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Gray whales survived Ice Age with different diet

Doug O'Harra

Pacific gray whales may have numbered as many as 120,000 before the arrival of humans in their waters -- as much as five times the present population -- and often subsisted on a much more varied diet that enabled the species to survive multiple ice ages over millions of years.

The bottom-feeding, Alaska-bound species -- one of the world's great conservation recovery stories after being hunted nearly to extinction -- may have a lot more tricks in their repertoire than biologists ever realized, according to new research published this month in the online, peer-reviewed journal PLoS ONE.

In particular, the authors speculate that tens of thousands of east Pacific grays once skipped their annual epic migration to Alaska and remained year-round in coastal enclaves. These "resident" populations would have been among the first hunted to extinction by shore-based whalers, leaving their migratory cousins to dominate the scene.

One intriguing clue to this capacity: Some grays have started staying put in recent decades, hanging out and eating herring near Vancouver Island and Southeast Alaska all year round.

The grays show "a lot more evolutionary plasticity than anyone imagined," said lead author David Lindberg, an evolutionary biologist at the University of California Berkeley.

That's particularly good news for the species, he added in this story posted by UCB, because it could help whales survive the dramatic rise in sea level predicted by some climate models during the next few centuries if Greenland and Antarctic ice sheets melt.

"I suspect the gray whales will be among the winners in the great climate change experiment," added co-author Nicholas Pyenson, a former student of Lindberg's who is now curator of fossil marine mammals at the Smithsonian in Washington, D.C.

'Fantastical' abundance: Gray whales once thrived

The study -- "What Happened to Gray Whales during the Pleistocene? The Ecological Impact of Sea-Level Change on Benthic Feeding Areas in the North Pacific Ocean" -- also raises the startling notion that humans might grossly underestimate just how abundant certain marine mammals, fish and turtles used to be before decimation by centuries of overharvest, an idea that's cropped up in other research during the past decade.

As a team of scientists wrote in 2001 in the journal Science: "Historical abundances of large consumer species were fantastically large in comparison with recent observations."

Although some scientists estimate that grays have returned to their pre-whaling population of about 22,000 (leading to the removal of the species from the Endangered Species list), the genetic component of the new research suggests the species must have been far more numerous and versatile in the distant past.

"We propose that gray whales survived the disappearance of their primary feeding ground by employing generalist filter-feeding modes, similar to the resident gray whales found between northern Washington State and Vancouver Island," the scientists wrote.

"A combination of low population numbers and a species migrating between places where humans didn't bother them gave us the impression that gray whales have a stereotypical migratory and feeding behavior that may not be historically correct," Lindberg added in the UCB story. "There almost certainly were higher gray whale populations in the past."

The grays are baleen cetaceans that can grow up to 50 feet in length and 40 tons in weight, almost always on a diet of soft-bodied crustaceans and worms suctioned from sea-bottom muck. Although nearly wiped out by commercial whaling, the eastern Pacific population is now considered recovered and may number as many as 22,000 animals. Only a few hundred animals remain in a western Pacific population and are considered critically endangered.

Gray whales are popular megafauna during their Pacific Coast migration, and the new study has already generated news coverage, mostly outlets reprinting the original UCB press release (which is worth checking out for its interesting photo of a gray whale scafting down chow on the sea floor.)

Where did they dine when the Bering was a land bridge? 

In their study, Lindberg and Pyenson tackled a peculiarly North Pacific conundrum: Just how could our gray whales survive all those ice ages without their favorite feeding grounds?

Every year, thousands of these animals travel from winter mating areas in Baja California up the west coast of North America and around the southern rim of Alaska toward the benthic buffets of the Bering and Chukchi seas. This 10,000-mile round trip is the longest known annual migration by any mammal and one of the Pacific Coast's most extraordinary natural spectacles.

And yet, as recently as 12,000 years ago, many of these feeding grounds simply didn't exist.

When continental glaciers locked up vast quantities of the Earth's water, the ocean level dropped up to 400 feet, transforming what is now sea bottom into wind-scoured steppes. At the height of the ice age, much of the modern gray whale lunch box would have been high and dry, rumbling with the likes of woolly mammoths, bison and carnivorous cats.

And yet, gray whales thrived out in the ocean. Where? How? What did they eat?

"The invertebrates that form their primary prey are restricted to shallow water environments, but global sea-level changes during the Pleistocene eliminated or reduced this critical habitat multiple times," the authors wrote.

If gray whales could only survive by eating sea-floor amphipods and worms -- as most do in modern times -- the whales would have crashed in number during the ice age peaks. But a technical, detailed analysis of gray whale genetics and former populations suggests that such crashes, or genetic "bottlenecks," did not occur.

So the whales had to expand their menu.

The once and future grays 

"Gray whales survived the Pleistocene because of a greater range of feeding modes and a less canalized migratory behavior that allowed for them to feed outside of the Sea of Okhotsk and Bering region and away from ice cover and glaciers,” the authors concluded in the paper.

With new climate change bearing down on the North Pacific and Arctic, gray whales may again need to draw on that capacity to subsist on fish and squid in more southern locales. As a result, people should try to make sure that these new resident groups of grays -- the whales no longer migrating to Alaska each year -- continue to get sufficient protection, the scientists said. If the resident grays continue to grow, Pacific grays may reach huge numbers again.

"We proposed that with continued human diligence, gray whales might return to a more typical interglacial distribution and abundance."

Contact Doug O'Harra at doug(at)alaskadispatch.com