A new census of past whale populations suggests that some whale populations were once about 10 times larger than previously estimated. That means the ocean can sustain more of the leviathans than thought, say the authors, who argue that goals for whale recovery efforts should be set higher. But some scientists question the findings.
Whale populations have risen and plummeted with changes in market demands, hunting technology, and ocean conditions. To tally past populations, researchers traditionally relied on the sometimes spotty whaling logbooks compiled over the past few centuries.
The new estimates, published in the 25 July issue of Science, are based on genetic evidence. Marine conservation biologists and geneticists Joe Roman of Harvard University in Cambridge, Massachusetts, and Steve Palumbi of Stanford University in California used databases of a small section of mitochondrial DNA (mtDNA) to reconstruct population histories for humpback, minke, and fin whales mostly in the North Atlantic. The team estimated how quickly whale mtDNA had mutated over millions of years and measured the genetic diversity within each species. Given those numbers, they could calculate how many breeding females would have been necessary to accumulate the genetic variability observed in the mtDNA samples (mtDNA is inherited via eggs, not sperm). Based on the relative fraction of a whale population that consists of breeding females, they could then estimate the total population.
According to their calculations, humpback whales in the North Atlantic once numbered about 240,000 individuals, about 12 times more than previously thought, while populations of minke and fin whales were two and six times greater, respectively, than previous estimates. Although the researchers admit that these large populations might have only existed hundreds of thousands of years ago, they speculate that huge numbers of whales could have roamed the oceans until relatively recently, when industrialized hunting began taking its toll.
But many geneticists and whale biologists object. "The proposed reduction in abundance could have happened at any point in [recent] evolutionary time," says Per Palsbøll, a whale population geneticist at the University of California, Berkeley. That calls into question the work's relevance to present management efforts, he says. Another key uncertainty is tied to setting the molecular clock of mtDNA. Karen Martien, a conservation geneticist and fisheries biologist at the Southwest Fisheries Research Center in La Jolla, California, says Roman and Palumbi's estimated mtDNA mutation rates may have artificially inflated their population estimates.