Most mercury in fish is methylmercury , a highly toxic substance that can build up in predatory fish such as walleye, swordfish, and tuna, and in animals that eat these fish. Methylmercury can damage the brain, nervous system, and kidneys. The risk is probably very low for adults who eat fish only occasionally. The risk is greatest for developing fetuses, children, and people who depend on sport fish for food. Mercury also threatens the health of fish-eating wildlife such as loons, eagles, otters, and raccoons.
Some evidence suggests that at very high concentrations it harms the fish themselves.
And, mercury-contaminated sport fish may adversely affect a local economy that depends on recreational fishing. It's the amount that counts Your health risk depends on your age, weight, and health, in combination with how much fish you eat, how often you eat fish, and how much mercury is in the fish. You can reduce your health risk from mercury-contaminated fish by following recommendations for eating local sport fish. All waterbodies contain some mercury, but not all sport fish are tested for mercury. They're one of the most popular catches at Lake Roosevelt.