"Factories, apparently, dump lots of mercury in the ocean. What's up with that? In a worst case scenario, Japanese scientists found that one piece of legally "harvested" whale meat potentially contains a shocking 5000 times the legal limit of mercury. But if you can't afford whale sashimi, you can work your way up to it by eating lots of dolphins (not the Flipper kind, though). A single exposure to mercury saturateed whale meat could be hazardous even for an adult, by the way. For the little ones, mercury can (and does) lead to irreversible brain damage among other things.
The good news is that salmon, the fish we eat most in Japan, is generally lower in mercury than other kinds of fish. The bad news, I learned, is that the orange colored water that washes off cheap salmon is dye. Apparently, krill don't naturally swim into the net enclosures where farmed salmon spend their lives, so they eat pellets instead. Salmon that eat krill have nice, pink meat. Salmon that eat pellets have pale meat which consumers don't like. To solve the pale meat problem, dye is added to the pellets. Wellah, oishii pink salmon!"
--Andy Gray--from Japan Window.com