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'. It has been estimated that one million or more dolphins and porpoises die each year in these types of nets. Some of the modern methods of fishing, especially in the tuna industry, have had a great toll on dolphins. The use of up to 40 mile long monofilament drift nets to encircle and trap tuna has killed numerous dolphins. Fortunately in 1992 the drift nets were banned.