Säsongsmässig variation i gruppstorlek hos knubbsäl (Phoca vitulina) på isflak i Johns Hopkins Inlet, Glacier Bay National Park, Alaska

Individuals are believed to benefit in various ways from living in groups. Some species aggregate to increase foraging efficiency and others for social benefits. Why pinnipeds rest in groups is yet not fully understood, but the most common ecological explanation is the benefit of enhanced predator detection. An alternative explanation for grouping, however, is limitation of resting sites, which might force individuals to aggregate on whatever sites are available, despite increased competition for food or mates. My study is focused on group size of harbor seals (Phoca vitulina) hauled out on glacial-ice sites in Glacier Bay National Park (GBNP), southeastern Alaska. Previous studies have focused on group size on terrestrial haul-out sites, and to date there have been few studies of group size on ice haul-out sites. In GBNP, peak numbers of harbor seals haul out during the pupping season (June) and the molting season (August). During the breeding season, glacial-ice haul-out sites are used primarily by females and pups, and during the molting season by molting seals. Because seals may group as an anti-predator strategy, I hypothesized that group size would differ between seasons, with smaller groups of mainly mothers and pups during June, and larger-sized groups during August. The aim of this study was to quantify seasonal differences in the group size of harbor seals hauled-out on glacial ice in Johns Hopkins Inlet (JHI), GBNP, southeastern Alaska, and in addition, map and evaluate the large-scale (inlet-level) spatial distribution of seals for both seasons. I used digital aerial survey photographs to map seal distributions. Four replicate aerial surveys were conducted in JHI during both June and August, 2007. Digital images were georeferenced and imported into ArcGIS 9.3 where the location of each seal was digitized, and group size calculated. To assess within- and between-season differences in spatial distribution of hauled-out seals, the location of each seal for each survey was plotted in ArcGIS, and compared to a visual assessment of ice coverage. My results show that harbor seals in JHI tend to haul out as single individuals during both seasons, with the exception of nursing mothers with pups in June, and a few larger-sized groups in August. There was no significant difference in group size between seasons. Group size during June varied very little, with almost all seals being either single or in mother-pup pairs. Group size varied more during August, and although most seals hauled out alone, there were a few larger-sized groups during each survey. The relative lack of grouping behavior is contrary to previous research, and it suggests that predation pressure on seals within JHI may be lower than at other sites. The greater frequency of larger-sized groups in August appeared to be related to reduced availability of glacial-ice haul-out sites. Long-term reduction in the availability of glacial-ice haul-out sites in JHI, therefore, might lead to shifts in haul-out behavior of harbor seals, with more seals being forced to haul out in larger groups