So many fish today — including tuna, grouper, marlin, mackerel and Chilean sea bass — have very high levels of mercury.
How did that happen — and aren’t fish supposed to be healthy to eat?
Fish and mercury absorption
Fish absorb mercury from fresh or sea water when it passes over their gills. When larger predatory fish eat smaller fish, their mercury levels rise because they take on all the mercury their prey had absorbed (this is called bioaccumulation).
By the same token, older predatory fish — those who have been around longer and have eaten lots of the little guys — tend to have the highest mercury levels of all.
Unfortunately, you can’t clean or cook the mercury out — it’s interspersed through every part of the fish.
It’s rainin’ mercury
So why are there high levels of mercury in the water in the first place? Mercury doesn’t naturally occur in bodies of water in any great quantity – just what is delivered by volcanic eruptions, due to the weathering of rocks, and via deep-sea vents.
When mercury gets into the atmosphere, it travels anywhere from a few miles to halfway around the world before being deposited on land and water bodies. As a result, major point sources often deliver mercury both to nearby locations and to the global atmosphere. Mercury falls to earth with rainwater and with dry particles. It lands on water bodies and on surrounding watersheds. It may also be discharged directly into receiving waters by factories or waste sites, although most of these “point sources” have been curtailed or eliminated.
Once introduced to water (oceans, rivers, lakes, etc), mercury is naturally converted to the very toxic methylmercury due to the action of certain bacteria, and now, most fish have at least some methylmercury.