I’d turn red and shrug my shoulders, muttering something like, “I don’t know, depends where it comes from, I guess.” Truth was that I studied the impacts of chemicals on reproduction in fish not humans, so really, I could only answer as an expert for fish concerned about their reproductive health.
But even for those who study the human health impacts of chemicals, the issue of evaluating the risk associated with contaminants in seafood has always been tricky. Risk from contaminant exposure depends on the contaminant, the particular health effects associated with the contaminant, the species of fish (some fattier than others), the age of the fish, where it was caught (if wild), if farmed, what it was fed, how much one eats fish how often, and even on who’s eating the fish!
An recent analysis by Sam Luoma and Ragnar Lofstedt titled "Contaminated Salmon and the Public's Trust" published in Environmental Science and Technology addressed the complexity of that simple question “Should I get the fish?” and “If so, what kind?” They refer to a study published in Science several years back, which reported on concentrations of PCBs and similar chemicals in farmed and wild salmon, and which reported that farmed salmon were, in general, more highly contaminated than wild-caught salmon.
According to Luoma and Ragnar, this set off a “contentious dialogue….mostly because the risk analysis for salmon did not consider a balance of risks,” the end result (at least for a time) was a drop in consumer confidence for farmed salmon resulting in a heavy burden on wild salmon populations.
While the authors don't answer the question "to eat or not to eat", they do provide an interesting discussion about communicating and evaluating risk for complex scientific issues, even ones that seem simple, check out it.