Fine-scale variability in iceberg velocity fields and implications for an ice-associated pinniped

Abstract

Icebergs found in proglacial fjords serve as important habitats for pinnipeds in polar and subpolar regions. Environmental forcings can drive dramatic changes in the overall reduction in ice coverage across fjords in the circumpolar regions, with implications for pinnipeds that use ice for critical life-history functions, including pupping and molting. To better understand how pinnipeds respond to changes in iceberg habitat, we combine (i) iceberg velocity fields over hourly to monthly timescales, derived from high-rate time-lapse photogrammetry of Johns Hopkins Glacier and Inlet, Alaska, with (ii) aerial photographic surveys of harbor seals (Phoca vitulina richardii) conducted during the pupping (June) and molting (August) seasons. Iceberg velocities typically followed a similar diurnal pattern: flow was weak and variable in the morning and strong and unidirectional in the afternoon. The velocity fields tended to be highly variable in the inner fjord across a range of timescales due to changes in the strength and location of the subglacial outflow, whereas, in the outer fjord, the flow was more uniform, and eddies consistently formed in the same locations. During the pupping season, seals were generally more dispersed across the slow-moving portions of the fjord (with iceberg speeds of < 0:2 ms−1). In contrast, during the molting season, the seals were increasingly likely to be found on fastmoving icebergs in or adjacent to the glacier outflow plume. The use of slow-moving icebergs during the pupping season likely provides a more stable ice platform for nursing, caring for young, and avoiding predators. Periods of strong glacier runoff and/or katabatic winds may result in more dynamic and less stable ice habitats, with implications for seal behavior and distribution within the fjord.