While the amount might sound alarming, it’s worth a reminder that there are about 326 million cubic miles of water. To put that roughly, if all the world’s ocean water was a basketball, 150 million gallons would be about the size of a head of a pin.
The real danger, though, is what that amount of mercury could do to the food chain. Salmon would be immediately affected, and would quickly carry toxic (if not immediately lethal) amounts of mercury. Any increase in mercury intake would carry with it sharply increased risks of certain types of cancers.
Source: Mercury making its way into global drinking water thanks to global warming