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.
Next - Section VI. Information Gaps
Marine Mammals Table of Contents
