After filleting a fish in a boat or on shore, why can't I throw a fish carcass back into the water body from which it was caught?

It is illegal to litter due to sanitation and aesthetic concerns. Not all parts of the fish sink, and body parts may wash up a shoreline. If everyone chose to throw carcasses into the water, areas around ramps and popular shore-fishing spots could become a big mess. Some facilities have modern fish cleaning stations that should be used. Otherwise, fish carcasses should be disposed of properly (e.g. double bagged and placed in dumpsters, buried, etc.). This includes filleting on the ice; the carcasses cannot be left on the ice or in the water.