The ecology and population biology of the northern elephant seal, Mirounga angustirostris Gill 1866, on the Southern California Channel Islands

I studied the patterns of seasonal abundance, behavior, and population biology of northern elephant seals (Mirounga angustirostris) on the Southern California Channel Islands from 1979 through 1987 and investigated the influences of crowding on colony growth and neonatal survival.;Four primary peaks in terrestrial abundance of elephant seals are related to reproduction in winter and age- and sex-related differences in timing of the molt in spring and summer. The magnitudes of these peaks on San Nicolas Island varied among years. Non-breeding season peaks in abundance are unreliable for assessing absolute abundance or trends in colony size, but annual fluctuations at one rookery may be useful indicators of temporary or long-term oceanographic or environmental changes or suggestive of demographic changes on other rookeries. An understanding of the demographics of all colonies is, however, essential for interpreting seasonal changes in abundance on each.;Births continued to increase exponentially at average annual rates of 11.5% and 7.8% at San Nicolas and San Miguel islands, respectively. Crowding during the breeding season had insignificant effects on increases in births in most habitats and the greatest rates of increase in births occurred at densities very near the carrying capacities (K) of the breeding habitats (i.e., at about.75K). Pre-weaning pup mortality was independent of densities on rookeries and remained low at about 4% on San Nicolas Island and generally less than 6% on San Miguel island. Crowding on some beaches where female distribution was determined by the influence of beach topography on adult male abundance and distribution did, however, influence the tenure of lactating females and growth of their pups.;Females were evidently in poorer physical condition in 1983 and 1984 than in other years presumably because of decreased food abundance or altered prey distribution which may have required them to range further when at sea from late 1982 through early 1984 during an intense ocean warming (El Nino Southern Oscillation) event. Consequently, parous females arrived on rookeries later in 1983, nursed their pups fewer days in 1983 and 1984 which resulted in pups being weaned in poorer condition and were at sea feeding longer in spring 1983 than in other years. They also conceived a significantly greater proportion of male progeny in 1984 than in other years.