Effects of external tags on maternal postpartum, offspring body mass and breeding frequency in gray seals Halichoerus grypus

Few studies have examined the impacts of externally fitted data-loggers and telemetry tags on pinnipeds. We tested for instrument effects on body mass of lactating female gray seals and their offspring and probability of pupping in the next breeding season. Known-age adult females (n = 216) were fitted with instruments in winter, spring, and fall from 1992 to 2018 at Sable Island, Nova Scotia. Of those tagged in spring and fall, 61 of 135 returning females and 59 of their offspring were weighed within 5 days postpartum and 79 pups were weighed at weaning. Instrumented females were assigned to treatments based on tag frontal area sums, tag mass, deployment duration, and acoustic tag presence compared to control females without instruments using linear mixed-effects models. None of the treatment effects were included in the preferred models predicting birth mass of offspring or probability of breeding in the following year. The small negative effect (−3% to −7%) on postpartum maternal mass and pup weaning mass (−4.7%) for females instrumented in fall may be an artifact as longer spring deployments showed no effect. Overall, we found that the instruments deployed had no detectable negative effects on the maternal and offspring variables measured.