Estimation of an unobservable transition: from dependence to weaning in the California sea lion (Zalophus californianus)
Harris, Jeffrey D.
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For wild mammals, weaning begins the most vulnerable life-history period. Success during this transition hinges on an individual’s ability to elude predators while learning to handle and locate prey. In most mid-latitude marine ecosystems there is a strong seasonal influence on productivity and prey availability. Thus, the timing of weaning should play a critical role in a successful transition in these systems. In some wild marine mammal populations, however, weaning is largely an unobservable event and, therefore, its timing is poorly understood. Accordingly, we initiated a study in 2010 to estimate the timing of weaning using a single cohort of California sea lions (Zalophus californianus) on San Miguel Island, CA (SMI). We marked 244 sea lions (n = 119 males, n = 125 females) so that they were individually identifiable when less than four months of age and then conducted 19 sighting occasions for marked individuals prior to, during, and after weaning, ending in August 2013. There were 2,179 resights collected for use in analysis, and the data were analyzed using a hidden Markov multi-state model with two levels of state uncertainty. Models included combinations of weight, sex, age, area, and time to assess the influence of these explanatory factors on the probabilities of survival, state detection, sighting, movements between resight areas, and the timing of weaning. The majority of weaning occurred abruptly during a 4-week period starting in late April. The timing of weaning was not affected by the sex or weight of pups, but survival was reduced by 30% for individuals with below-average weight. Sexual segregation occurred immediately after weaning, with males being 3.68 times more likely to disperse from the rookery island. The abrupt weaning transition in late spring revealed by this study coincides with peak upwelling in the California Current Ecosystem, illustrating the importance of prey availability to successful weaning in the California sea lion. Recently, young-of-the-year California sea lions have experienced four years of unusual and largely unexplained increases in mortality, most notably in 2013 and 2015, when record numbers of pups stranded as early as January. Our findings offer new insight into a putative driver of these mortality events – mismatch between seasonal upwelling and weaning – and suggest that pup weights and stranding rates may serve as an index of the health of the California Current Ecosystem.
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