Mark-resight estimates of seasonal variation in harbor seal abundance and site fidelity

Line S. Cordes*, Paul M. Thompson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

20 Citations (Scopus)


Monitoring trends in abundance of pinnipeds typically involves counting seals at terrestrial haul-out sites during the breeding season. Counts of seals made at other times of the year are typically lower; however, it is often unknown whether this is because fewer animals are present or whether lower counts simply reflect a reduction in haul-out probability. Here we illustrate how photo-identification data from an individual-based study of harbor seals (Phoca vitulina) can be used to estimate seasonal variation in abundance and site fidelity. Monthly data collected over a two-year period were analyzed using a mark-recapture mark-resight model accounting for individuals transitioning between observable and unobservable states. Levels of site fidelity were high throughout the year and abundance estimates showed no seasonal pattern. This suggests that individual seals used haul-out sites to the same extent throughout the year, and that peaks in counts during the breeding season are a result of seasonal changes in haul-out probability. The results of this study have implications for understanding population sub-structuring, gene flow and disease spread.

Original languageEnglish
Pages (from-to)467-472
Number of pages6
JournalPopulation Ecology
Issue number3
Early online date9 Jun 2015
Publication statusPublished - Jul 2015

Bibliographical note

Acknowledgments We would like to thank the Mammal Conservation
Trust and Talisman Energy (UK) Ltd. for financial support.


  • Haul-out probability
  • Individual-based
  • Mark-recapture
  • Phoca vitulina
  • Photo-identification
  • Population dynamics
  • Southern elephant seals
  • Hawaiian monk seal
  • Haul-out patterns
  • Phoca-vitulina
  • Mirounga-Leonina
  • Tugidak Island
  • Grey seals
  • Population
  • Survival
  • Conservation


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