It takes a LOT of mercury to be harmful. If you lived by fishing near an urban center, I can see it being a problem, but not so much with the occasional wild-caught salmon or trout, or cod and halibut shipped in. While there are toxins, yes, life is just like that. I don't think it warrants freaking out and never eating a wild fish. It does maybe warrant avoiding a total seafood diet (especially in problem areas) and by all means reducing mercury pollution.
By my logic, things in nature generally don't need changing unless there's an obviously real problem with them or you see a clear way to improve them. Say that God doesn't make mistakes, or people have evolved to live that way or whatever. Some things are just part of life. This includes eating meat, the sun, seasonal floods (plan around them, not against them!), and even the OCCASIONAL extinction. Generally we should only be mucking with nature if we mucked with it accidentally in the first place. And our lifestyles in which we have a surplus of food (here anyway) and climate control and medicine clearly let us live longer and happier, so that's something we can change. But IMO, anyone who suddenly discovers that the sun is bad for you or we shouldn't have been eating meat for the last 200,000 years is fear mongering. IE, people who tan and don't burn were usually exposed to the sun and sunburn at a young age.
Now, not to sound like some kind of rambo-hippy, like I said, some changes to our environment are logical. But as a general rule, if no one changes anything, that's what would be 'natural' and we can usually be fine with it, exceptions being the bubonic plague and famine.