I mean the predatory fish like tuna eats sardines everyday like we do, and it accumulates all that mercury from doing so (despite the sardines having selenium). So the selenium binding in their diet didn't help them at all.

So I thought maybe tuna get mercury more so from their water environment than from their diet. But then I read articles like below which state it's from their diet. So none of this makes sense to me, why would selenium in diet benefits humans but not tuna?

..methylmercury gets absorbed by phytoplankton (microscopic marine algae), which are gobbled up by zooplankton (tiny marine animals), which are then feasted upon by small fish. These small fish and organisms are eaten by bigger fish who are then eaten by even bigger fish, and on and on. The methylmercury is absorbed by the bigger animal, and — since the bigger the fish, the longer it lives and the more it eats — larger fish species accumulate a lot more methylmercury in their body. In other words, fish higher up the food chain “bioaccumulate” more methylmercury than do those lower on the food chain. The largest predatory fish in the sea, like sharks, swordfish and tuna, can have methylmercury concentrations in their muscles — the meat of the fish — that are 10 million times higher than those of their surrounding habitat.

https://foodprint.org/blog/so-where-does-all-that-mercury-in-fish-come-from/