Abstract
Tidewater glaciers calve icebergs into the marine environment which serve as pupping, molting, and resting habitat for some of the largest seasonal aggregations of harbor seals (Phoca vitulina richardii) in the world. Although they are naturally dynamic, advancing and retreating in response to local climatic and fjord conditions, most tidewater glaciers around the world are thinning and retreating. Climate change models predict continued loss of land-based ice with unknown impacts to organisms such as harbor seals that rely on glacier ice as habitat for critical life history events. To understand the impacts of changing ice availability on harbor seals, we quantified seasonal and annual changes in ice habitat in Johns Hopkins Inlet, a tidewater glacier fjord in Glacier Bay National Park in southeastern Alaska. We conducted systematic aerial photographic surveys (n = 55) of seals and ice during the pupping (June; n = 30) and molting (August; n = 25) periods from 2007 to 2014. Object-based image analysis was used to quantify the availability and spatial distribution of floating ice in the fjord. Multivariate spatial models were developed for jointly modeling stage-structured seal location data and ice habitat. Across all years, there was consistently more ice in the fjord during the pupping season in June than during the molting season in August, which was likely driven by seasonal variation in physical processes that influence the calving dynamics of tidewater glaciers. Non-pup harbor seals and ice were correlated during the pupping season, but this correlation was reduced during the molting season suggesting that harbor seals may respond to changes in habitat differently depending upon trade-offs associated with life history events, such as pupping and molting, and energetic costs and constraints associated with the events.
Highlights
The distribution of abiotic and biotic resources required for animal survival and reproduction is seasonally dynamic and can change in space and time (Fretwell, 1972)
Understanding how ice habitat changes is important for understanding patterns in the distribution and abundance of harbor seals, given the expected future changes to tidewater glaciers and associated fjord ecosystems
During the pupping period, when there was more ice and the distribution of ice was more extensive, non-pup harbor seals and glacier ice were more strongly correlated; this correlation was reduced during the molting season
Summary
The distribution of abiotic and biotic resources required for animal survival and reproduction is seasonally dynamic and can change in space and time (Fretwell, 1972). Harbor Seals as Ice Sentinels can be complex interactions between life history events, the physiological state of the animals, and the abiotic and biotic environment which can result in trade-offs during the annual cycle (Stephens et al, 2014). Tidewater glaciers undergo substantial seasonal variation which can influence freshwater flux, vertical mixing, nutrient input (Bartholomew et al, 2010; Howat et al, 2010; Moon et al, 2014; Arimitsu et al, 2016; Amundson and Carroll, 2017; Fried et al, 2018), biological productivity, marine food webs, and create important habitats for invertebrates, fish, seabirds, and marine mammals during critical phases of the annual cycle (Etherington et al, 2007; Lydersen et al, 2014; JuulPedersen et al, 2015; Meire et al, 2017). Ice that is discharged or calved from the terminus of tidewater glaciers produces icebergs that serve as resting, pupping, and molting habitat for several species of pinnipeds including ringed seals (Pusa hispida), bearded seals (Erignathus barbatus), and harbor seals (Phoca vitulina) (Calambokidis et al, 1987; Lydersen et al, 2014; Hamilton et al, 2016, 2019)
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