Marine Mammals
V. Human Impacts

(also see Human Influences section)

Pinniped populations can be affected by human-induced disturbance (Suryan 1995), manufactured toxins (e.g. organochlorines; Kopec and Harvey 1995) and fisheries conflicts. Pinnipeds require secluded and undisturbed areas for land-based resting and reproductive activities. If human disturbance is too great, pinnipeds will abandon haul-out sites and rookeries, thereby, possibly decreasing reproductive success. The effects of disturbance can be illustrated using two examples within the MBNMS. After the State of California protected much of the area surrounding Point Año Nuevo, northern elephant seals began using the mainland beaches for breeding, pupping, and rest, where traditionally elephant seals had only used the island. Births of harbor seals in Elkhorn Slough, occurred only after the California Department of Fish and Game protected the haul-out site from human encroachment.

Fishery interactions have significantly impacted some local marine mammal populations. In 1992, an estimated 356 common dolphins, 230 other small cetaceans, 114 elephant seals, and 68 California sea lions were killed in the shark/swordfish drift net fishery (Julian 1992). Gillnet and trammel fisheries in the Monterey Bay area have been responsible for deaths of sea otters, harbor porpoises and other marine mammals (Wendell et al. 1985, Jefferson et al. 1994, Lennert et al. 1994). Ten percent of the California sea lions treated by the California Marine Mammal Center have been shot or caught in a net (Gerber et al. 1993).

Boat traffic may affect the behavior of marine mammals. During their migration, gray whales will change their course to avoid vessels (Wyrick 1954). Gray whales changed their course slightly to avoid a vessel producing various noises that was anchored in the middle of the migration path along the California coast (Malme et al. 1984). Approximately 50% of the gray whales showed some avoidance behavior when the received signal strength was >120 decibels. Although there is a great deal of vessel traffic throughout the MBNMS, many marine mammals probably have habituated to vessel noise, and may avoid collisions using their excellent acoustic capabilities.