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Research Document 2017/054

Estimating Changes in Vital Rates of Sable Island Grey Seals Using Mark-recapture Analysis

By den Heyer, C.E., and Bowen, W.D.


After more than four decades of growth at 13%, the rate of pup production of Grey Seals (Halichoerus grypus) at Sable Island has declined to about 4% per year. As resource limitation becomes more acute, life history theory suggests that first juvenile survival, then adult fertility, and finally adult survival will change. Previously, mark-resight analysis of Grey Seals on Sable Island found that juvenile survival had been reduced by almost 50% between the early 1990s and early 2000s, suggesting that resources may have become limiting for this population. Here, we fit a Cormack-Jolly-Seber model to the resighting history of individually marked Grey Seals that have recruited to the Sable Island breeding colony since 1978 to estimate age- and sex-specific adult survival. Of those initially marked, 562 males and 1728 females were resighted in the breeding colony between 1978 and 2016. Average adult survival was high (male=0.943, Standard Error (SE)=0.003; female=0.976, SE=0.001), but male Grey Seals had lower survival at all ages. Resighting probability has remained between 60 and 80% since the late 1980s. Males are more likely to be sighted in a breeding season than females. Only female Grey Seals with pups are regularly sighted on the breeding colony; thus, those females that skip breeding are unobservable (temporary emigration). A multi-state open robust design model was used to estimate the transition probabilities between breeding (observable) and non-breeding (unobservable) states for individually marked females that were observed on the colony from 1992 to 2016. The first-order Markov state-dependent transition model was preferred over random transition probabilities.  Females that gave birth had, on average, an 85% chance of pupping in the following year. However, females that did not give birth had a 56% chance of giving birth in the following year, suggesting that female quality plays a role in breeding probability. Although breeding probability varied among years, there was no trend over time suggesting the average natality rate has not changed and is not contributing to the slowing of the rate of growth in pup production.

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