BALANCING FORAGING AND REPRODUCTION IN THE MALE HARBOR SEAL, AN AQUATICALLY MATING PINNIPED

Aquatically mating male harbour seals, Phoca vitulina, must balance th e competing demands of foraging and reproduction while at sea during t he breeding season. Time-depth recorders (TDRs) were attached to 31 ad ult male harbour seals to investigate changes in diving behaviour at S able Island, Nova Scotia, during the 1992-1994 breeding seasons. Male seals were captured, fitted with TDRs and weighed at the beginning of the season in late May, then recaptured for TDR removal and reweighing at the end of June. Males made deep dives (to maximum depths >20 m) m ore frequently early in the breeding season, then switched to shallow (less than or equal to 20 m) diving later during the mating period. De ep dives (38.8 +/- 2.2 m; 4.6 +/- 0.1 min) were fairly uniform in shap e, appearing flat-bottomed with rapid rates of descent and ascent and long bottom time, but shallow dives (10.5 +/- 0.5 m; 3.0 +/- 0.1 min) were more variable in shape. Rates of mass loss varied inversely with time spent in deep dives, indicating that deep diving behaviour reflec ts foraging activity. Males lost mass while making shallow dives assoc iated with mating behaviour later in the breeding season. Deep diving occurred more frequently during daylight hours. Shallow dives were pre dominant at twilight and at night when females are likely to be depart ing and returning to the island from foraging trips in late lactation. At Sable Island, males may maximize their encounter rates with oestro us females by ceasing to make offshore foraging trips, and increasing the time spent patrolling home ranges and displaying in shallow water near the breeding colony in late lactation. Relatively larger males ma y have a competitive advantage since they can afford to spend less tim e making foraging trips away from the concentration of females and mor e time displaying in shallow water near the shore. (C) 1997 The Associ ation for the Study of Animal Behaviour.