Age-specific recruitment and natality of California sea lions at San Miguel Island, California

摘要

We conducted a 15 yr mark-resight study of branded California sea lions (Zalophus californianus) at San Miguel Island, California, to estimate age-speci?c recruitment and natality of the population. We used the Schwarz and Stobo model to estimate sighting, survival, recruitment, timing of births, abundance, and agespeci?c natality from sighting histories of 1,276 parous females. The advantage of this approach was that the reproductive status of females did not have to be known for all females of reproductive age. Probability of recruitment into the reproductive population began at age 3 or 4, peaked between ages 5 and 7, and slowly declined. Age-speci?c natality was similar for ages 4–16 but declined after age 17, suggesting that reproductive senescence occurs in older females. The average annual natality for parous females 4–16 yr of age was 0.77 (SE = 0.03); natality declined to 0.56 (SE = 0.10) for parous females 17–21 yr of age. Natality for both age classes was reduced during El Nino conditions by 24% and 34%, respectively. In addition to ? reducing natality, El Nino events may result in a delay of recruitment if females experience El Nino conditions before they turn 4 yr of age.