In terms of mercury biomagnification (wikipedia) works like this. Organisms absorb mercury very efficiently, but it takes far longer to excrete. Algae absorb mercury from seawater readily, so even small amounts in the water are absorbed and retained. Fish eat the algae, bigger fish eat the smaller fish, and so on. At all levels the mercury is retained (in fatty tissues) at a far better rate then it is excreted. Ultimately it accumulates and amplifies in predator fish like swordfish and sharks as well as birds of prey like eagles and osprey. And, of course, at the apex of the food chain is humans. A mother breast feeding would, of course, pass it on in concentration to her baby.
See fish you should avoid eating (very much of) here.
It turns out that we are building up pollutants at the bottom of the ocean at an ugly and alarming rate. This is emphatically demonstrated by a recent study in Nature. An easier short read is in the WSJ by Kincaid. Animals at the bottom of the ocean (4 miles deep) had amazingly high concentrations of pollutants, 50 times more than one of the worst polluted rivers in the world (in China). The pollutants included chemicals that don't naturally decay but have not been produced (much) in decades. These POPs should mostly be in landfills; they were largely used in electronics. But they will continue to reek havoc to the environment for centuries to come.
With about 65% of POPs in landfills, there's a huge amount out sloshing around in the environment, all the way from the North Pole to the Mariana Trench. Plus, as the landfills fail, as they always do, eventually, ... That's ugly... so very ugly...
Sorry, no positive spin for this. Not enough lipstick to cover this ugly pig.