There are several different ways you could approach this. For your thesis statement, consider which direction you want to take. If you're looking to show how this is bad for the environment, you'll want to consider statements regarding how the mercury gets into the fish. If you're focused on how mercury in fish is bad for human consumption, focus your statement on how much mercury (if any) is safe, how much fish can (or should) be eaten, or pick a particular group on which to focus (such as children or pregnant women). Then you can narrow it to something such as "women who are pregnant should avoid eating more than (amount) of (what kind of) fish per (time period) because of the concentration of mercury in that fish. Make sure your thesis is specific, but not so detailed or specific that you can't find enough information on it or make your paper long enough to meet your requirements.
