THE DIEL HAULING-OUT CYCLE OF HARBOR SEALS IN AN OPEN MARINE-ENVIRONMENT - CORRELATES AND CONSTRAINTS

A colony of harbour seals in the Pacific north-west was monitored over two years, concurrent with a variety of environmental variables. Regr ession models described diel hauling-out activity as: i) a photoperiod ic cycle; ii) a function of other environmental factors, or iii) a cyc le modified by environmental constraints. Throughout the year, the num ber of seals on shore followed a diel pattern with a midday peak. Seal s hauled-out in lower numbers in winter than in summer, and for a smal ler proportion of the day (although for about the same proportion of t he photoperiod). During the annual moult, numbers hauled were elevated around the clock, and the midday peak was skewed to late afternoon/ea rly evening. Models that defined hauling-out in terms of environmental factors were significant, but did not fit the data as well as models based on photoperiod. The strongest environmental correlates (such as tidal height) owed much of their explanatory power to artefactual simi larities with the photoperiodic cycle. Four general conditions are pre sented which, if met, should always result in a diel hauling-out cycle with a midday peak. The most fundamental of these involves a proposed 'cost of immersion' which motivates pinnipeds to haul-out when not fo raging. Two likely candidates for such a cost involve risk from aquati c predators and the energetic expense of sleeping while immersed.