Gray whales winter amid Arctic ice

July 21, 2007

Courtesy of  Far North Science
By Doug O'Harra

Gray whales swimming
Credit: Sue Moore / NOAA Magazine
Photo: Tad Pfeffer.

And then, as darkness fell and the ocean froze, the whales would exit though the Bering Strait and the sea would go silent.

After all, the great whales can die if caught in thick ice - three gray whales trapped in ever shrinking leads near Barrow triggered an international rescue in the fall of 1988. At least one animal disappeared before a Russian ice-breaker plowed a path to open water.

But a few gray whales didn't get the memo.

Instead of joining 10,000 other Pacific grays on their 5,000-mile fall migration to wintering grounds in Mexico, the intelligent bottom-feeding invertebrate munchers spent the winter amid Alaska's Arctic pack.

In a stunning finding that raises questions about accelerating climate change and undermines assumptions about gray whale behavior, an autonomous acoustic device anchored 4,100 feet beneath the surface of the frozen Beaufort Sea recorded gray whale calls throughout the winter of 2003-04.

"Because this is the first-ever winter-long acoustic study, we cannot be certain that gray whales have not over-wintered in the Beaufort Sea in the past," the authors wrote in a report published in June in the journal Arctic. "However, a combination of increasing population size and habitat alteration associated with sea ice reduction and warming in the Alaskan Arctic may be responsible for the extra-seasonal gray whale occurrence near Barrow."

The discovery that gray whales overwintered in the Arctic was made possible by a revolution in marine field biology: tracking whales by sound with special underwater recorders.

Over the past decade, the devices helped identify and pinpoint a tiny population of North Pacific right whales in the eastern Bering Sea, offering hope for the most endangered large whales on the planet. Other devices have outed blue whales in the Gulf of Alaska. In an environment where mammals depend on sound, the technology promises to do for deep ocean exploration what satellite coverage offers on the surface.

In Gray whale calls recorded near Barrow, Alaska through the winter of 2003-04, Stafford and three co-authors (including long-time Pacific Ocean whale biologist and acoustic pioneer Sue Moore) reported that they collected more than 1,500 "pulses" from 202 calling bouts by the grays.

The device anchored at the 1,000 foot depth collected calls every week from Oct., 6 until it stopped working on Dec. 29. The deeper device continued to catch gray whales through May 12, though the number of calls fell off somewhat as time passed "suggesting that fewer whales were producing sounds at that time," the authors wrote.

No one knows for sure what gray whales eat northeast of Barrow, though the animals have been seen along the shelf break only about 25 miles out in summer. Nor do the researchers quite know what to make of the presence of the whales all winter.

"We had assumed that the wide extent and great thickness of sea ice in the western Beaufort Sea would exclude gray whales from this area in winter," they wrote.

Could they have been trapped, just like the whales in 1988? Satellite images showed enough plenty of leads - making the winter of 2003-04 one of the highest open-water winters on record. The scientists concluded the whales were not trapped.

Other explanations? Gray whales have been expanding in population, and seasonal migration has drifted later in the season in recent decades. Climate warming, changes in prey and foraging habits, and less sea ice might have contributed.

"Clearly, we need further long-term acoustic monitoring," they concluded. "We propose that autnomous acoustic monitoring become a standard practice."