Polar Bears and Narwhals Are Using Three to Four Times as Much Energy to Survive as Arctic Sea Ice Declines

Warming temperatures in the Artic and resulting sea ice loss are impacting several species well-adapted to the environment, putting their survival at risk. The animals have to work harder to adjust to their changing habitats. A new paper sheds light on just how much effort this is taking.

A report published February 24th in the Journal of Experimental Biology says polar bears and narwhals, which are especially vulnerable to the speed and unpredictable sea ice deterioration in the Arctic, are expelling three to four times as much energy to survive as they typically would. The scientists behind the paper say this is largely due to disruptions of their hunting patterns, with narwhals going deep under the surface to get their meals and polar bears hunting on the top of ice. Polar bears are consuming fewer calories as ice changes, and narwhals run the risk of being trapped within the ice because of unreliable breathing holes.

Polar bears do much of their hunting on sea ice over continental shelf habitats with depths under 300 meters. There, they avoid large energy output for hunting by waiting at breathing holes to catch seals. Changes to their habitat are making this more difficult.

The paper says, “As climate change results in progressively earlier sea ice break-up and retreat, polar bears are displaced from their primary foraging habitats earlier, with diminished opportunities for catching seals. Additionally, as the return of sea ice in the autumn becomes progressively later, the period of seal accessibility becomes further reduced. Ice seals represent a substantial energetic pay-off, making other prey less profitable to energetic intake.”

Meanwhile, narwhals have adapted to dive to depths past 1500 meters to reach their prey. They get large amounts of their energy intake in the winter by feeding on halibut at the sea’s bottom. They also feed on Arctic cod, squid, shrimp, and capelin, primarily in the summer. Their feeding habits have allowed their bodies to adapt to holding their breath for long periods of time. Due to these long stretches of being submerged, it’s important for them to find reliable breathing holes.

Dr. Terrie Williams from the department of ecology and evolutionary biology at the University of California, Santa Cruz, is one of the paper’s authors. She says narwhals are increasingly having trouble locating good breathing holes.

She explains, “The Arctic world is so much more unpredictable for these animals now. With a finite amount of oxygen in their muscles and blood, we find that the narwhals budget their speed, depth, and duration of dives to match the capacity of their internal scuba tanks. One miscalculation could result in drowning.”

The paper also mentions that the loss of summer ice has attracted more killer whales, which can kill narwhals, and more human activity, which narwhals are sensitive to.

As apex predators, declines in polar bears and narwhals are apt to have a large impact on the Arctic ecosystem.

The paper’s conclusion says, “Declines in these species are likely to foreshadow declines in other ice-dependent marine mammals and some of their principal prey, such as Arctic cod that rely on sea ice-associated zooplankton.”

The paper comes after 2020 saw the second-lowest Arctic sea ice cover since records began in the late 1970s. An analysis of satellite data by NASA and the National Snow and Ice Data Center at the University of Colorado Boulder found that the ice measured as low as 1.44 million square miles last September. This was 958,000 square miles below the average minimum between 1981 and 2010. The only time it had been lower was in 2012. NASA also says that September Arctic sea ice has been declining an average of 13.1% per decade since 1979.