Just what this may mean for human health is murky, given that the risk appears to be small and that the health benefits of eating salmon, whether farm-raised or caught in the wild, are thought to be considerable.
But the message to fish farmers was unmistakably clear: Stop feeding your penned-up salmon the fish meal that seems to be causing the problem.
The bad tidings about farmed salmon came via a scientific report published recently in the journal Science. The researchers sampled 700 salmon from around the world and found that concentrations of organochlorine chemicals were far higher in farmed salmon than in salmon caught in the wild.
They also found marked geographical differences; farmed salmon from Europe was more contaminated than farmed salmon from North or South America. These differences can be traced to feeding the salmon ground-up smaller fish that have themselves been contaminated with toxic chemicals from polluted ocean waters.
Focusing on polychlorinated biphenyls, or PCB's, the researchers suggested that consumers should probably limit themselves to one meal of farmed salmon per month.
They reached that conclusion by applying a formula developed by the U.S. Environmental Protection Agency to estimate the cumulative impact of ingesting several chemicals at once. But this looks like a case where risk estimation has outrun common sense.
The one-meal-a-month advice seems at odds with the standard recommendation from heart experts that people should eat two fish meals a week, preferably fatty fish like salmon, tuna or mackerel, to help combat cardiovascular disease.
What's more, the contamination levels found in the new study were well below the tolerance levels for some individual pollutants set by the U.S. Food and Drug Administration and must be viewed against the background of a huge drop in recent decades in the amount of PCB's, dioxins and similar chemicals found in the food supply.
Cooking the salmon and trimming off its skin would reduce the contaminant levels significantly.
Those who worry about toxic chemicals, especially pregnant women, may want to pay extra for wild salmon or limit their consumption of farmed salmon, always aware that many other foods contain these same contaminants as well. But those who are more concerned about healthy hearts will see no reason to give up this tasty source of nutrition.
The real message of this study is that the fish farming industry needs to clean up its feeding materials to reduce the level of contaminants.
It would also be desirable for salmon to be labeled clearly to show whether it was farmed or wild, and where it came from. That would help consumers make wise choices and put pressure on the dirtier parts of the fish farming industry to clean up.