If tuna is hatched and raised in non-mercury containing water, does that make it mercury-free?



Will heavy metals travel from parent to offspring? If not, why aren’t fish that contain heavy metals hatched in places where water can be monitored?

In: Biology

Mercury accumulates through the food chain, so if it’s fed non-Mercury containing fish, it won’t accumulate it. Tuna can’t be farmed, they are wild caught.

It’s only in the grand sense that tuna collect mercury from the water. Tuna get the mercury from their food. The mercury absorption happens at the bottom of the food chain. Mercury is normally very rare in the environment so pretty much no living things have evolved ways to get rid of it. This means it builds up over the animals lifetime, and is what the term ‘bioaccumulation’ means. Tuna are top predators and both live a long time and eat things that have accumulated mercury themselves.

To get mercury free tuna you would need to give them food from an entire food chain raised in special low mercury water which would be outrageously expensive. To avoid mercury, move down the food chain. A pound of sardines that only ate plankton and lived for nine months will have less mercury than a pound of a tuna that spent 10 years eating those sardines.

Tuna accumulates mercury from seawater, where it’s found in trace amounts, and food (think all the water ever filtered by many smaller fish eaten). A minute amount of mercury can be there from the egg, but that’s nothing compared to how much it will store over the years of growth.

Tuna are an ocean predator, not some calm aquarium fish. You have to feed them fish, and those fish are how the tuna picks up mercury, not from the water they swim in.

Tuna cannot presently be farm raised. They are huge fish that live in the open ocean, where they roam very long distances. The closest thing to captive raised tuna are open ocean “farms” that are large netted areas that keep the tuna in one area. The nets are open enough for other fish to swim through, which feed the tuna.

Tuna are very high on the food chain. They are apex predators that rely on a pretty large food web to survive. Because they are such active swimmers, they don’t thrive in enclosed spaces.

So, that’s why we can’t raise mercury-free tuna.

The mercury comes from their food. Mercury is hard for living tissue to get rid of. Once it gets into your fatty tissues, it tends to stay there. The organisms lowest in the food web that are filter feeders accumulate some low levels of mercury from the environment. The predators that eat them take in all that mercury and can’t get rid of it. Since they eat a lot of those small organisms, they accumulate much more mercury than their food does individually.

Larger predators eat those predators and accumulate even *more* mercury, and so on. Each step up in the food web means more mercury accumulates. Since tuna are pretty much at the top of the food web, they end up consuming a lot of mercury, which ends up in their own tissues. They also have a lot of oil and fat, which is where the mercury ends up, so they can hold a lot of it.

If it were possible to raise tuna without any of these natural food sources, yes, we could raise tuna to be much lower in mercury. However, since they are so high in the food web we would first have to raise their food source without much mercury, which itself would have its own food that would have to be raised without mercury. Since we’re already overfishing the oceans just to feed ourselves, it wouldn’t be feasible to grow that much food just to feed the food that feeds our food.