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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