Diving behaviour of lactating harbour seals and their pups during maternalforaging trips

Female harbour seals (Phoca vitulina) undertake foraging trips during mid t o late lactation. We show that they are accompanied by their pup during man y of these foraging trips. Time-depth recorder data were obtained from 20 l actating females and 14 of their pups in 1995 and 1996 at Sable Island, Nov a Scotia. Overall, females spent 55.4 +/- 4.68% (mean +/- SE) of their time at sea compared with 39.8 +/- 2.29% for pups. Like those of their mothers, pups' dives occurred in clusters or bouts: 71.4 +/- 4.4 dives, 2.5 +/- 0.1 5 h in duration. Bouts of diving by females and pups began 0-3 days post pa rtum. Mean dive duration of pups increased from about 1 to 1.5 min over the course of lactation, but was still shorter than that of adult females (1.5 -2.25 min). Both females and pups appeared to dive within their theoretical aerobic dive limits (TADL) of 8.9 and 2.6-3.1 min, respectively. Up to 3.6 % of dives by some pups may have exceeded their TADL. Pups appeared to comp ensate for their lesser diving ability by making more and shorter dives per bout than females, particularly during early lactation.