Global Warming May Be Keeping Gray Whale Numbers Down

Stefan Lovgren
for National Geographic News

September 10, 2007

Eastern gray whales were once three to five times more numerous in the Pacific Ocean than today, and climate change may be the cause of the population decline, a new study says.

According to a recent census, about 22,000 eastern gray whales currently live in the Pacific.

Experts had thought this to be around the whales' natural population level, because of widespread die-offs of gray whales between 1999 and 2001 that plunged the animals to 18,000 individuals.

But scientists studying whale DNA have concluded that the Pacific whale population previously ranged from 76,000 to 118,000.

(Related: "Climate Change Harming Bering Sea Mammals, Birds, Study Shows" [March 9, 2006].)

"Our results suggest that rather than looking at this historical carrying capacity as the cause behind these die-offs, we might be better off looking at the changing conditions on the gray whale feeding ground that we know are occurring," said Elizabeth Alter, a doctoral biology student at Stanford University in Palo Alto, California.

Alter is the lead author on the study, which appears this week in the journal Proceedings of the National Academies of Science.

DNA Discovery

Gray whales, which can reach a length of more than 50 feet (16 meters) and a weight of 36 tons, were extensively hunted in the 19th century.

The western, or Asian population remains one of the world's most endangered whale populations. Only 120 western individuals now remain, primarily because of continued whaling.

The eastern gray whale population, however, is one of the few that is believed to have fully recovered to its pre-whaling numbers. In 1994 the whales were even removed from the U.S. list of endangered and threatened species.

But the new study suggests that the eastern population may in fact still be depleted from its historical levels.

Sampling DNA from 42 whales, scientists found a much higher genetic variation in the whale population than expected, indicating a much larger gene pool of contributing individuals than conventional theory.

The experts concluded that historically the average gray whale population was about 96,000.

"The DNA analysis helps us reconstruct what our oceans used to look like before human impact," Alter said.

Scientists don't know, however, when exactly the population declined.

Between 1999 and 2001 the gray whale population suffered an unusual mortality event throughout its geographic range, from Alaska to Baja California, with frequent strandings and lower calving rates.

Scientists had linked the die-offs to the whale population reaching its carrying capacity.

But the new study shows that the Pacific Ocean used to be able to accommodate many more whales than today, leading scientists to search for other causes, such as global warming.

Recent studies have shown that changing climate conditions in the Bering Sea and nearby areas have reduced prey populations for gray whales, for example.

"Our estimate of typical gray whale abundance suggests that recent problems in gray whale feeding—including reports of thin adults or high calf mortality—may result from changing conditions in northern feedings grounds," the study authors write.

Phillip Clapham works at the Alaska Fisheries Science Center, part of the National Oceanic and Atmospheric Administration's National Marine Mammal Laboratory in Seattle, Washington.

"It's clear that future climate change will significantly impact the Arctic and sub-Arctic ecosystems on which gray whales and other species depend," he said. "But quite how that colossal unintentional experiment comes out is anyone's guess."

That means raising recovery targets based on historical figures is pointless if the oceans simply can't support those numbers, he added.

"If there were a hundred thousand gray whales at some point, the carrying capacity of the North Pacific ecosystem had to support them, and it is likely that the current ecosystem cannot do that," pointed out Steven Swartz of NOAA's National Marine Fisheries Service in Silver Spring, Maryland.

Food for Thought

The gray whale decline may be having a profound impact on the ocean's ecosystem.

"Gray whales don't feed in the way that most people think of whales feeding, by filtering krill from the water column," lead study author Alter pointed out.

"Instead they scoop up these gigantic mouthfuls of mud from the sea bottom," she said. "In the process they stir up a lot of sediment and recycle nutrients ... and they also end up bringing a lot of food to the surface."

According to the study, at their historical high, gray whales may have helped dredge up food for as many as a million seabirds.

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