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

Abstract

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.

Birth dates vary with fixed and dynamic maternal features, offspring sex, and extreme climatic events in a high‐latitude marine mammal

“…Our findings add to a small but growing number of large‐mammal studies reporting repeatability of birth dates for individuals (Lunn and Boyd ; Boyd ; Ellis et al. ; Cordes and Thompson ; Plard et al. , ; Wolcott et al.…”

“…). Even after such factors are considered, some individuals repeatedly give birth earlier or later than others (Cordes and Thompson ; Plard et al. ; Wolcott et al.…”

“…Thus, birth timing for a given population depends on the individuals in the population and the environmental conditions those individuals face (Sydeman et al. ; Boyd ; Cordes and Thompson ; Friebe et al. ).…”

Age‐specific variation in timing of parturition in Steller sea lions at Forrester Island Complex, Alaska

“…), and declining populations were associated with either later mean parturition dates when subject to bottom‐up control (Bowen et al . ) or earlier mean parturition dates when subject to top‐down control (Cordes and Thompson ).…”

Age–length relationships in UK harbour seals during a period of population decline

“…Interestingly, despite large fluctuations in the population size during this time, due to a combination of factors (Matthiopoulos et al, 2014;Thompson, Mackey, Barton, Duck, & Butler, 2007), the length-at-age data did not show any substantial or biologically significant variation in the residuals around the growth function over time. Similarly, Cordes and Thompson (2013) concluded that an advance in pupping date during a period of decline was likely to be related to top-down (direct removals) rather than bottom-up processes. Similarly, Cordes and Thompson (2013) concluded that an advance in pupping date during a period of decline was likely to be related to top-down (direct removals) rather than bottom-up processes.…”