Mercury concentration of the sedentary fishes in Minamata Bay in 1985

Methylmercury in sea water is taken into aquatic animals through gill or intestine directly.
On the other hand, the concentration of mercury tends to be higher as the trophic ranking be higher.
It is called bio-accumulation.

The figure shows relation between trophic ranking mercury level of sedentary fishes in Minamata Bay when mercury-polluted sludge still remained in the bay.