How are mercury batteries a hazard to the environment?

If not disposed of correctly they can leech Mercury into the environment this, through a process of oxidation, is absorbed or taken in by fish and shell fish and poisons them, It then poisons the birds and animals (e.g. Humans) that eat those animals

http://www.environment.gov.au/atmosphere/airquality/publications/sok/mercury.html

more there and around the place if you look hard