Mercury poisoning occurs when exposure with the element Mercury (Hg) occurs. It can cause insanity, weakness in sight, change in sensations, and impairment of certain abilities. Extreme exposure to this element could affect your kidney, cause respiratory failure, and in some cases death.
In fish mercury poisoning causes inability to produce many offspring.
Mercury pollution mainly occurs with coal powered plants when they burn coal, it goes into the air. After some time, the mercury gets rained down and runs into rivers and oceans. The mercury then gets exposed to the fish and they get poisoned. After fishermen catch the fish, they still contain mercury. That mercury then gets consumed when eaten.