Using the data collected from the transmitters, Laidre has learned that narwhals are true creatures of habit. Many of the narwhals’ biological patterns—when they eat, migrate, and reproduce—are orchestrated in tandem with the annual expansion and retreat of Arctic sea ice.
As summer transitions to fall, the coastal summering grounds start to freeze over and the narwhals begin their migration southward. The map above shows sea ice concentration in the region from August 10, 2005, to December 31, 2005, as sea ice expands from the north into the water east of Baffin Island and west of Greenland. The arrows show the *approximate* paths of narwhal pods migrating south. By winter, large populations of narwhals are gathered in Baffin Bay and Davis Strait, virtually surrounded by dense winter pack ice. The sea ice in Baffin Bay reaches its maximum in March. Sea ice data provided by Advanced Microwave Scanning Radiometer — Earth Observing System (AMSR-E) sensor on the NASA Aqua satellite, processed by researchers at the University of Bremen. Credit: NOAA climate.gov
In the summer, retreating ice allows narwhals and their newly born calves to access their summering grounds in the sheltered inlets, bays, and fjords along the Canadian Arctic coast and northwest Greenland. As fall begins and the coastal summering grounds begin to freeze over again, the narwhals move out and migrate southward.
By mid-November, large populations of narwhals are gathered in the offshore areas of Baffin Bay and Davis Strait where Greenland halibut—their main food source—are plentiful. Narwhals spend the winter in this region, virtually surrounded by dense winter pack ice, and diving to feed on halibut near the sea floor.
“The pack ice in winter works like a conveyer belt, forcing the animals to constantly move,” Laidre says. “The narwhals have the largest ranges and highest velocities during years with extensive ice cover.”
A recent expedition gave Laidre an even greater appreciation of how challenging it is to survive in this winter habitat. In 2008, Laidre and her colleagues flew over the narwhal wintering grounds in a Twin Otter plane. They recorded their own narwhal sightings from the aircraft, as well as sightings documented by video recorders and digital cameras. They estimated that there were approximately 18,000 narwhals in their survey region. Most of that area, however, was covered in dense ice, and only two percent was open water available to narwhals for breathing.
Narwhals are mammals and need air to survive, so they tightly pack themselves in stretches of open water and cracks—called “leads”—in the lighter, newer ice. The researchers estimated that about 77 narwhals were clustered below every one square kilometer of open water in the survey region.
The frozen landscape can shift quickly; changes in ocean currents, winds, and temperature can cause leads to open up and then refreeze again rapidly.
Recently, even larger changes have been taking place in the narwhals’ wintering grounds. Satellite data have revealed a 9 to 11 percent decline in sea ice extent per decade in the Baffin Bay and Davis Strait areas since 1979. Scientists have also documented the earlier springtime breakup of ice in Baffin Bay.
“The whales are used to living in the pack ice during the winter,” Laidre says. “We don’t yet know what they will do if, in the future, the ice isn’t present where it normally is.”
While sea ice loss would create more open water space for the narwhals, shifts in the melting and freezing cycles may alter their migratory, feeding, and reproductive patterns. Changes in the ice regime could also directly translate to changes in the Arctic ecosystem that could indirectly harm the narwhals.