Low and Delayed Recruitment at Two Grey Seal Breeding Colonies in the UK

Abstract

This study is concerned with changes in the recruitment of UK grey seal pups into the adult breeding population. Pups were marked over two decades at North Rona and the Isle of May. The proportion of these animals observed to recruit as adults was at best 0.10 on North Rona and 0.31 on the Isle of May. Double-tagged cohorts were re-sighted at the highest rates, compared with cohorts that were marked with single tags or with brands. There was also evidence of substantial interannual variation, and no individuals were ever re-sighted for certain cohorts. Estimates of absolute tag loss were higher at North Rona than at the Isle of May, but not sufficient to explain the low re-sight rates there. Recruitment at the Isle of May appears to be occurring later in recent years and this is consistent with the effects of density dependence. There are too few tag returns from North Rona to allow the investigation of any time-dependence in recruitment, but this lack and the continued decline of pup production on North Rona suggests that recruitment there may be low. These findings have direct implications for models of UK grey seal population dynamics.