This article has a nice comparison of omega-3 vs. mercury, including the quote below:
Quote:
Salmon, oysters, whitefish, sea bass, freshwater trout, and sardines contain both high levels of heart-healthy omega-3s and low mercury levels, below 0.13 parts per million.
Other low-mercury choices include perch, king crab, flounder, sole, pollock, catfish, croaker, scallops, crawfish, shrimp, clams, and tilapia. They contain less omega-3s, but servings can add up.
Tuna is controversial, because different varieties contain different amounts of both mercury and heart-healthy fats. Canned light tuna contains a small amount of omega-3, about as much as shrimp, and fairly low 0.13 ppm mercury. But fresh tuna steaks and the more expensive canned white or albacore tuna contain three times as much mercury, and almost as much omega-3 as salmon.