Earlier quoted context omitted.
That's what happens when you're effectively an apex predator - you eat the things that eat the things with contamination and much of it gets concentrated into you, particularly the things that bioaccumulate over time. Outside of pesticides but similar, that's why it's important to limit things like tuna consumption particularly from larger species that can build to high mercury levels.
IIRC, there has never been a case of mercury toxicity from normal consumption of commercial seafood recorded in any American medical journal. These larger species you mention could exist outside of that sphere.
Fatigue, insomnia, and a host of other symptoms are fairly common and not easy to spot as a part of a bigger problem.
Anecdata: when I went to a large hospital they laughed off my idea that I have heavy metal poisoning and didn't even have the facilities to test for it. Only months later after visiting a naturopath and getting tested did my suspicions get confirmed. This case will never be on any official health records.