Provisional regional Potential Biological Removal (PBR) values for UK seals in 2025

This briefing paper provides Potential Biological Removal (PBR) values for the grey and harbour seal “populations” that haul out in each of the Seal Monitoring Units (SMUs) in the UK. Sets of possible values are tabulated for each SMU using the equation in Wade (1998) with different values of that equation’s recovery factor. A value, and associated justification, is suggested for this parameter for each SMU and the resulting PBR is highlighted. The PBR values are calculated using the latest composite counts in each SMU; it should be noted that some of these counts are over 5 years old. The PBR estimates, especially for declining SMUs should be considered in that context. There are numerous changes compared to SCOS 2024. Other than the extension to all UK SMUs with notable populations (previously only Scottish SMUs were considered), the other main change compared to SCOS 2024, is that instead of using the August count directly as Nmin for harbour seals, the count is scaled. Specifically, the count is raised using the 20th percentile of the distribution of the scalar used to account for seals at sea during the survey windows. This is equivalent to using the 20th percentile of the population estimate, aligning with the method used for grey seals and recommended by Wade (1998). The counts have been updated for SMUs 1 and 2 (Southwest Scotland and West Scotland). Moreover, the recovery factors for harbour seals have been adjusted for these SMUs to reflect that abundance is no longer increasing. Indeed, harbour seal recovery factors across all considered SMUs are < 1. Grey seal recovery factors were all set to 1 on the basis of the stable or increasing trends. This is with the exception of SMUs 4 and 5 (North Coast & Orkney and Shetland) for which the recovery factor was set to 0.5 on the basis that the available data are > 5 years old.