Man has introduced countless types of marine debris of which the plastics and discarded fishing gear have the worst effect on dolphins. Plastics are non biodegradable and will float around the globe for an eternity.
Dolphins die from eating plastics and get tangled up in discarded fishing line, ropes and nets.
Marine debris, however does not claim nearly as many dolphins as pollutive toxins and certain fishing methods. The ancient coastal method of fishing using gill nets stretched out for miles on buoys have claimed many dolphins. Fish enter the net head first and are trapped by their own gills when trying to withdraw. Dolphins do the same and are caught by their teeth and fins and drown. For the most part, the dolphins are not even desired by the fishermen and are considered 'bycatch'.