Phenology of harbor seal pupping and the influence of weather on pup counts, investigated by UAV

Abstract

Surveys of harbor seals to assess pup production in Denmark are conducted at the assumed breeding season peak between 9 and 15 hr at winds <10 m/s, with no precipitation at least 6 hr before observations and at low tide in areas with a significant tidal range. Harbor seal pups can swim immediately after birth, so the fraction available for counting likely varies according to conditions. In the Limfjord, a separate harbor seal population is living in a large inlet in northern Denmark where >95% of pups are born at two haul-out sites situated 4 km apart. Drone surveys of these two haul-outs were conducted throughout the breeding seasons during 2017–2019. The effects of weather conditions and timing of surveys on counts were analyzed by generalized additive mixed models. The best-performing model included date and wind speed. Pup counts grew from the onset of the observation period and were predicted to peak on June 22. Wind speed had a linear relationship with pup counts and > 50% more pups were counted at 5 m/s than at 1 m/s. To achieve representative pup counts careful consideration of survey conditions and possibly correction for these effects are necessary.