Aggregation Behaviour of Harbour Seals at Forillon National Park, Canada

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Abstract
To explain the aggregation behavior of harbor seals (Phoca vitulina) at their landing sites, 2 possibilities were investigated: grouping in response to resting site distribution or in response to changes in individual time-energy budget. To verify if seals aggregated according to the distribution of their landing sites, site selection was studied. Although seals select sites with distinctive features, they frequented only those whose area was sufficient or that were close enough to other suitable sites to hold a tight resting group. Distribution of landing sites was ruled out as the sole factor explaining aggregation. Membership in a group generated an individual benefit. This takes the form of an increased time allocation to sleep in larger and tighter groups. While individual scanning rate decreased with group size and proximity, overall vigilance increased with group size and decreased with proximity. Although evidently desiring to join larger and tighter groups, seals saw their density regulated on each site by aggressive interactions. Rate of aggressive interaction did not vary with density but the proportion leading to departures increased with density. No fighting advantage was found in larger size but the 1st animal to land on the site was generally the winner. Changes in the landing behavior in relation to weather were consistent with other studies. Seasonal weather trends could not fully explain the increase in group size from spring to autumn during which time the proportion of used space remained roughly constant.