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

Abstract

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.

Haul-out patterns and effects of vessel disturbance on harbor seals (Phoca vitulina) on glacial ice in Tracy Arm, Alaska

“…However, Hoover-Miller et al (2011) reported peak pup counts in Aialik Bay, KFNP, in early-to midJune, indicating regional variation in pup birth dates. The maximum proportion of pups from our counts (30-36%) was also similar to the high values reported for seals in other tidewater glacial fjords (KFNP: 21-34%, Hoover, 1983, Hoover-Miller et al, 2011GBNP: 34-40%, Calambokidis et al, 1987, Mathews and), but ~3 times greater than the proportion of pups in Disenchantment Bay (Jansen et al, 2015). Note also that the proportion of pups can be affected by immigration or emigration of nonpups in addition to changes in productivity.…”

“…Seal counts and vessel numbers Higher vessel counts were not associated with reduced seal counts in 2001 (Table 1), a pattern also noted by Jansen et al (2015) in Disenchantment Bay. However, our total counts included seals in the water, and therefore they were less sensitive to vessel disturbance.…”

“…Other studies also have found that harbor seal counts are affected by environmental factors, but such factors can be site specific or might not be detected in a study because of the limited range of conditions under which surveys were conducted (e.g., Boveng et al, 2003;Simpkins et al, 2003;Jemison et al, 2006, Jansen et al, 2015. Although harbor seal haul-out patterns on glacial ice typically are not affected by tide stage (Calambokidis et al, 1987;Boveng et al, 2003;Mathews and Pendleton, 2006;but see Hoover-Miller et al, 2011), they are affected by the availability of floating ice.…”

“…Calambokidis et al (1987) and Young et al (2014), both working in GBNP, found that seal counts were positively related to percent ice cover, the proportion of seals counted in the water was negatively related to percent ice cover, and, when ice cover was low, hauled-out seals were concentrated on the few remaining icebergs. Jansen et al (2015) reported that seals were most abundant with intermediate ice densities (5-7 tenths coverage).…”

“…Research on harbor seals in glacial fjords in Alaska has been concentrated in GBNP (Streveler 5 ; Calambokidis et al 1 ; Mathews and Kelly, 1996;Mathews and Pendleton, 2006;Young, 2009;Womble et al, 2010;Blundell et al, 2011;Womble and Gende, 2013), Kenai Fjords National Park (KFNP) (Hoover, 1983;Jezierski, 2009;Hoover-Miller et al, 2011, and Disenchantment Bay (Jansen et al, 2010(Jansen et al, , 2015. Tracy Arm is a glacial fjord that is used by more than 1000 harbor seals during the summer and that has high tourist visitation.…”