Are Polar Bears on Thin Ice?

By: Kaylie Anne Costa, SRC Intern

When you think of polar bears what comes to mind? Is it a mama bear and a cub struggling to swim miles to find a piece of sea ice? Because that is exactly what is beginning to occur in the Arctic. With the rise of the sea surface temperatures, more and more sea ice is thawing causing the polar bears breeding and hunting grounds as well as means for transportation to disappear.

Figure 1: Polar bears using sea ice for transportation (By NOAA Photo Library – anim0115, Public Domain,

Polar bears have a varied diet consisting of seals, birds, fish, whales, and other marine resources. They also utilize a mixture of hunting methods. For example, polar bears may stalk seals in the open ocean or sneak up on seals that are drifting on sea ice. When there is not sea ice, polar bears must rely much more heavily on their swimming skills for transportation and hunting.

In a recent study, Lone et. al (2018) studied the time that female polar bears spend in the water to gain understanding as to how polar bears might react to future decreases in Arctic sea ice. 57 adult polar bears were tagged with devices to gather data on their locations, the amount of time spent swimming, and the diving depths. This study showed that polar bears’ choice of hunting strategies, and therefore amount of swimming, greatly depends on the individual. In addition, environmental factors and if the females had cubs also impacts the time a polar bear spends swimming. Polar bear cubs lack the thick layer of fat that insulates their bodies leaving them more susceptible to hypothermia. As a whole, the main variable that influence the swimming behaviors of the polar bears was the seasonal variation in sea ice. The most swimming occurred in summer and fall with less swimming occurred during the winter and spring. Modeling techniques were also used to correlate increased swimming with decreased levels of sea ice.

Figure 2: Polar bear swimming (

Overall the polar bears appeared well adapted to arctic marine environments and were able to complete long distance swims and dive greater than 10 meters. As sea ice continues to disappear, more polar bears will be required to alter their choices of hunting strategies to adapt to the new environment. This study shows promise in polar bears’ ability to adapt to reduced sea ice, at least to a certain extent. Further studies will need to be completed to analyze the impacts that additional swimming behavior will have on the polar bears health overall.

Works Cited

Lone, K., Kovacs, K. M., Lydersen, C., Fedak, M., Andersen, M., Lovell, P., & Aars, J. (2018). Aquatic behaviour of polar bears (Ursus maritimus) in an increasingly ice-free Arctic. Scientific reports8(1), 9677.

Effects of Global Warming on Polar Bears in the Arctic

by Dani Ferraro, RJD intern

Global warming and the loss of Arctic sea ice is affecting populations of polar bears (Ursus maritimus) in Hudson Bay. Localized rises in sea surface temperatures (SST) have lead to mortality events and habitat changes for several marine species (Dulvy et al. 2008). While some species have adaptations that allow them to tolerate warming events, the loss of habitat and consequent die-offs of prey species is devastating.  The Hudson Bay Lowlands (HBL), the second largest inland sea in the world and home to polar bears, has warmed approximately three degrees Celsius since the 1990s (Ruhland et al. 2013).  With warmer air temperatures and increasingly rising SST comes the loss of winter ice-cover and reduced snow depth. This has directly caused the mortality of polar bear cubs and their prey, the ringed seal (Phoca hispida) and the bearded seal (Erignathus barbatus). As the forage and movement patterns of ringed seals and closely linked with sea ice, loss of this habitat could explain this mortality. The latest population estimates are about 21,500-25,000 individuals throughout the circumpolar Arctic (Luque et al. 2014).

 Ice formation in early November in Hudson Bay, Canada. Image Source: Wikimedia Commons

Ice formation in early November in Hudson Bay, Canada. Image Source: Wikimedia Commons

As a k-selected species, polar bears have delayed maturation and high adult survival rates, but smaller litter sizes. Sea ice acts as a polar bear’s hunting grounds, with terrestrial habitats as their maternity and breeding grounds. For female polar bears, impacts beyond loss of habitat exist. With reduced sea ice, females will have a cascading loss of adipose stores, causing lowered reproductive rates. This loss of adipose means that females have less fat to invest in their cubs throughout the winter season and subsequent fasting season. With reducing sea ice thickness, it becomes thinner and more pliable to winds and currents. Polar bears will respond with increased walking or swimming, using higher energy in order to retain their habitat range.

It’s important to acknowledge the differences in sea ice thickness and location. Polar bears prefer the annual sea ice located over the inter-island archipelagos and continental shelf surrounding the polar basis. This sea ice has declined in near shore areas and in amount of multiyear ice. With this decline comes the decrease in preferred habitat locations for polar bears, as well as other pagophilic species throughout the arctic marine ecosystem. Large expanses of open water due to melting sea ice often separates terrestrial maternity dens from residential pack ice. Pregnant females have a tendency to leave their residential areas during ice break-up and remain separated throughout the summer. In order to endure the summer before they can return to sea ice to feed, females need to have built up sufficient fat stores to sustain themselves for at least 8 months. However, considering the preferred location of polar bears: the deep polar basin, where there is a lower seal density, females will find difficulties obtaining sufficient fat stores. Without having accumulated adequate adipose stores, females have fewer nutrients to pass along to nursing cubs. Due to lower energy and fat stores, females are more likely to give birth to single cub litters, often with low survival rates caused by small body mass (Derocher).

Image 2 Ferraro

Polar Bear (Ursus maritimus) Image Source: Wikimedia Commons



With increasing SST and breaking sea ice, polar bears use more energy moving against the direction of ice drift. If ice moves more quickly, more energy is needed to move and hunt accordingly. Once sea ice concentration falls below 50%, polar bears tend to stick to terrestrial environments. Hunting and hauling prey onto land is energetically costly, requiring older polar bears to consume more, leaving fewer scraps for juveniles to scavenge. Combined with lower female productivity, the loss of food for juveniles doesn’t bode well for polar bear populations in the future. The impacts of climate change and global warming are already being seen with increasing sea surface temperature and decreasing sea ice depth. These habitat changes cause a cascading shift down the Arctic ecosystem, from habitat loss to mass mortality and reduced productivity. There will be shifts in survival rates, maturation age, and reproductive rates in populations of polar bears as well as that of its prey, both the bearded seals and ringed seals. With such a limited habitat in the circumpolar Arctic, global warming and climate change have a drastic effect on their populations, environments, and breeding habits.



Derocher, A. (2004). Polar Bears In A Warming Climate. Integrative and Comparative Biology, 163-176.

Dulvy, N.K., Rogers, S.I., Jennings, S., Stelzenmuller, V., Dye, S.R. & Skjoldal, H.R. (2008) Climate change and deepening of the North Sea fish assemblage: a biotic indicator of warming seas. Journal of Applied Ecology, 45, 1029–1039.

Luque, S., Ferguson, S., & Breed, G. (2014). Spatial behaviour of a keystone Arctic marine predator and implications of climate warming in Hudson Bay. Journal of Experimental Marine Biology and Ecology, 504-515.

Ruhland, K., Paterson, A., Keller, W., Michelutti, N., & Smol, J. (2013). Global warming triggers the loss of a key Arctic refugium. Proceedings of the Royal Society B: Biological Sciences, 20131887-20131887.