Pollution can do several things to animals--it can kill fish and bugs, cause birds beaks to be deformed, reduce egg production and animal reproduction, and
eliminate species all together. People study the "gross" effects on animals, such as deformed beaks, and thenstudy the
sublethal effects on reproduction and try to determine the long-term effects on the population by species, and by the "food chain"
groups. They also know that certain chemicals accumulate in fish, in their fat, and that humans should not eat fish from the 
Great Lakes daily. There are guidance documents that show how often to eat the fish, such as trout, smelt and such.
When humans eat fish that have such chemicals in them, we can measure it in the milk that mothers produce for their babies, and some of these chemicals
are thought to be responsible for cancers and deformities or learning disabilities in babies.