That is exactly, for instance, the scientifically dishonest approach that David Suzuki and the Pew Trust have taken with their "research" showing farmed salmon have more pcb's than wild salmon. What they really in fact showed was that both farmed and wild salmon have pcb levels many times lower than the FDA's threshold level... and their work bore out what scientists already knew---that different species of salmon (whether farmed or wild) are likely to have slightly different levels of pcb's. But they achieved their "mission," which was to create misinformation and consumer panic.
So that's why the FDA recommended eating farmed salmon no more frequently than 1x/month but made no similar recommendation for wild salmon eh