Chemicals in salmon vary by species
Some farmed and wild salmon from British Columbia differ only slightly with respect to their contaminant loads, but species can be very different.
Farmed salmon from the Pacific Northwest compare favorably with wild-caught salmon in terms of their drawbacks and benefits, researchers conclude in an assessment published today on ES&T’s Research ASAP website (DOI: 10.1021/es060409). The team’s comparison, however, notes that some species may be better to eat than others, whether farmed or wild, because they have acceptable levels of contaminants but higher concentrations of beneficial compounds such as omega-3 fatty acids.
The researchers conclude that farmed Atlantic salmon from British Columbia tend to have the most PCBs, whereas wild pink salmon and chum have the least. Farmed chinook and coho had PCB levels comparable with those of wild salmon. However, farmed Atlantic salmon provide the most fatty acids, the team finds. David Higgs, a nutrition scientist from Fisheries and Oceans Canada and a coauthor of the new research, calls the analyses “far more comprehensive, especially for farmed salmon species,” than any previously published.
One of the first surveys documenting toxic compounds, PCBs in particular, in wild versus farmed salmon appeared in Science in 2004. That research showed that salmon from the Pacific Northwest are cleaner than Norwegian and other fish farmed in the North Atlantic (farmed Chilean fish are the cleanest). Atlantic salmon, Salmo salar, are the most common species farmed, and they carry the most PCBs and their congeners.
In the new ES&T study, Michael Ikonomou of Fisheries and Oceans Canada and colleagues took a closer look at various salmon species harvested in British Columbia. They conducted laboratory analyses of skinned fish fillets taken from three farmed and five wild-caught species, harvested from different places. The levels of PCBs and other contaminants such as mercury fell well below standards from the U.S. Food and Drug Administration, the U.S. Agency for Toxic Substances and Disease Registry, and Health Canada. The team found negligible mercury levels.
“Species differ tremendously in terms of life histories and biology,” says Sandie O’Neill, a research scientist with Washington State’s Department of Fish and Wildlife. O’Neill stresses that everything from marine distribution—something not considered in the new ES&T research—and diet to the life expectancy of different species contributes to how much of a toxic compound an animal accumulates in its tissues. That biological variability can contribute to “huge differences in the amounts of contaminants that are accumulated. This current paper addresses some of that,” she says. The data collected by Ron Hites and colleagues that was published in 2004 in Science lumped wild-caught species together, she notes, missing such variations.
Salmon in Puget Sound, for example, are more contaminated than coastal fish, O’Neill points out, probably related to the migration patterns of the fish. That difference could lead to slightly different human health risks, and Washington’s health department recently released a guide by species and location for people eating fish caught in the state. Rob Duff, director of Washington’s Office of Environmental Health Assessments, notes that the new results from Ikonomou’s team are consistent with the state’s fish guide. However, “we don’t support FDA’s 2-parts-per-million limit [for PCBs],” Duff says. “It’s just much too high.”
Action levels for PCBs set by U.S. and Canadian regulatory agencies are not protective of human health, agrees Jeffery Foran, president of Environmental Science and Health International LLC, a consulting firm. Foran, who was a coauthor on the initial Science report and lead author on a recent statistical risk–benefits analysis published in the Journal of Nutrition, also objects to how Ikonomou and co-workers report PCB congeners. By separating them out instead of combining them, he says, they miss a fuller picture of risk. The World Health Organization recommends calculating the total toxic equivalents of PCBs.
In the end, Foran says, the risk analysis performed by Ikonomou’s team is not rigorous enough. For example, “with regard to their attempt to compare the benefits to the risks,” no evidence exists “just based on a regular diet that we [humans] lack adequate intake of omega-6.” The authors “lump in” the more prevalent omega-6 with the harder-to-get omega-3 essential fatty acids in their assessment of the benefits of eating fish. Ikonomou and his team note that they “provided levels of omega-6, omega-3, as well as the important highly unsaturated members of the omega-3 family of fatty acids, EPA and DHA, for all sources of salmon, unlike previous work published.”
Although contaminant loads trigger controversy, researchers agree that consumers should not stop eating fish, particularly salmon, because the fish are quite high in omega-3 fatty acids, which have been linked to preventing cardiovascular disease. Rigorous statistical analyses of the benefits and risks of eating salmon have appeared in a series of articles in the November 2005 American Journal of Preventive Medicine and elsewhere, and many reviews further support the benefits, such as in the October 18 Journal of the American Medical Association.
“It’s clear that people need to eat fish,” Foran says, “but people need to make informed choices about which fish they eat.”
