Photo-ID-based estimates of reproductive patterns in female harbor seals

Variations in reproductive patterns offer important insights into the dynamics of pinniped populations, but collecting data on reproduction for species that spend much of the breeding season in the water is problematic. We used land-based photo-identification techniques to collect individual-based data on the timing of pupping, total pup production, and lactation duration in a population of harbor seals in NE Scotland. Capture-Mark-Recapture (CMR) techniques were used to overcome potential biases due to changes in probability of capture, and provide estimates of lactation duration based upon changes in the survival of mother-pup bonds. A mean birth date of 20 June is the first direct estimate of parturition date for UK harbor seals. Information on cumulative births indicated that the peak daily haul-out count accounted for 77% of total pup production. CMR-based estimates of lactation duration suggest that 50% of mothers had weaned their pups, when pups were 21-d old. These results highlight the potential for using photo-ID techniques to study harbor seal reproductive patterns at sites where intensive capture and marking is not possible for logistic, legislative, or ethical reasons.