The Potential Role of Glacial Dynamics in the Life History of Two Contrasting Fjord Populations of Harbor Seals (Phoca vitulina)

Energy economy is a fundamental constraint on the evolution of ice-breeding seals, with adaptations reflected in life history tactics for storage and timely allocation of energy to rear pups. New capabilities in remote sensing and spatial analyses raise the prospects of measuring life history characteristics of populations once considered inaccessible, or prone to disturbance. We developed new tools to estimate pupping phenology and pup growth in harbor seals over two years at two tidewater glacial fjords in Alaska, USA: Icy Bay [IB], an aggregation of up to 11,000 seals on ice in a fjord complex of 3 retreating glaciers; and Disenchantment Bay [DB], an aggregation of about 2,000 seals on ice calved from a single advancing glacier. We used vertical photography to estimate seal body length (N=9,421 pups and 8,607 mothers over 35 surveys) and quantify features of their floating ice habitat. We found pup growth was faster (2.9 vs 1.2 mm/day) and phenology advanced (2-7 days) at IB. IB had 2-fold greater ice cover and ice for hauling out was 50% larger than DB. Typical of phocids, the largest pre-parturient females arrived first; females at IB were significantly larger. Hauling out is critical in pinnipeds, serving to conserve energy in adults and promote efficiencies in milk delivery and pup growth. At IB, glaciers retreating since the late 19^th century would have led to increased calving, a growing area of floating ice, and more reliable haul-out conditions than DB where the only remaining glacier has been advancing and producing less calved ice. In conjunction with rising human impacts at DB, we hypothesize that ideal, more isolated habitat at IB promoted energy conservation ultimately leading to higher pup survival and abundance. The use of glacial ice by the largest aggregations of harbor seals in the world suggests distinct benefits over tidally-influenced islets. We believe the formation of tidewater glaciers after the last ice age promoted life history strategies that led to large source populations which probably affected regional distributions of harbor seals. The present rapid retreat of Alaska’s tidewater glaciers may have repercussions for the state-wide abundance of harbor seals.