We’re supposed to be careful about not eating too much tuna because of all the mercury they accumulate by being very high up on the food chain. But if the mercury level in tuna is risky for us, isn’t it even riskier for the tuna themselves? Are tuna showing any signs of mercury poisoning?
There may be some long-term effects that haven’t shown up or been noticed yet, but given the fact that millions of adult tuna appear to be living “the good life” in the middle of the ocean, at least until they get caught, leads me to believe that any ill effects mercury may cause aren’t that serious. Someone will come along soon to tell me I’m full of it.
Full of tuna, mercury or something else? 
From The Yale School Of The Environment:
Mercury’s Silent Toll On the World’s Wildlife - Yale E360
" While mercury doesn’t kill many animals outright, it can put a deep dent in reproduction, says David Evers, chief scientist at the Biodiversity Research Institute (BRI), who serves on a scientific committee informing the process. “It is a bit of a silent threat, where you have to kind of add up what was lost through studies and demographic models.”
Harmful levels of mercury have turned up in all sorts of animals, from fish and birds living around the world to pythons invading the Florida Everglades and polar bears roaming far from any sources of pollution. In recent years, biologists have been tracking mercury’s footprints in unexpected habitats and species. Their research is illuminating the subtle effects of chronic exposure and is showing that ever-lower levels cause harm."
If the mercury levels in tuna gave 10% of them mild cognitive issues and reduced peripheral nerve sensitivity (I’m making up effects here) and consumption led to 1% of consumers getting the same, we probably wouldn’t notice the first, but still be concerned about the latter.
@Czarcasm’s cite includes this description of effects on wildlife:
In the earliest studies of these sublethal effects in the 1970s, Heinz reported that captive mallards fed mercury-laced food laid fewer eggs than control ducks and laid them outside the nest. Also, their ducklings didn’t respond well to their calls. Numerous examples have accumulated since. Fish form loose, sloppy schools and are slow to respond to a simulated predator. Several bird species sing different songs. Loons lay smaller eggs, and they incubate their nests, forage, and feed their chicks less. Salamanders are sluggish and less responsive to prey, Hopkins and colleagues found. Egret chicks are similarly lethargic and unmotivated to hunt.
The population loss from such issues would probably be more visible if we weren’t doing such intense work to reduce population size and diversity in myriad other ways.
The problem with mercury is that it’s a bioaccumulant. Ingest some, and it stays in you nearly permanently. And this means that it tends to get concentrated more, the higher up the food chain you go.
It starts with a little mercury in the water. Plankton ingests the water, and gets a little mercury in it, but not much. But then the plankton get eaten by small fish, and the fish ends up with all of the mercury from all of the plankton it’s eaten. And then those small fish get eaten by larger fish, and the larger fish ends up with all of the mercury that was in all of the smaller fish it ate, and so on. Humans, of course, are very high on the food chain. You’re not getting one tuna’s worth of mercury; you’re getting all of the mercury from all of the fish you ever eat.
And then there are other factors that make it even worse for humans: First, one of the major effects of lead poisoning is cognitive impairment, and cognition is kind of our major schtick. Second, we care a lot more about what happens to humans than we do about what happens to other animals.
Said another way, mercury does to animals just about what weed does to humans. Lazy stoners the lot of 'em. 
Bluefin tuna lifespan is 15-30 years, if they aren’t caught first.