Recent discussion centered on a bass-fishing competition in a nearby lake, and how silly it was to have it there because there was an environmental disaster a while back and people are discouraged from eating the lake fish because of mercury poisoning.
So - the question that emerged is whether the metals impact fish brains like they do human brains, whether that would impact their behavior, whether the amounts were high enough to possibly cause that impact in the first place, and how we would even study that possibility and whether anyone even has.
Related question - can other mammals (dogs, cats, bears) get heavy-metal poisoning and would it make them crazy?
I’m not a fish biologist, so I don’t know the answer directly, but as I understand it, part of the problem is that of bioaccumulation. The idea is that you might eat one fish and be OK, because it only has a little mercury (presumably, in my hypothetical, too little to affect the fish’s health). But as you eat more and more, the toxic stuff is retained in your system and eventually builds up to the point that it affects YOU, even though each individual fish contributed only a little bit.
As Smeghead mentioned, it’s more a danger to us than the fish because, humans being relatively long-lived creatures, the heavy metals can accumulate in our bodies and reach dangerous levels.
That said, animals certainly can get heavy metal poisoning. In fact, pets, being small, will often show signs of heavy metal poisoning in a household before the humans living there will (similarly, children will usually show symptoms before adults). Not only can pets eat paint chips (like children) but pets will often eat fishing sinkers or lead shot (as will children on occasion) and get lead poisoning.
Pets can often act kinda crazy by human standards anyway (our puppy routinely does laps at high speed around the coffee table, for example, and we nicknamed one of our cats Psycho Kitty for a reason), so changes in behavior like sudden aggression or depression are usually the most noticeable. Weight loss and diarrhea are other common symptoms.
I had to poke around on google to answer the fish part, but apparently the symptoms of heavy metal poisoning in fish are gasping, flashing (which I learned today is the fish equivalent of coughing), reddening gills, erratic swimming, and general confusion.
I know nothing about fish (except that salmon is very yummy), I have been advised by several veterinarians to limit the amount of fish I feed our cats. Their small bodies can’t deal with mercury filled fish on a regular basis. I think I’ve also read that pregnant women shouldn’t eat a lot of fish because the mercury could harm the fetus.