services Ltd., British Columbia, Canada. Funding for the study came from the Pew Charitable Trusts.