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Keywords:

  • birth rate;
  • population dynamics;
  • reproductive rate;
  • fecundity;
  • reproduction;
  • recruitment;
  • survival;
  • mark-recapture;
  • pinniped;
  • sea lion;
  • El Niño

Abstract

We conducted a 15 yr mark-resight study of branded California sea lions (Zalophus californianus) at San Miguel Island, California, to estimate age-specific recruitment and natality of the population. We used the Schwarz and Stobo model to estimate sighting, survival, recruitment, timing of births, abundance, and age-specific 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-specific 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 Niño conditions by 24% and 34%, respectively. In addition to reducing natality, El Niño events may result in a delay of recruitment if females experience El Niño conditions before they turn 4 yr of age.