Mercury poisoning doesn't cause autism, in case you were wondering. It does, however, cause other neurological issues, especially in very young kids.
Ocean ecosystems have very long food chains, and environmental toxins such as mercury get concentrated with each step of the food chain. For that reason, it's recommended that pregnant women and young children avoid eating fish that are near the top of their food chains (predatory fish, and especially those who eat other predators). Fish that are closer to the bottom of the food chain, such as plant-eating fish, have much less mercury and are fine to eat.