How does mercury get into the fish and where does it come from?
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Bioaccumulation.
Mercury exists in the environment for various reasons, mostly burning of fossil fuels. There are some natural ways like volcanos, natural soil runoff, or other geologic activates but mostly from human activity like mining or fossil fuel burning.
Generally levels are higher in older fish that are high on the food chain.
This happens from it building up, say a plankton only has 1 unit of mercury. A medium fish eats 10 plankton, and now has 10 units of mercury in it, then a bigger fish like a tuna eats 2 medium fish, now the tuna has 20 units of mercury in it.
Then because tuna can live relatively long, it eats many more medium fish, and ends up with enough mercury for it to be concerning to humans eating it.
Since we also have long lifespans, so eating 3 tunas, ends up with a all the mercury from billions of plankton.
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