Assessing Seasonal and Annual Changes in Glacier Ice Habitat for Harbor Seals in a Tidewater Glacier Fjord in Glacier Bay National Park, Alaska

Tidewater glaciers calve icebergs into the marine environment, which then serve as resting and pupping habitat for some of the largest seasonal aggregations of harbor seals in Alaska and the world. Although tidewater glaciers are naturally dynamic, advancing and retreating in response to local climatic and fjord conditions, most of the ice sheets that feed tidewater glaciers are thinning and, as a result, many of the tidewater glaciers are retreating. Models predict continual and rapid loss of glacier ice with unknown impacts to organisms that rely on glacier ice as habitat. To quantify seasonal and annual changes in the availability of glacier ice as habitat for harbor seals, we conducted systematic aerial photographic surveys (n = 55) of seals and ice during June and August (2007 to 2014) in Johns Hopkins Inlet, a tidewater glacier fjord in Glacier Bay National Park, Alaska. Object-based image analysis was used to quantify ice habitat and multivariate spatial models were developed to jointly model stage-structured seal location data and ice. Seal distribution was more extensive in June and corresponded to increased ice coverage in the fjord, which was likely influenced by an increase in the frontal ablation rates during late spring. The spatial distribution of ice in the same month was fairly consistent, particularly during August with intensity changes among years. However, ice coverage was more variable in June than in August. During June, adult harbor seals and glacier ice were highly correlated, but this correlation diminished in August after pups were weaned. Harbor seals exhibit high seasonal fidelity to tidewater glacier fjords. Thus, understanding mechanistic linkages between glacier advance and/or retreat, factors that influence calving dynamics of glaciers, and iceberg production will be essential for predicting the response of seals to changes in ice habitat in tidewater glacier fjords.