Harbor seal breeding behavior and habitats constrain opportunities for individual-based studies, and no current estimates of both survival and fecundity exist for any of the populations studied worldwide. As a result, the drivers underlying the variable trends in abundance exhibited by harbor seal populations around the world remain uncertain. We developed an individual-based study of harbor seals in northeast Scotland, whereby data were collected during daily photo-identification surveys throughout the pupping seasons between 2006 and 2011. However, a consequence of observing seals remotely meant that information on sex, maturity-stage, or breeding status was not always available. To provide unbiased estimates of survival rates we conditioned initial release of individuals on the first time sex was known to estimate sex-specific survival rates, while a robust design multistate model accounting for uncertainty in breeding status was used to estimate reproductive rate of multiparous and 3-yr-old females. Survival rates were estimated at 0.95 (95% CI = 0.91-0.97) for females and 0.92 (0.83-0.96) for males, while reproductive rate was estimated at 0.89 (0.75-0.95) for multiparous and 0.69 (0.64-0.74) for 3-yr-old females. Stage-based population modeling indicated that this population should be recovering, even under the current shooting quotas implemented by the recent management plan.
Bibliographical noteWe would like to thank Dr. William Kendall for statistical advice. Also thanks to Helen Wheeler and Laila Aranda for carrying out data collection in 2006 and 2011, respectively, and to the Mammal Conservation Trust and Talisman Energy (UK) Ltd for financial support. Line Cordes was partly supported by a University of Aberdeen College of Life Science & Medicine Postgraduate Studentship. We thank Kelly Hastings and two anonymous referees for their constructive comments.
- Phoca vitulina
- vital rates
- population dynamics