Variation in temporary emigration and survival rates and implications for recruitment for female Weddell seals
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Date
2012
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Montana State University - Bozeman, College of Letters & Science
Abstract
The life-history of many colonial-breeding species includes a period of absence from the natal colony followed by a period of attendance as prebreeders prior to first reproduction. During this prebreeder period, survival rates, and the probability of temporary emigration is variable, and patterns of attendance can have implications for future reproduction. We used 26 - 28 years of encounter data of female Weddell seals (Leptonychotes weddellii) in Erebus Bay, Antarctica to estimate variability in survival and temporary emigration rates and to evaluate implications for recruitment rates. Temporary emigration rates were nearly 1 at age 1, decreased until age 8, and increased slightly thereafter. Annual variation in temporary emigration rates was substantial, and rates were positively related to the distance from the fast-ice edge to Erebus Bay and to the population of adult females in the previous year. Recruitment rates varied annually, and were typically, but not always, positively related to the number of years (0, 1, 2+) that a prebreeder had previously attended reproductive colonies in Erebus Bay. Survival rates varied by birth-cohort and were positively related to the extent of current-year winter sea-ice in the Ross Sea. The influence of birth-cohort on survival rates was persistent for several years but did not last indefinitely. Survivorship from birth to age 6 was related to the specific sequence of sea-ice conditions experienced by each cohort, and cohort-specific rates varied from 0.13 (SE = 0.04) to 0.42 (SE = 0.06), and averaged 0.25 (SE = 0.02). Our results suggest that (1) the influences on survival of conditions experienced in early life, along with later influences, act as a filter that determines what proportion of a cohort of female Weddell seals reaches reproductive maturity, and (2) there are benefits and potential costs to prebreeders that are associated with attending reproductive colonies, and the decision to attend or not likely depends on the balance of consequent tradeoffs. Useful avenues for additional research include (1) inter-annual movements within and outside Erebus Bay, especially in relation to previous-year conditions, and (2) implications of prebreeder attendance patterns and age at first reproduction for reproductive success after recruitment.