Age-Specific Natality of Northern Elephant Seals at the South Farallon Islands, California

Abstract

We monitored 233 tagged, known-age female northern elephant seals ( Mirounga angustirostris ), age 3–16 years, that gave birth on the South Farallon Islands, 1975–1986. Females were from the 1970–1982 cohorts born on South Farallon, Ano Nuevo, San Nicolas, and San Miguel islands, California. Age-specific natality was significantly lower for 3- and 4-year olds (0.13 and 0.75, respectively), then stabilized at about 0.80 for females age 5–10 years. Natality was significantly lower in 1984 and 1985 than in other years. For the 1971–1980 cohorts, mean age at first reproduction ranged from 3.9 to 4.6 years. During this study, the age structure of females giving birth on the South Farallon Islands changed; mean age rose from 4 years in 1975 to 7 years in 1986. At least 20% of Farallon-born females survived to reproduce, ≥17% returned to the South Farallon Islands, and ≥ 3% emigrated to other rookeries. Females born at Ano Nuevo, San Miguel, and San Nicolas islands immigrated to the South Farallon Islands to give birth (≥3, ≥2, and >1% respectively). Mature, nonpregnant females were seen during the breeding season (6%), during the autumn haul out (15%), and during spring molt or a later breeding season (79%).