Balancing foraging and reproduction in the male harbour seal, an aquatically mating pinniped

Abstract

Aquatically mating male harbour seals,Phoca vitulinamust balance the competing demands of foraging and reproduction while at sea during the breeding season. Time-depth recorders (TDRs) were attached to 31 adult male harbour seals to investigate changes in diving behaviour at Sable 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 >20m) more frequently early in the breeding season, then switched to shallow (≤20m) diving later during the mating period. Deep dives (38.8±2.2m; 4.6±0.1min) were fairly uniform in shape, appearing flat-bottomed with rapid rates of descent and ascent and long bottom time, but shallow dives (10.5±0.5m; 3.0±0.1min) were more variable in shape. Rates of mass loss varied inversely with time spent in deep dives, indicating that deep diving behaviour reflects foraging activity. Males lost mass while making shallow dives associated with mating behaviour later in the breeding season. Deep diving occurred more frequently during daylight hours. Shallow dives were predominant at twilight and at night when females are likely to be departing and returning to the island from foraging trips in late lactation. At Sable Island, males may maximize their encounter rates with oestrous 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 may have a competitive advantage since they can afford to spend less time making foraging trips away from the concentration of females and more time displaying in shallow water near the shore.