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Scientists have found that narwhals are altering their steadfast migration patterns in response to climate change, offering some hope that the tusked whales and other species may be able to adapt to a rapidly warming Arctic.
Researchers analyzed satellite tracking data from 40 narwhals that had been tagged between 1997 and 2018. They determined that with sea ice forming later in the year due to rising temperatures, the ice-dependent whales have delayed migrating to their deep-ocean winter feeding grounds by 10 days per decade. The marine mammals are also taking longer to make the 900-mile (1,500-kilometer) journey from the coastal areas of the Canadian Arctic where they spend the summer.
“The fact that narwhals are changing their behavior over what we assume are individual lifetimes could be an indicator that potentially these other animals might be able to leverage behavioral changes to bolster themselves against the effects of climate change," said Courtney Shuert, a post-doctoral fellow at the University of British Columbia and lead author of the peer-reviewed study published in the Proceedings of the National Academy of Sciences last month.
The long-term success of this kind of adaptation may depend on how fast humans reduce greenhouse gas emissions that are melting the Arctic.
“The rate of climate change is almost impossible for species to adapt to through mechanisms of evolution," said Shuert.
For narwhals, climate change poses an extinction-level threat due to the loss of sea ice on which they depend for food and protection. As ice disappears, predatory orcas have expanded their range further into the Arctic while huge ships ply now-open Arctic waters. Warming seas also endanger narwhal’s preferred prey — polar cod, Arctic cod and halibut — which thrive in a narrow temperature range.
Known as the unicorns of the sea for their single, long spiraled tusk, narwhals are estimated to live as long as 100 years or more. But they’re very particular about where they live and where they travel. Researchers demonstrated in a 2020 paper published in the journal Scientific Reports that narwhals have a “narrow thermal preference," with a small rise in ocean temperature reducing their abundance.
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Research has shown that narwhals in the past resolutely stuck to the same migratory routes and schedules throughout their lives, according to scientists. The whales, which can dive to depths of more than a mile hunting for prey, spend their winters in the deep ocean. They prefer areas where ice covers more than 95% of the sea but contains fissures that allow them to surface to breathe.
When that ice cover breaks up in the spring and exposes narwhals to predators, they migrate to coastal areas because, scientists believe, shallower bays and fjords there give them refuge while rearing young.
When the weather starts to turn colder in September, narwhals depart before “fast ice" can form in their summer grounds. Fast ice is attached to land and can block their migratory routes, suffocating them under a frozen expanse with no openings to the surface.
But as the Arctic Ocean warms and delays ice formation, narwhals are lingering longer in the summer and taking more time to reach their winter habitat, according to the satellite tracking data analyzed by Shuert and her colleagues. How the whales know to postpone their journey and what they’re doing during their extended summer sojourn remains largely unknown.
“We can't really say for sure exactly what they're sensing on a day-to-day basis," said Shuert. “They're very intelligent animals, and some of the data suggests that they're probably sensing the environment that they're in currently plus have a memory of past events, such as if the spring ice breakup was really early that year."
She said it’s possible that narwhals spend the extra time exploring new areas for prey.
Determining the impact of a decline in winter ice on narwhals remains difficult due to the limits of satellite tag technology. Narwhals are captured and tagged in shallower areas but by the time they reach their winter grounds, the tags’ batteries typically have died, according to Shuert.
If narwhals overstay their summer break, the paper states, they could risk being entombed by a sudden formation of ice triggered by climate-driven temperature swings. A 2011 study published in the journal Polar Biology documented four instances between 2008 and 2010 where hundreds of narwhals died after being trapped by ice when temperatures quickly plunged.
Mads Peter Heide-Jørgensen, a co-author of the 2011 paper and a professor at the Greenland Institute of Natural Resources, said no entrapments, which typically are discovered in remote areas by Inuit hunters, have been found over the past decade.
“If the ice is generally disappearing (not just less predictable), then there should be less ice entrapments," Heide-Jørgensen, a veteran narwhal researcher who is also an author of the new migration paper, said in an email.
Ship collisions pose a mortal danger to whales worldwide but Heide-Jørgensen said narwhals are at less at risk than other species. “There is generally more ship traffic all over the Arctic but the risk for collisions with narwhals is small as narwhals react at long distance to ship noise," he said.
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- FIRST PUBLISHED25.11.2022 | 05:00 PM IST