Diving behaviour of harbour seal Phoca vitulina pups from nursing to independent feeding

To study the diving activity of harbour seal (Phoca vitulina) from nursing to independent feeding, we VHF radio tagged and tracked seven pups. The pups gradually developed the diving skills necessary to obtain enough food after weaning and to avoid natural and man-made dangers. The maximum dive duration remained unchanged with pup age, indicating that pups have considerable breath-holding capacity already from an early age. However, mean dive duration increased with age, from 0.92 min in pups ≤25 days old to 2.55 min in 1–2 month olds, and 3.09 min in 2–3 month olds. The increase continued for weeks after the assumed weaning, and indicates that the capacity to perform long-lasting dives steadily increases as the pups grow older. Mean and maximum surface duration remained unchanged with pup age. Mean surface duration was 0.32 min with a maximum value of 6.10 min. An increase in mean surface duration with mean dive duration was found when the pups were ≤25 days old, but not when they were older. This may indicate that young pups need to rest more after diving than older ones. No relationship was found between dive duration and the subsequent surface interval. The proportion of time spent submerged increased with age, from 71% when the pups were ≤25 days old to 86% when they were older. The pups hauled-out less frequently as they grew older. The pups were never recorded outside the study area (defined by the 30-km station range). Time of day, tidal height and weather conditions seemed not to influence the diving behaviour of the pups.