Nuclear pollution in parts of the world also harms marine life. In addition, human introduction of exotic species threatens native populations. As a conservative estimate, more than 30 million tons of marine mammals, sea turtles, seabirds, sharks, and other unwanted fish are thrown back into the oceans, dead or dying, annually. The actual number is almost certainly far higher. Methods used by large-scale fisheries often prove deadly to the many other marine creatures who share their home with commercially targeted fish species.
Some fisheries use large nets with small mesh, trapping or fatally wounding many non-target species.
Significant negative interactions occur between fisheries and marine mammals such as porpoises, dolphins, whales, sea lions, seals, manatees, and dugongs. A particularly bad example of this occurs in the eastern tropical Pacific Ocean, where dolphins are deliberately chased and encircled by huge nets used to catch the tuna swimming below them. Millions of dolphins experience harassment, trauma, and injury each year and thousands die as a result of the use of this fishing method. Another critical interaction involves the highly endangered northern right whale and New England fisheries. The vaquita, a highly endangered harbor porpoise, is killed in gillnets in the Gulf of California.