The danger is being dramatically highlighted this year by a colossal, 70 kilometre-long “ice island” that broke away from a Greenland glacier and is now drifting toward Baffin Bay — the principal habitat for the world’s estimated 80,000 narwhals.
The study has calculated that narwhals must surface for air at least every 1.4 km, meaning that whole pods of whales could drown if they swim too far out beneath large ice floes like the 250-sq.-km Petermann Ice Island — the single biggest free-floating mass of ice in Arctic waters since the early 1960s.
The U.S. study, published last week in the journal Marine Mammal Science, was co-authored by biologists Terrie Williams and Shawn Noren of the University of California and researcher Mike Glenn from Sea World of San Diego.
“Rapid changes in sea ice cover associated with global warming are poised to have marked impacts on polar marine mammals,” the U.S. researchers state.
They note that several of the narwhal’s physical traits — such as muscles built for long journeys rather than rapid swimming speeds — could place the air-breathing creatures at serious risk in their increasingly ice-choked Arctic habitats.
“We found that extreme morphological and physiological adaptations enabling year-round Arctic residency by narwhals limit behavioural flexibility for responding to alternations in sea ice,” the study states.
The researchers found that the narwhal’s maximum swimming distance of about 1,450 metres between breathing holes “permits routine use of only 2.6 to 10.4 per cent of ice-packed foraging grounds in Baffin Bay” off the northeast coast of Baffin Island.
“These first measurements of narwhal exercise physiology reveal extreme specialization of skeletal muscles for moving in a challenging ecological niche,” the researchers conclude.
Williams, the team’s lead author, couldn’t be reached for comment Wednesday. But she told the BBC earlier this week that warming Arctic temperatures have been producing more, larger and “highly mobile” ice floes in recent years.
“That makes icebergs that are too big for these animals to swim beneath, and changes the reliability of known breathing holes,” she said.
“A wrong decision (by a narwhal) or a shifting wind moving ice could be fatal.”
The risk of drowning faced by narwhals was vividly demonstrated in November 2008 when an estimated 600 whales died in Nunavut after becoming trapped by ice in a bay near Pond Inlet on northern Baffin Island.
While changing ice conditions are widely seen as a major threat to the species, Canadian researchers provided some good news about narwhal numbers earlier this year.
Using a new method of estimating the whale’s population size in eastern Canadian waters, the scientists concluded that previous counts suggesting narwhals numbered about 30,000 around Baffin Island were far too low.
Instead, they found, the population is probably closer to 60,000.
In 2004, Canada’s committee on endangered wildlife had recommended listing the narwhal as a “species of special concern” because of “uncertainty about its numbers, trends, life history parameters and levels of sustainable hunting.”
And in 2008, a major international study identified the narwhal as the species most susceptible to impacts from the retreat and thinning of Arctic sea ice because of its reliance on specific ice conditions for various behaviours and its specialization in feeding on halibut.