Wintering narwhals thrive amid the pack ice

          I’m really beginning to feel the chill here in Rhode Island as we approach the winter solstice and the coldest time of the year. But I’ve got it easy compared to those living in the Arctic.  And yet narwhals seem to thrive at this time of year in the frozen conditions of the waters between Greenland and Baffin Island, Canada. They spend their days repeatedly diving 1,800 meters beneath the surface on a 30-minute round trip to the seafloor in search of food. And if that’s not remarkable enough, they then have to surface for air in tiny openings in the sea ice that are often few and far between.

To get a sense for the habitat narwhals live in during the winter and how many are out there, biologist Kristin Laidre and others flew an aerial survey of the pack ice in late winter 2008 to see for themselves what it looked like.  Flying in a small plane from Kangerlussuaq, Greenland, they flew eight zigzag transects back and forth over the wintering grounds – as many as they could do with the fuel available in such a small plane – before returning to the mainland.

 “During their migration, the ice begins to chase them south and they arrive in their wintering grounds and the ice just forms right around them,” explained Laidre.  “The narwhals become enveloped by the ice.  It’s very dense ice, but it’s moving very fast; there’s a strong current that moves the ice to the south, and the ice floes are constantly changing, the leads are constantly opening and closing.”

Photos by Flip Nicklin

Laidre’s objective in conducting the aerial survey was, in part, to quantify the amount of open water where the narwhals are found.  By combining the data collected on the aerial survey with satellite images of the sea ice, she determined that just two percent of the area surveyed was open water, and there were between 17,000 and 19,000 narwhals there, or 73 narwhals per square kilometer of open water.

“That means you have this large density of animals that need open water to breathe packed into a very small amount of habitat,” she said.  “The overall habitat area is large, but what’s actually usable to them is quite small.”

Listening to Laidre describe what anyone would agree was an immensely challenging environment in which to live, it got me wondering why the narwhals stay there, when just 10 or 20 or 30 miles further south there is far less ice and the living conditions would be much easier with far fewer concerns about a sudden freeze making it impossible to reach the surface for air.  Greenland halibut must surely be found in open water as well as beneath the pack ice, right?  It must be easier living in ice-free water than in the dense ice pack, so why do they remain where the risk of meeting their death in an ice entrapment is so high?

The answer, Laidre said, is partly because that’s simply how they have evolved.  “They really have a niche, they’re totally adapted to this pack ice, more than any other northern hemisphere cetacean, and they don’t have many competitors.  Why go further south when you’re adapted to live in the pack ice and don’t need to go further?  It’s evolution.  They’ve become adapted to being in a certain climate and exploiting it and being successful, and I think that’s just what they’ve done.”

          But there’s more to it than that, she added.  It probably also has a great deal to do with competition and the partitioning of resources.  Narwhals, belugas and bowheads are the only whales that spend their entire lives in the Arctic, but there is a large pool of more southerly whales and marine mammals – minke and fin and humpback and blue and killer whales among them, as well as several varieties of smaller whales, porpoises and seals -- that come to the Arctic in summer to feed in its highly productive ecosystem.  Those subarctic species avoid the Arctic when it’s dark and ice-covered and miserably cold in the winter but move in during the spring and stay throughout the summer and early fall.  The narwhal has developed a strategy to exploit the ecosystem at a time when there are few competitors in an area where they know they have a reliable food supply available.  Belugas and bowheads do the same thing – they feed intensively in the winter and early spring when the other subarctic species aren’t there to compete with them.  And when the ice recedes, the Arctic whales move north just as the slew of subarctic species arrive in the area they just left.
          It is a strategy that has served them well, despite how cold it makes me feel.