Only 13 years ago, scientists announced that an important population of gray whales was back after having been nearly hunted to extinction by the early 20th century. But a new genetic study indicates that this stock, which migrates between Alaska and Mexico, is still far from its peak.
Scientists typically base their estimates of whale populations on historical catch records. Whalers' logs of the number of animals they killed and the number they harpooned but lost provide a minimum estimate of abundance for demographic modeling. Based on those data, researchers concluded that the gray whales (Eschrichtius robustus) in the eastern North Pacific Ocean had returned to their original population size, which models suggested was between 19,480 and 35,430 individuals. No longer threatened with extinction, the population of eastern North Pacific gray whales was removed from the endangered species list by the federal government. Western North Pacific gray whales, which now live only in the Sea of Okhotsk, remain critically endangered.
But missing from records is the number of eastern gray whales targeted by subsistence hunters, who have been killing the animals for some 1500 years. Thus, estimates of past gray whale numbers "probably are not complete," says C. Scott Baker, a molecular ecologist at Oregon State University's Marine Mammal Institute in Newport.
To test whether the whales had actually recovered, a team of researchers from Stanford University in Palo Alto, California, sequenced the genes from 42 eastern North Pacific gray whale tissue samples in the Southwest Fisheries Science Center archives in San Diego, California. Their analysis revealed a "surprisingly high genetic diversity," a sign of a far larger population in the past, says S. Elizabeth Alter, a graduate student in ecological genetics, who led the team.
The results, presented online this week in the Proceedings of the National Academy of Sciences, suggest that the whales' numbers prior to commercial whaling ranged between 76,000 and 118,000 for the western and eastern stocks combined. "Our study indicates that the gray whales were once three to five times more plentiful than they are today," says Alter.
Climate change is adding to the bad news. Although the whales' numbers are increasing, the animals may never regain their past abundance, says Alter, because their feeding grounds in the northern Bering Sea are being altered by warming ocean temperatures (Science, 10 March 2006, p. 1461). "The whales' prey has moved steadily north, and that stresses the whales," says Alter, noting that between 1999 and 2000, the eastern North Pacific gray whales were shockingly thin, and their numbers dropped, perhaps because that year's El Niño further disrupted other potential food resources by warming the water along their migration route.
Although the population has recovered from that plunge, researchers still see "skinny whales," says Steven Swartz, a population biologist at the National Oceanic and Atmospheric Administration’s National Marine Fisheries Service in Silver Spring, Maryland. "They're also feeding in their breeding lagoons and along their migratory route," he says, which is unusual and suggests they're being "pressed to find new resources." All in all, says Alter, it may be time for a review of the gray whales' "recovered" status.