Bowhead whales are delaying migration as Arctic ice declines

Bowhead whales are the only whales to spend their entire lives in the Arctic and sub-Arctic, where they use their massive baleen plates to filter out planktonic crustaceans and other prey from the water. They are known to have the largest mouth of any living animal, and the greatest longevity of any mammal – they may live to well over 200 years.

The population of bowhead whales that is found off Alaska spends the winter months in the southwestern Beiring Sea and migrates northwards through the Beiring Strait in spring, as the sea ice opens up. The group spends summer and fall in the Chukchi Sea off Alaska, and in the Beaufort Sea off Canada. Since the movements of the whales depend on there being gaps in the sea ice, it is possible that their migration patterns may be changing as global warming affects the extent and timing of sea ice formation.

The Arctic has experienced more rapid climatic change than the global average, with near-surface air temperatures increasing four times faster since 1979, and annual minimum sea ice extent decreasing by 13 percent per decade in the same period. Sea ice that was once perennial in the Chukchi Sea is now considered annual – meaning that the ice is no longer surviving through the melt season.

A new study by researchers from the Marine Mammal Institute at Oregon State University has now investigated whether the migratory patterns of the western Arctic bowhead whales is being affected by these changes in sea ice. This group is also known as the Bering–Chukchi–Beaufort (BCB) group of bowhead whales and its migration pathway has already been well established from multiple sources of data, including aerial surveys, satellite tagging, acoustic monitoring, and Indigenous knowledge.

Sea ice is believed to play an important role in the bowheads’ survival. Slow-moving animals may use sea ice as shelter from potential predators, and the ice-covered water might also lend itself to improved communication among the individuals. In the past, the whales would follow the sea ice south through the Bering Strait to their wintering grounds. The Strait would close up again as ice formed in the Chukchi Sea. 

But warming temperatures in the Arctic over the past decade have led to sea ice decline and kept the Bering Strait open increasingly into the winter months, said the study’s lead author, Angela Szesciorka, a research associate at the Marine Mammal Institute. The researchers had also received information for the first time that some of the BCB bowhead whales had spent the entire 2018–2019 winter at what is normally their summer foraging grounds in the Amundsen Gulf and eastern Beaufort Sea.

In order to investigate the ways in which whale migratory patterns might have changed, the researchers analyzed 11 years of recordings of bowhead whale calls and songs, made using passive acoustic monitoring devices placed in the Chukchi Sea near the entrance to the Beiring Strait. These recordings were made between 2009 and 2021 and they captured the songs and the simple, low frequency sounds made by whales as they keep in contact with one another. 

“Bowheads make a number of non-singing calls, but in the fall, winter and into spring, they are singing,” Szesciorka said. “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.” 

A total of roughly 22,235 h of bowhead song, and 4,669 h of simple, non-song calls was recorded during the study period. These calls gave the researchers an indication of when the whales were present in the vicinity of the Beiring Strait, each year over 11 migration cycles. Data on daily sea ice extent in the Beiring and Chukchi Seas was derived from Sea Ice Index V3.

The results, published in the journal Movement Ecology, showed that the timing of the whales’ southward movement through the Beiring Strait in fall was significantly correlated with the amount of sea ice cover in the Chukchi Sea. If there was a lot of sea ice present then the southward passage took place early, but it was delayed in years when the extent of sea ice cover was less. In recent years, the diminishing ice cover has also been linked to the first documented cases of whales remaining in their summer grounds throughout the winter.

“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,” said study co-author Kathleen Stafford, an associate professor at the Marine Mammal Institute, “Not all of the bowheads are passing through this turnstile anymore.”

The researchers also found that spring northward migration was earlier in years when there was less sea ice. Indigenous Traditional Knowledge also suggests that less ice and more open water has shifted the timing of the spring migration by about a month. Those changing migration patterns could impact the Indigenous communities that rely on bowhead whales for nutritional, cultural and spiritual subsistence, the researchers said.

“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 said. “That has the potential to decrease food security in these communities, and that is problematic.”

“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 said. “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.”

“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.”

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By Alison Bosman, Earth.com Staff Writer

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