I love tuna so this question has been bugging me senselessly.

I am aware that larger fish tend to have more mercury concentration in their flesh than their prey, however, I don't understand how this can be possible.

I understand that there will be more mercury in the large fish than the small fish as it collects in the large fish over time, but why isn't it distributed across a much greater volume of animal tissue, therefore keeping the overall concentration the same?

The only thing I can think is that the larger fish have some reduced capability for excreting mercury. Whatever the reason, why don't we hear about it? Why is it always "bioaccumulation" this or "biomagnification" that?


1 Answer 1


Bioaccumulation occurs when organisms aren't able to excrete/eliminate/metabolize something as fast as they take it in. The specific circumstance of predators higher on the food chain accumulating even more is called biomagnification.

Every organism needs to eat more than it weighs to grow, as a simple consequence of conservation of energy. If you have a predator fish that eats smaller fish, it will have eaten far more than 10 kg worth of smaller fish by the time it grows to 10 kg in size. If it ate 100 kg worth of prey fish that each had 1 unit/kg of mercury and it can't excrete mercury effectively, it would now have 10 units/kg of mercury. The biggest fish eat prey that already experience the biomagnification effect in their own diets, taking the overall effect even further.

  • 3
    $\begingroup$ You hit the nail on the head! It didn't occur to me that the fish would have to eat substantially more than their body weight to grow (duh). $\endgroup$ May 20, 2022 at 15:06
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    $\begingroup$ @JordanRegan This is also the same reason you get more food by eating the vegetables that an animal would otherwise eat than eating the animal itself. $\endgroup$
    – DKNguyen
    May 21, 2022 at 1:53
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    $\begingroup$ [+1] Now I learnt the difference between "bioaccumulation" and "biomagnification". $\endgroup$ May 21, 2022 at 10:33

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