Reduced Adult Survival Estimated in Areas of Decline of Harbour Seal Populations in Scotland

TL;DR

Harbour seal populations in Scotland are declining, and the study finds that reduced adult survival, not lower reproduction, is likely the main cause.

Contribution

The study uses mark–recapture models on photo-ID data to identify demographic drivers of population decline in harbour seals.

Findings

Apparent adult survival was significantly lower at the declining site of Burray compared to stable or increasing sites.

Fecundity rates were high across all sites, suggesting that reduced reproduction is not the main driver of the decline.

The results support the hypothesis that adult survival, not fecundity, is the key demographic factor behind the population decline.

Abstract

Understanding the demographic drivers behind observed changes in wild populations is key to inferring intrinsic and extrinsic causes behind such changes. In Scotland, harbour seal populations have undergone regional declines since the early 2000s. Here, we apply mark–recapture models to photo‐identification data collected during the breeding season at haulout sites representative of three areas with contrasting population trajectories to estimate sex‐specific apparent adult survival and fecundity rates. Apparent adult survival rates were lower at the declining site of Burray, located within the North Coast and Orkney Seal Monitoring Unit (SMU), which has declined by 85% since the mid‐1990s: female survival = 0.844 (95% CI 0.803–0.878) and male survival = 0.826 (95% CI 0.751–0.883) (photo‐ID data collected in 2016–2022). At stable or increasing sites, estimated apparent adult survival rates were higher: at Dunvegan, located in the West Coast SMU, a region that has been generally increasing since monitoring started in the mid‐1990s: female survival = 0.878 (95% CI 0.810–0.924) and male survival = 0.842 (95% CI 0.756–0.902) (photo‐ID data collected in 2016–2022); at Loch Fleet, located in the Moray Firth SMU, which has shown no trend since 2003: female survival = 0.941 (95% CI 0.922–0.956) and male survival = 0.919 (95% CI 0.888–0.942) (photo‐ID data collected 2006–2021). Mark–recapture fecundity rates were generally high at all sites (0.809 to 0.883), with the lowest estimated fecundity at the declining site of Burray. The results indicate that the causes of the decline are likely acting on adult survival, while evidence that a decrease in fecundity is driving the observed declines had less support. The estimated vital rates inform current research into potential causes of the declines and can be incorporated into stage‐structured population dynamic models to investigate whether the hypothesised mechanisms for decline are supported by the data.

We applied mark–recapture models to photo‐ID data from harbour seals at three sites of contrasting population trajectories in Scotland and estimated apparent adult survival and fecundity rates. The results indicate that the causes of the decline are likely acting on adult survival, while evidence that a decrease in fecundity is driving the observed declines had less support.

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