Colony specific implications of individual mass changes for survival and fecundity in female grey seals (Halichoerus grypus). Report for the Special Committee on Seals

We present the results of a hidden process model fitted to long-term observational data from mark- recapture studies at grey seal breeding colonies on the Isle of May (IM) and North Rona (NR). We assume that mass changes between years are dependent on environmental factors and on the breeding status of animals, and explore the influence of an individual?s mass on apparent survival, and fecundity. There was general annual variation in mass gain, especially at IM, presumably due to fluctuating resource availability. We find that females whose mass is low are less likely to breed, but that there is no strong evidence for a similar effect on survival. We are also able to arrive at general estimates of fecundity for females using each of these 2 colonies, including years in which they are not observed to attend the breeding colony. Overall fecundity estimates were different at the two colonies (0.770 NR, 0.860 IM).