What's Happening to Polar Bears and Narwhals as Arctic Ice Melts

Polar bears and narwhals are particularly vulnerable to the threats from climate change. As Arctic sea ice melts, their hunting and eating patterns have had to shift, threatening their survival.

Researchers recently studied the impact of warming temperatures on these iconic polar species. They released their findings in part of a special issue in the Journal of Experimental Biology focused on climate change.

Climate change has had an immense impact on Arctic sea ice. Arctic sea ice reaches its minimum each September. September Arctic sea ice is now declining at a rate of 13.1% per decade, according to the U.S. National Snow and Ice Data Center (NSIDC).

The timing of sea ice breakup in the spring is happening earlier each year and sea ice return in the fall is occurring progressively later, points out Anthony Pagano, review coauthor and postdoctoral research fellow in population sustainability for San Diego Zoo Global.

This change in sea ice decreases the length of time that polar bears have to hunt seals on the ice.

“In particular, the main feeding period for polar bears is in the late spring and early summer when seals are giving birth and weaning their pups and the concern is that earlier ice breakup will reduce the amount of time polar bears have to catch seals during this time,” Pagano tells Treehugger. 

“Additionally, polar bears are increasingly reliant on summer land use due to declines in Arctic sea ice. Polar bears will consume land-based food, but the energy available from most prey on land is not adequate to compensate for lost feeding opportunities of seals on the sea ice.”

Polar Bears and Dining Changes

When polar bears have to hunt on land instead of ice, they rely on lower-calorie diets. The researchers write, “A polar bear would need to consume approximately 1.5 caribou, 37 Arctic char, 74 snow geese, 216 snow goose eggs (i.e. 54 nests with 4 eggs per clutch) or 3 million crowberries to equal the digestible energy available in the blubber of one adult ringed seal.”

They add, “Few resources exist on land within the polar bears' range that could compensate for declines in seal feeding opportunities."

Relying on terrestrial dining instead of seals has consequences for the health and longevity of polar bears.

“As bears are increasingly reliant on summer land use and are displaced from the sea ice earlier in the summer, they are likely to experience declines in body condition, which may lead to reduced reproductive success and survival,” Pagano says. “In some polar bear populations, increased summer land use has already been linked to decreased body condition, survival, and abundance.”

In some cases, declines in sea ice have forced bears to swim long distances in order to find food. Some bears have had to swim as long as 10 days.

“These swims are energetically expensive for polar bears and are likely to threaten reproductive success of females and survival,” Pagano points out. “Additionally, in some regions of the Arctic, polar bears appear to be moving greater distances to follow the pack ice as it retreats further into the Arctic Basin than it did historically. Any increase in energy expenditure combined with potential decreases in access to prey threaten their long-term energy balance and survival.”

Narwhals Face Threats

Narwhals also face consequences due to sea ice loss. They are exposed to the negative results of human activities such as pollution from shipping and fisheries, and there’s an increased presence of killer whales.

“Narwhal responses to both of these threats include a decrease in routine diving behavior and an increase in energetically expensive swims away from these threats,” Pagano says. “In combination, the preferred prey of narwhals are expected to decline with continued sea ice declines, which similar to polar bears, further threatens their energetic balance.”

In addition, because of the high amount of energy they spend from diving, and the loss of breathing holes they depend on due to sea ice shifts, many more narwhals have become trapped beneath the ice as their migration seasons have become more unpredictable.

As the population of polar bears and narwhals drop, the changes affect the Arctic ecosystem. Both species are apex predators in the Arctic, Pagano points out.

“They are also highly dependent on the Arctic sea ice, which makes them important sentinels of the impacts of climate change on the Arctic marine ecosystem,” he says. “Declines in polar bears will impact ice seals and their prey (primarily Arctic cod), but ice seals themselves are also likely to be challenged by forecasted declines in Arctic sea ice.”

Similarly, population declines in narwhals will likely indicate declines in their fish prey.

Pagano warns, “Overall, future declines in polar bears and narwhals are likely to foretell large changes in the Arctic marine ecosystem.”