A study claims that

Farm-raised salmon, a growing staple of American diets, contains significantly higher concentrations of PCBs, dioxin and other cancer-causing contaminants than salmon caught in the wild, and should be eaten infrequently

Not surprisingly, fishing industry officials immediately took issue with the findings, but then so did the Food and Drug Administration. Moreover, the study was funded by the Pew Charitable Trust, which is enamored of such causes. Moreover, the researcher David O. Carpenter has convinced himself that PCBs are evil:

A lot of my work focuses on some of the detrimental effects PCBs have on IQ. PCBs cause many of the same kinds of problems that exposure to lead does. We see children with ADHD, shorter attention spans and more disruptive school behavior. PCBs are carcinogenic, and the resulting cancers are seen primarily in adults.

Asked if there a "toxic" level of exposure to PCBs, or a threshold amount at which health is affected, he says,

I interpret the evidence to suggest that there is not a threshold effect. If it's not normally present in the body, it's not doing anything that's good for you.

What happened to "the dose makes the poison?" As I understand it, PCBs aren't proven human carcinogens:

Several regulatory and advisory agencies have categorized PCBs as animal carcinogens; however, studies of workers exposed to high doses of PCBs over long periods of time have not demonstrated an increased cancer risk. In fact, the only health effects that could be attributed to PCBs were skin and eye irritation.

Of course, that's from the American Council on Science and Health, whose opponents will claim you can't believe because it's industry funded.