The mercury "scare" is evidently very real, not imagined. A little chart I had never seen before is beneficial: Nearby anthropogenic sources, such as coal burning and mining of iron, can contaminate water sources with methylmercury, which is efficiently absorbed in the bodies of fish. Through the process of biomagnification, mercury levels in each successive predatory stage increase. Source: https://en.wikipedia.org/wiki/Mercury_in_fish
There used to be some gold mines in north Georgia. They recommend not eating the largest (oldest) fish caught in some of the lakes. In fact the Georgia EPA puts out a booklet, Guidelines For Eating Fish From Georgia Waters, on how many, and what kind of fish you should eat from which lakes, because of different contaminants.
As far as the oceans go, China is by far the biggest producer of methyl-mercury. Of course, they were exempted from the Kyoto treaty, as they are a "developing nation" just as India. They won't be restricted until 2030 or something.
A couple of decades ago, I got mercury poisoning from just eating too much tuna fish out of the can. It just became my lunch for work and I ate it almost every day in a snack box that came with crackers. So, I know lots of things we eat have hidden dangers if we aren't aware of them.