There is actually a correlation: large, predatory fish accumulate the most mercury because they're highest on the food chain, and they also tend to be rare and

There is actually a correlation: large, predatory fish accumulate the most mercury because they're highest on the food chain, and they also tend to be rare and slow to breed, and hence more endangered. With farmed fish it basically depends on where it's farmed. It can be very healthy and sustainable, or quite terrible.