Natural and human effects on harbor seal abundance and spatial distribution in an Alaskan glacial fjord

摘要

Tidewater glacial fjords support the largest populations of harbor seals (Phoca vitulina richardii) in Alaska and are a prime destination for tour ships. Chronic disturbance from ships, however subtle, could impact long-term population stability. We examined variation in abundance and distribution of harbor seals on floating ice in Disenchantment Bay, Alaska, a tour ship destination for over a century with near daily visitation by ships in the spring/summer over the last decade. Counts of seals by aerial transect showed a sharp decline in May, prior to pupping and the first ships arriving; counts rebounded by the end of June remaining high until August. Seal distribution and abundance peaked in 5-7 tenths ice cover; total area of ice cover showed no effect. Despite regular flushing of seals by ships, we found no broad-scale patterns in seal abundance and distribution that could be explained by ship presence. We cannot rule out mechanisms of long-term disturbance, difficult to detect and that might explain notable differences with other, similar sites. Population declines at disturbed glacial sites and the still rising popularity of vessel-based tourism indicate a need for individual-based studies on how seals respond to the dynamics of glacial ice environments and human-caused stresses.