Friday, April 29, 2011

Ice Whale winter habitat 98% ice-covered

A research paper published in February but which received little attention in the popular press is a fascinating example of why narwhals could be considered the most extreme marine mammal on the planet.  The paper reports on an aerial survey of Baffin Bay in April 2008 that highlights the challenging conditions that narwhals live under in winter.
“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 biologist Kristin Laidre, who conducted the study.  “They become enveloped by the ice.  It’s very dense ice...and the ice floes are constantly changing, the leads are constantly opening and closing.”
Laidre’s objective in conducting the aerial survey was, in part, to quantify the amount of open water where the narwhals were 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 18,000 narwhals there, or 77 narwhals per square kilometer of open water.
Narwhals surface to breathe in narrow opening in the sea ice. Photo by BBC.

“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.”
It is amazing to me that these remarkable animals can thrive in these conditions, where the risk that the ice could freeze over and provide no access to catch their breath seems absurdly high, where the area often appears totally frozen over to the naked eye, but somehow they eke out a living there.
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. 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?”
But there’s more to it than that.  She said that 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 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.  When the ice recedes, the Arctic whales move north just as the slew of subarctic species arrive in the area they just left.
“There are also theories that the narwhals avoid killer whales by living in the pack ice,” Laidre said.  “I don’t necessarily think that’s the case, because there are basically no killer whales in the Arctic in the winter.  There’s no good reason to just hide in the ice when, at that time, there is nothing to hide from.”
It’s just one more reason to be in awe of the ice whale of the Arctic.

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