Found this article - explaining the food chain - and how it ends up in the fish.
"But about
halfof all mercury released into the atmosphere today comes from the burning of fossil fuels like coal, oil and natural gas, with contributions from waste incineration, mining and other industrial activities. This mercury pollution falls directly into the ocean and other water bodies or onto land, where it can be washed into waterways. In this form, mercury poses little danger because living things can get rid of it quickly. But bacteria convert mercury as it’s carried down from the ocean surface, turning it into a highly-toxic form called methylmercury.
The food chain takes it from there, as methylmercury is absorbed by phytoplankton, which are gobbled up by zooplankton, which are then feasted upon by small fish and onwards and upwards as the amount of the toxin grows in ever-accumulating quantities. The largest predatory fish in the sea, like sharks and swordfish, can have mercury concentrations in their muscles – the meat of the fish – that are 10 million times higher than those of their surrounding habitat."
www.gracelinks.org/blog/584/so-where-does-all-that-mercury-in-fish-come-from