How does mercury get into the fish and where does it come from?
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Tiny mercury particles in the water get absorbed by plankton.
Small fish eat the plankton.
Bigger fish eat the smaller fish.
The bigger the fish, the higher the amount of mercury built up in it.
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.
Just adding that some fish farms feed fish to their stock, and this can lead to particularly high levels.
Mercury gets into lifeforms from what they eat, and the “higher up” in a food chain they are (the more types of meat they eat) the worse the mercury gets.
In its pure form, mercury isn’t too big of a deal to most things. It’s just a heavy liquid. But if it evaporates (which it is always doing, slowly; don’t mess with pure mercury without a professional) or combines with certain chemicals, it changes into a form that can mess up important chemical reactions in a living body. The times in life with the most delicate bodies (children, their creation, already sick, old age, etc.) can have the balance tipped away from healthy with this interference, and with enough even healthy things become sick. This is why mercury is bad for organics.
The mercury in the ocean has combined with some of those certain chemicals, which is why it can stay ‘floating’ in the water, but still in a form and amount that just being in ocean water doesn’t have enough of it to bother most things. It mostly comes from industrial era pollution, and it will stay in the ocean for a long time to come.
While simply being in the water isn’t a bother, there is enough of the ocean mercury that it gets into plants when they absorb water for photosynthesis. When this happens, it stays there for a long time. Long enough that more mercury is absorbed overtime, and more and more mercury is built up. Then, if the plant is eaten, all of that mercury build up is transferred into whatever fish (or crab or whatever small creature) ate the plant, which then starts storing mercury in a very similar way to the plant. Only, because the plant already concentrated the mercury through its life, the small fish starts out with all of that mercury already and builds it up even faster than the plant. Still only in amounts that are a problem for sensitive fish, but eventually quite a lot.
The same relation of concentration happens to bigger fish eating smaller fish. This is why some types of fish are considered worse for mercury sensitive conditions (like pregnancy); tuna eat a lot of smaller fish that eat smaller fish etc., so their mercury concentration is higher than anchovies that only eat plants and plankton. Tuna also aren’t the largest fish people eat, so something like a halibut or albacore (a type of larger tuna) has enough mercury that even healthy people should be cautious about how much they eat.
Mercury has lots of natural sources, but most of the mercury found in fish comes from the burning of coal, run-off from mining, and industrial waste. It gets into water that washes into the sea where plankton and small fish absorb it. It’s fairly easy to absorb and very difficult to flush out, so it tends to accumulate in living things exposed to it. When larger things eat smaller things, they get even more.
Eventually people eat the fish that’s absorbed mercury and they absorb the mercury themselves. The human body will get rid of the mercury it absorbs, but slowly; it cuts the amount of mercury in your body in half every 50 days or so.
That means, if you eat 100g of fish containing 100 micrograms of mercury, you’ll absorb 100 micrograms of mercury. After 50 days, 50 micrograms will be in your body. 50 days later, it will be 25 micrograms; then 12.5, 6.25, … A year after eating that small piece of fish, you’ll have about 0.625% of the mercury that was in that fish still in your body. If you don’t eat seafood very often, perhaps it’s not a big deal. You won’t notice much in the way of health effects. If you seafood frequently, however, you will build up more and more mercury in your body over time and can get it to levels where it starts to affect your health.
Of course, there are animals that rely on fish as food, and they are constantly building up mercury in their bodies and begin to see toxic effects at some point.
Mercury is liquid. Water is liquid. Mercury can resolve in water. Fish lives in water. Fish absorbs mercury.
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