Effects of Ice Floe Area and Density on Harbor Seal (Phoca vitulina) Haul-Out Behavior in Glacier Bay National Park, Alaska

Abstract

The sustained decline of harbor seals (Phoca vitulina) in Glacier Bay National Park (GBNP) remains unexplained, despite strong protections and limited evidence for direct anthropogenic or natural drivers. Accelerated tidewater glacier retreat provides a possible explanation, as harbor seals depend on the ice floes they generate for critical life stages. Here we quantify the relationship between fine-scale ice floe structure and harbor seal haul-out behavior using high-resolution drone imagery collected in the Johns Hopkins Inlet during June 2023 and 2024. Machine learning approaches were used to detect seals, delineate ice floes, and estimate age classes. Resource selection functions and mixed effects models reveal that seal presence, including young-of-the-year animals, is strongly associated with ice floe area and local ice density, with a significant interaction indicating sensitivity to spatial configuration. Group size and social structure were similarly constrained by the availability of preferred ice habitat. These findings demonstrate that fine-scale ice morphology is a primary determinant of habitat use during the pupping season. Ongoing glacier retreat, and associated changes in floe size distributions and spatial arrangement, may reduce pupping habitat quality and have negative implications for the persistence of GBNP’s harbor seal population.