Nicholas W. Pilfold

Age and Sex Composition of Seals Killed by Polar Bears in the Eastern Beaufort Sea

Background Polar bears (Ursus maritimus) of the Beaufort Sea enter hyperphagia in spring and gain fat reserves to survive periods of low prey availability. We collected information on seals killed by polar bears (n = 650) and hunting attempts on ringed seal (Pusa hispida) lairs (n = 1396) observed from a helicopter during polar bear mark-recapture studies in the eastern Beaufort Sea in spring in 1985–2011. We investigated how temporal shifts in ringed seal reproduction affect kill composition and the intraspecific vulnerabilities of ringed seals to polar bear predation. Principal Findings Polar bears primarily preyed on ringed seals (90.2%) while bearded seals (Erignathus barbatus) only comprised 9.8% of the kills, but 33% of the biomass. Adults comprised 43.6% (150/344) of the ringed seals killed, while their pups comprised 38.4% (132/344). Juvenile ringed seals were killed at the lowest proportion, comprising 18.0% (62/344) of the ringed seal kills. The proportion of ringed seal pups was highest between 2007–2011, in association with high ringed seal productivity. Half of the adult ringed seal kills were ≥21 years (60/121), and kill rates of adults increased following the peak of parturition. Determination of sex from DNA revealed that polar bears killed adult male and adult female ringed seals equally (0.50, n = 78). The number of hunting attempts at ringed seal subnivean lair sites was positively correlated with the number of pup kills (r2 = 0.30, P = 0.04), but was not correlated with the number of adult kills (P = 0.37). Conclusions/Significance Results are consistent with decadal trends in ringed seal productivity, with low numbers of pups killed by polar bears in spring in years of low pup productivity, and conversely when pup productivity was high. Vulnerability of adult ringed seals to predation increased in relation to reproductive activities and age, but not gender.

Migratory response of polar bears to sea ice loss: to swim or not to swim

&NA; Migratory responses to climate change may vary across and within populations, particularly for species with large geographic ranges. An increase in the frequency of long‐distance swims (> 50 km) is one predicted consequence of climate change for polar bears Ursus maritimus. We examined GPS satellite‐linked telemetry records of 58 adult females and 18 subadults from the Beaufort Sea (BS), and 59 adult females from Hudson Bay (HB), for evidence of long‐distance swimming during seasonal migrations in 2007–2012. We identified 115 swims across both populations. Median swim duration was 3.4 d (range 1.3–9.3 d) and median swim distance was 92 km (range 51–404 km). Swims were significantly more frequent in the BS (n = 100) than HB (n = 15). In the BS, subadults swam as frequently as lone adult females, but more frequently than adult females with offspring. We modelled the likelihood of a polar bear engaging in swims using collar data from the BS. Swims were more likely for polar bears without offspring, with the distance of the pack ice edge from land, the rate at which the pack ice edge retreated, and the mean daily rate of open water gain between June–August. Coupled with an earlier study, the yearly proportions of BS adult females swimming in 2004–2012 were positively associated with the rate of open water gain. Results corroborate the hypothesis that long‐distance swimming by polar bears is likely to occur more frequently as sea ice conditions change due to climate warming. However, results also suggest that the magnitude of the effect likely varies within and between populations.

Mass Loss Rates of Fasting Polar Bears

Polar bears (Ursus maritimus) have adapted to an annual cyclic regime of feeding and fasting, which is extreme in seasonal sea ice regions of the Arctic. As a consequence of climate change, sea ice breakup has become earlier and the duration of the open-water period through which polar bears must rely on fat reserves has increased. To date, there is limited empirical data with which to evaluate the potential energetic capacity of polar bears to withstand longer fasts. We measured the incoming and outgoing mass of inactive polar bears (n = 142) that were temporarily detained by Manitoba Conservation and Water Stewardship during the open-water period near the town of Churchill, Manitoba, Canada, in 2009–2014. Polar bears were given access to water but not food and held for a median length of 17 d. Median mass loss rates were 1.0 kg/d, while median mass-specific loss rates were 0.5%/d, similar to other species with high adiposity and prolonged fasting capacities. Mass loss by unfed captive adult males was identical to that lost by free-ranging individuals, suggesting that terrestrial feeding contributes little to offset mass loss. The inferred metabolic rate was comparable to a basal mammalian rate, suggesting that while on land, polar bears can maintain a depressed metabolic rate to conserve energy. Finally, we estimated time to starvation for subadults and adult males for the on-land period. Results suggest that at 180 d of fasting, 56%–63% of subadults and 18%–24% of adult males in this study would die of starvation. Results corroborate previous assessments on the limits of polar bear capacity to withstand lengthening ice-free seasons and emphasize the greater sensitivity of subadults to changes in sea ice phenology.

Seasonal habitat selection by adult female polar bears in western Hudson Bay

Individual variation in habitat selection has emerged as an important component necessary for understanding population ecology. For threatened species, where habitat loss and alteration affect population trends, understanding habitat use provides insight into mechanisms of population change. Polar bears, Ursus maritimus, in the Western Hudson Bay subpopulation have experienced declines in body condition, survival, and abundance associated with delayed freeze-up and earlier break-up of sea ice due to climate change. Although this subpopulation has been intensively studied, sea ice habitat selection remains poorly understood. We developed a habitat selection model using a mixed conditional logistic regression to determine habitat selection across seasons (freeze-up, early winter, late winter, break-up) and assess individual variation in habitat selection. We used 8487 locations collected between 2004 and 2010 from 64 GPS satellite linked radio-collars on adult females to compare habitat selected to habitat available. Selection changed across seasons and varied the most among individuals during the freeze-up and break-up seasons. During later winter, there was less variation in habitat selection among individuals and bears showed the least amount of selection in habitat use. Distance to the denning area, a core terrestrial refuge habitat, was the top-ranked covariate in all seasons suggesting site fidelity plays a role in habitat selection. Some individual variation may have been due to reproductive status, though we could not account for this directly. Recognizing individual differences, especially in a rapidly changing environment, allows managers to identify critical habitats instead of simply average resources, and may lead to more successful efforts to protect habitats.

Polar bear ( Ursus maritimus ) Migration from Maternal Dens in Western Hudson Bay

Migration is a common life history strategy among Arctic vertebrates, yet some of its aspects remain poorly described for some species. In February-March, post-parturient polar bears ( Ursus maritimus ) in western Hudson Bay, Canada, migrate from maternity den sites on land to the sea ice with three- to four-month-old cubs. We investigated this migration using data from 10 adult females fitted with satellite-linked global positioning system collars tracked in 2011 – 16. Directed movement towards the coast began on average on 1 March (range: 31 January to 23 March) and took a mean of 7.8 days to reach the coast. Bears traveled 18 to 100 km from their dens to the coast (mean = 63 km) at a mean rate of 6.7 km/d. Movements were highly directed, with an approximate northeast orientation, but did not follow the shortest path to the coast. Observed migration patterns were broadly similar to those previously documented, although mean departure date from dens was about four days earlier and mean movement rate was only 40% of that from the late 1990s. Given the sensitivity of polar bears to climate change, the phenology of denning may be a meaningful parameter for long-term monitoring.