Climate change has led to abrupt declines in sea ice over the past three decades. Polar bears (Ursus maritimus) rely on sea ice as their primary habitat to hunt marine mammal prey. Due to their position at the top of the Arctic marine food web, polar bear foraging patterns can provide insights on ecosystem structure and function both spatially and temporally. This thesis used quantitative fatty acid signature analysis (QFASA) to estimate the diets, and adipose tissue lipid content to estimate the body condition, of polar bears in three western Canadian Arctic subpopulations: Northern Beaufort Sea, Southern Beaufort Sea, and Viscount Melville Sound. This thesis identified spatial, temporal, and intraspecific variation in the relationships between sea ice conditions, and polar bear diet and body condition. Polar bears with the greatest ecological constraints on diet composition may be most vulnerable to climate-related changes in ice conditions and prey availability.

The ecological response of climate change is expected to be especially pronounced across the circumpolar Arctic. Predicted declines in sea ice extent and seasonal duration are expected to affect the foraging ecology of marine species, particularly polar bears (Ursus maritimus) that rely on stable sea ice patterns for various aspects of their life history. The goal of this thesis was to quantify body condition and characterize diet composition of polar bears in western Hudson Bay over a broad temporal scale (2004-2014), and identify potential environmental factors that may influence dietary shifts in one of the southernmost subpopulations of this species. Body condition was higher in adult and subadult females than males, consistent with energetic demands of gestation and lactation. Body condition also declined over time in adult and subadult males and females and was influenced by sea ice breakup and freeze-up dates. These trends suggest that the historical climate-driven declines in polar bear body condition documented in western Hudson Bay have continued. Variation in diet composition and dietary niche breadth across age, sex and reproductive groups suggest foraging behaviour is structured by energetic demands, intraspecific competition and sexual body-size dimorphism. Specifically, variation in diet and niche breadth across females was influenced by age (experience), energetic state and avoidance behaviours. Variation in diet composition and niche breadth between male and female bears, however, was more likely structured by body size, whereby capture of larger prey types and a broader range of prey species occurred with increasing body size. Body condition was positively related to niche breadth in adult males but negatively related to niche breadth in females with dependents, suggesting that less-selective foraging (scavenging) does not benefit body condition among reproductive females. Inter-annual fluctuations in diet composition reflected shifts in local prey availability during the study period, and sea ice breakup date influenced the diets of subadults and family groups, suggesting an increased sensitivity to sea ice conditions. Therefore, inter-annual variability in diet and declines in polar bear body condition likely reflect contemporaneous changes in sea ice availability and population demography, and have implications for the long-term conservation of this subpopulation.

In Arctic Canada, Hudson Bay is a site of great exploration history, aboriginal culture, and a vast marine wilderness supporting large populations of marine mammals and birds. These include some of the most iconic Arctic animals like beluga, narwhal, bowhead whales, and polar bears. Due to the challenges of conducting field research in this region, some of the mysteries of where these animals move, and how they are able to survive in such seemingly inhospitable, ice-choked habitats are just now being unlocked. For example, are polar bears being replaced by killer whales? This new information could not be more salient, as the Hudson Bay Region is undergoing rapid environmental change due to global warming, as well as increased pressures from industrial development interests. A Little Less Arctic brings together some of the world’s leading Arctic scientists to present the current state of knowledge on the physical and biological characteristics of Hudson Bay.

Climate change is impacting the Hudson Bay region faster than any other portion of Arctic North America. As a consequence, polar bears ( Ursus maritimus) in western Hudson Bay, near the southern extent of their range, are already experiencing a phenological mismatch with their primary prey, ringed seals (Phoca hispida). These polar bears have relied on the energy stores amassed from hunting seal pups in spring to sustain them through the ice-free season on land for 4 to 5 months. As climate change causes the ice in Hudson Bay to melt earlier in spring, polar bears are projected to have less time to hunt seal pups on the sea ice, leaving them with smaller energy reserves to sustain them for longer periods on land. As a result, body condition is expected to deteriorate, leading to eventual declines in reproduction and survival, unless alternative energy sources are utilized.

"During spring 1976-79 we marked and recaptured polar bears in northern Labrador. Thirty-seven polar bears were captured and individually tagged. Most were found along the interface between the coastal landfast ice and the drifting pack ice, especially around the mouths of bays. The mark and recapture data indicated a low population (60-90 bears) in the area. Fifty to 80% fewer polar bears and tracks were seen per 100 km of potential habitat surveyed than recorded for other areas of the Arctic. This also indicates a smaller population"--Abstract.