Variation in breeding phenology provides insights into drivers of long-term population change in harbour seals

Line S. Cordes, Paul M. Thompson

Research output: Contribution to journalArticlepeer-review

26 Citations (Scopus)


Phenological trends provide important indicators of environmental change and population dynamics. However, the use of untested population-level measures can lead to incorrect conclusions about phenological trends, particularly when changes in population structure or density are ignored. We used individual-based estimates of birth date and lactation duration of harbour seals (Phoca vitulina) to investigate energetic consequences of changes in pupping phenology. Using generalized linear mixed models, we first demonstrate annual variation in pupping phenology. Second, we show a negative relationship between lactation duration and the timing of pupping, indicating that females who pup early nurse their pups longer, thereby highlighting lactation duration as a useful proxy of female condition and resource availability. Third, individual-based data were used to derive a population-level proxy that demonstrated an advance in pupping date over the last 25 years, co-incident with a reduction in population abundance that resulted from fisheries-related shootings. These findings demonstrate that phenological studies examining the impacts of climate change on mammal populations must carefully control for changes in population density and highlight how joint investigations of phenological and demographic change provide insights into the drivers of population declines.
Original languageEnglish
Article number20130847
Number of pages7
JournalProceedings of the Royal Society of London. B, Biological Sciences
Issue number1764
Early online date19 Jun 2013
Publication statusPublished - 7 Aug 2013

Bibliographical note

Funding statement
The Mammal Conservation Trust and Talisman Energy (UK) Ltd. supported financially. We thank the many funders and colleagues at both the University of Aberdeen and the NERC Sea Mammal Research Unit who have maintained the time series of breeding surveys since 1987. L.C. was partly supported by a University of Aberdeen College of Life Science and Medicine Postgraduate Studentship.

We would like to thank Helen Wheeler and Laila Aranda for carrying out data collection in Loch Fleet in 2006 and 2011, respectively. Thanks also to Stuart Piertney for valuable comments on earlier manuscripts. This research was conducted within the University of Aberdeen's Framework for Research Governance, and fieldwork was carried out in accordance with the Scottish Marine Wildlife Watching Code.


  • pupping phenology
  • population dynamics
  • top predator
  • individual-based
  • photo-identification
  • Phoca vitulina


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