The smaller the fish, the less mercury it has. The ones you have to watch out for are the big fish that eat lots of medium fish that in turn eat lots of smaller fish; each stage in the food chain concentrates more and more mercury. Sharks and tuna have a lot of mercury. And smaller tuna, like skipjack, have less mercury than larger tuna.

Also I think the threat of mercury in fish is greatly overstated anyway. Look at Japan; they eat tons of fish and they're not all brain damaged dummies.