Sea ice retreat affects migrations of bowhead whales | Polarjournal
Bowhead whales prefer proximity to sea ice and are the only baleen whales that live year-round in Arctic and subarctic waters. Photo: Heiner Kubny

Greenland whales are slowly recovering after the end of commercial whaling. There are now about 25,000 of them in the Arctic, in four populations. But their habitat is changing due to climate change. The decline in sea ice in recent years has already led to an adjustment in the migratory behavior of bowhead whales, which like to stay near sea ice, two Oregon State University researchers show in their new study. They published their findings in the journal Movement Ecology.

One of the four Greenland whale populations is the so-called Bering-Chukchi-Beaufort population. It is the largest and appears to be growing, according to Kathleen Stafford, associate professor at the Marine Mammal Institute at Oregon State University’s Hatfield Marine Science Center and co-author of the study. The whales normally winter in the northern Bering Sea and migrate north through the Bering Strait into the Canadian Beaufort Sea in spring. They spend the summer and fall there, returning south to the Bering Sea in winter with the expanding sea ice that blocks the Bering Strait.

Rising temperatures in the Arctic over the past decade have led to a decline in sea ice, keeping the strait increasingly open during winter months, explains Angela Szesciorka, a research associate at Oregon State University’s Marine Mammal Institute, and lead author of the study.

“The lack of ice means they are losing this critical habitat, and as a result, we’re seeing that these whales are not leaving the Arctic anymore for the winter,” Szesciorka says. “Without that ice, there could be changes in bowhead availability for the Indigenous people who rely on the whales. The lack of ice also opens the door for other species to move into the Arctic, resulting in competition for resources, potential predation and increased human interaction due to ship strikes or entanglement in fishing gear.”

The slow-swimming bowhead whales feed on copepods and krill. It is believed that they can live up to 200 years. Photo: Kathleen Stafford, Marine Mammal Institute, Oregon State University.

Scientists believe that sea ice plays an important role in the life of bowhead whales. According to Stafford, the slow-moving whales may be using the sea ice as protection from potential predators. In addition, the ice-covered water could also improve communication between animals.

However, Arctic sea ice has decreased by about 13 percent per decade since 1979. In the Chukchi Sea there was once perennial ice, now only annual, which does not survive the summer.

In spring, bowhead whales normally migrate north from the Bering Sea to Canada’s Beaufort Sea, where they spend the summer and fall. However, due to the decline in sea ice, some whales now no longer return to the Bering Sea in winter, but stay in the Chukchi Sea. The red circle marks the position of the acoustic monitoring devices. Map: Szesciorka & Stafford 2023

The two authors analyzed whale calls and songs recorded between 2009 and 2021 in the Chukchi Sea near the Bering Strait to track how whale migrations have changed with sea ice retreat. The sounds of passing ships were also recorded.

“Bowheads make a number of non-singing calls, but in the fall, winter and into spring, they are singing,” Szesciorka says. “We think it’s the males who are singing, and that the songs are for courtship purposes. They sing many different songs and they don’t tend to repeat. It’s beautifully complex.”

The researchers combined their findings with sea ice and weather data and found that the whales’ fall migration to the Bering Sea was delayed in years with less sea ice and that some animals wintered in the southern Chukchi Sea instead.

“The Strait is the only gateway between the Arctic and the Pacific – anything going between the two has to pass through there, like a turnstile,” Stafford says. “Not all of the bowheads are passing through this turnstile anymore.”

With their large skulls, bowhead whales can break through ice up to 45 centimeters thick. Photo: Heiner Kubny

In addition, whales migrated north earlier in years with little sea ice in spring. Indigenous traditional knowledge also suggests that less ice and more open water shifted the timing of spring migration by about a month. Because indigenous communities rely on bowhead whales for food and for their cultural and spiritual livelihoods, the researchers say the whales’ changing migration patterns could have an impact on them as well.

“Bowheads have been hunted for millennia by Arctic peoples, but in the fall of 2019, there were no whales in reach of Indigenous hunters in Utqiagvik, Alaska,” Stafford says. “That has the potential to decrease food security in these communities, and that is problematic.”

In addition, the lack of sea ice means that the Bering Strait is open to potential predators such as killer whales and to commercial ships, from which bowhead whales have previously been safe in winter.

“There are some big questions for future study: Will bowheads be at increased risk of ship strikes or fishing gear entanglement if the lack of sea ice leads to increased fishing or other ship traffic? Bowheads aren’t typically around vessels, and they may not know how to respond,” Szesciorka said. “This change is happening very quickly, and it is unclear what the potential impacts might be as the Arctic continues to warm.”

Julia Hager, PolarJournal

Featured image: Heiner Kubny

Link to the study: Angela R. Szesciorka, Kathleen M. Stafford. Sea ice directs changes in bowhead whale phenology through the Bering Strait. Movement Ecology, 2023; 11 (1) DOI: 10.1186/s40462-023-00374-5.

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