Mercury, and its more potent cousin, methyl mercury, is highly toxic and capable of causing serious damage to the developing brain. The most significant exposure pathway is fish and shellfish. But fish is also very healthy food. Fish consumption advisories warn children, pregnant women, and women of child-bearing age to avoid eating certain fish species. These advisories vary from location to location due to differences in the fish available. However, they can also be confusing because government health departments can be more or less precautionary in their advice. Advisories can vary for different age groups, for different fish species and even within the same species of fish, such as different forms of tuna.

Health Canada recommends that pregnant women, women of child-bearing age and young children limit consumption of shark, swordfish and fresh or frozen tuna to a single meal per month. In contrast, the US Food and Drug Administration and US Environmental Protection Agency recommend that these groups completely avoid shark, swordfish, king mackerel and tilefish. However, for fresh or frozen tuna, the US limit is less precautionary, at one meal per week. For fish with a lower mercury content, including canned light tuna, shrimp, salmon, pollock and catfish, the USEPA and USFDA recommend these fish be chosen within an overall limit of no more than two fish meals per week. For white (albacore) tuna, the advice is one meal per week as part of a two meals per week overall limit. Health Canada does not issue advisories for canned tuna or for overall weekly limits on fish consumption. Other fish likely to have lower mercury levels include farmed trout, haddock, tilapia and flounder.