Influence of maternal characteristics and oceanographic conditions on survival and recruitment probabilities of Weddell seals

Abstract

For long-lived animals, maternal age and breeding experience can vary widely and affect offspring survival and recruitment probabilities. In addition, these vital rates may be influenced by annual variation in environmental conditions. We evaluated various hypotheses regarding how offspring survival and recruitment probabilities vary as functions of maternal characteristics and oceanographic conditions, using 25 years of data from a study of individually-marked Weddell seals in Erebus Bay, Antarctica. We predicted that survival and recruitment would be positively related to maternal age and experience up to some threshold value and considered three hypothesized shapes for the relationship beyond the threshold age (steadily increasing, pseudo-threshold, or decreasing). We predicted an inverse relationship between maternal age at first reproduction and offspring survival and recruitment probabilities. We predicted that sea-ice extent, which positively influences primary productivity, would be positively related to annual recruitment probabilities. Results revealed contrasting influences of maternal age on probabilities of survival and recruitment of young. Survival rate was best modeled by a pseudo-threshold relationship with maternal age, e.g. in 1999, survival rate was estimated as 0.61, 0.69 and 0.72, respectively, for seals born to 6-, 14- and 22-yr-old mothers. In contrast, estimated recruitment probability was highest for seals born to young mothers, e.g. recruitment probability for a 7-yr-old who had not yet had a pup was estimated as 0.51 vs 0.30, respectively, if she was born to a 6- versus a 14-yr-old mother. The combined results for offspring survival and recruitment suggest countervailing selection where genotypes favored for reproductive success are generally selected against as juveniles, resulting in high recruitment probabilities for individuals that had low juvenile survival rates. Finally, we found support for our prediction that oceanographic conditions affected annual recruitment rates, but not survival rates. Specifically, annual recruitment probability was positively related to the sea-ice extent in September of the previous year.