Gray whale calves in Baja California, Mexico, are nourished only by their mothers' milk until they reach rich feeding grounds in the Bering, Beaufort and Chukchi Seas

In the 19th century, intensive hunting brought the gray whale close to extinction. Although whales in the western Pacific are still endangered, protection efforts in the eastern Pacific seemed, at first, to have brought the population there back to pre-whaling numbers.

But new genetic research shows gray whales may once have been much more abundant than they are today. The finding suggests the world's oceans may no longer be able to support such a large population.

The gray whales of the eastern Pacific have been considered one of the great success stories of species recovery. After being hunted to near-extinction by commercial whalers, their current population is now estimated to be about 22,000. But new data on historic gray whale populations are forcing scientists to reassess how well these whales have recovered.

Liz Alter is a Ph.D. student at Stanford University in California, and the lead author of a study that used genetic analysis to assess the historic population size of gray whales. She says the history of a population is "written in its DNA, because larger populations have much more genetic variability than smaller populations." By measuring genetic variation in gray whales, Alter was able to estimate the average, long-term, historical size of the Pacific population.

Earlier estimates of the pre-whaling population had been based largely on historical accounts from commercial whalers, and were in the same range as the current average population estimate of 22,000.

Alter found that the genetic diversity in gray whales today is much higher than would be expected. In fact, Alter says, the measured diversity would be more typical of a population of 78,000 to 117,000. "That indicates to us that there once were many more gray whales, as many as three to five times more in the Pacific Ocean than there are now."

Stanford University biology professor Steve Palumbi is co-author of a study which suggests that in the past, gray whales may have been three to five times more numerous than they are today

That difference between historic and current gray whale populations could have serious implications for other marine species, as well, according to Stanford biology professor Steve Palumbi, a co-author of the study. He says gray whales play a key role in the ocean ecosystem — through their unusual feeding behavior. He compares the whales to "marine bulldozers." As he puts it, "They feed by jamming themselves into the bottom […], and then scooping up a big mouthful of the sediments and clays and muds."

From the mud that they've scooped up from the ocean floor, the whales filter out and eat thousands of small shrimp-like crustaceans. They also lose a lot of them, along the way. "They're sort of messy eaters," explains Palumbi. As the whales rise to the surface, they trail a large plume of sediments and crustaceans behind them. "They're essentially bringing these bottom crustaceans up to the surface, where seabirds can get them."

In fact, at their historic population levels, gray whales may have stirred up enough crustaceans to support as many as a million seabirds.

Gray whales (Eschrichtius robustus) return from Arctic feeding grounds to lagoons in Mexico each winter to give birth

The new whale numbers may also help to explain more recent changes in the species' population dynamics. Between 1999 and 2001, gray whales began starving to death. Scientists hypothesized that the population might have recovered too well, that there were now more gray whales than the ocean could support.

But the new research suggests that the ocean once supported many more whales than exist today. Liz Alter says that this finding supports an alternative explanation for why the whales are starving: large-scale, ecosystem level changes are affecting the whales' feeding grounds in the Bering Sea. Other research has suggested that climate change may have reduced the gray whale's food supply, by warming deep arctic waters.

The eastern Pacific gray whale population has made a substantial recovery, coming back from near extinction. But according to Alter and Palumbi, the gray whales still need protecting, and more research on their complex ocean habitat will be needed to ensure their continued survival.