The bottlenose dolphin is protected in U.S. waters by the Marine Mammal Protection Act. Bottlenose dolphins are still generally plentiful in numbers, but are near depletion in some areas. Both incidental and direct exploitation of bottlenose dolphins are known to occur, generally at low to moderate levels. The largest direct kills have traditionally been in the Black Sea, where Russian and Turkish hunters apparently have reduced local populations. Bottlenose dolphins are accidentally caught in a variety of fishing gear, including gillnets, purse seiners used to catch tuna, and shrimp trawls. These dolphins also are occasional victims of harpoon and drive fisheries. Live captures of bottlenose dolphins for captivity have had effects on some local dolphin populations in the Gulf of Mexico and southeastern United States, but no commercial live captures have occurred in the U.S. since the 1980's. Bottlenose dolphins are vulnerable to pollution, habitat alteration, boat collisions, human feeding of and swimming with wild animals, and human disturbance (such as boating). Several die- offs of bottlenose dolphins have occurred. Retrospective analysis of tissues of dolphins that died in 1987-1988 during a large die-off (approximately 800-1,000 dolphins) on the Atlantic U.S. coast indicates that mortality may have been caused by a morbillivirus. This virus has been linked to dies-offs of Gulf of Mexico bottlenose dolphins as well. Dolphins with disease symptoms appeared to have elevated levels of PCB's, leading researchers to conclude that pollutants may be playing a role in these events. Preliminary evidence from other studies show links between contaminant residues in tissues and impaired immune system function.