Gray whale numbers in the eastern Pacific were once several times larger than they are today, say researchers – a fact that, combined with an adaptable diet, enabled them to survive previous climatic changes that might otherwise have spelled their doom.
Writing in the open-access online journal PLoS One, Nicholas Pyenson of the Smithsonian Institution and David Lindberg of the University of California at Berkeley note that, in 2007, Stephen Palumbi of Stanford University and colleagues calculated the initial population size of California gray whales as between 76,000 and 120,000. That estimate, which was based on the genetic diversity among the whales today, was resisted by some ecologists, who argued that the approximately 20,000 gray whales in the eastern Pacific today are as many as the Pacific coast ecosystems along the whales’ migratory route from Baja California to the Bering Sea could support.
But Pyenson and Lindberg reasoned that those ecosystems likely underwent plenty of change at various times during the gray whales’ existence. For example, about 120,000 years ago, Earth entered into its last great ice age, during which time sea level dropped by about 120 meters, or nearly 400 feet, as ocean water became locked up in glaciers. Gray whales feed primarily on bottom-dwelling, or benthic, organisms such as worms and amphipods, generally along coastal shelves at depths of 50 meters (160 feet) or less and sometimes in ocean floor channels about 75 meters (250 feet) deep. But during the ice age, water would have retreated from those shallow coastal shelves, forcing the gray whales into greater depths.
Reasonably then, they figured, gray whale numbers would have declined during that time. But if the initial population had been the size of the present one, it would have “bottlenecked” – a phenomenon that occurs when populations become so small that inbreeding becomes common, decreasing genetic diversity. Such a genetic bottleneck, had it existed, would show up in the whales’ DNA, but none has been found.
So how many whales were there? By estimating the amount of food a gray whale needs to survive, and calculating how much available food would have decreased as a result of changes in the ecosystem brought about by cooling, the two researchers concluded that, in order to avoid bottlenecking, the population would have had to have been around 70,000 during warm periods.
It could, they suggest, possibly have been even higher. They argue that, while grays may be predominantly migratory benthic feeders, they are not exclusively so. There is, for example, an assemblage of eastern gray whales, accounting for approximately 1 percent of the population, that spends the year in the vicinity of Vancouver Island, subsisting on herring and krill.
It is entirely plausible, they argue, that before humans came on to the scene, there were many more such “resident” populations of gray whales; but, as humanity colonized the region, these resident grays would have been hunted to extirpation and their existence forgotten. Modern commercial whaling nearly finished off the species entirely, but it recovered in numbers first by exploiting the niche to which it is best adapted and only then beginning to gain a flukehold in other areas. In other words, they argue, gray whales have a great deal more “evolutionary plasticity” than had been imagined – a trait that may hold them in relatively good stead as temperatures rise and ecosystems undergo more rapid change over the next several centuries.
Photograph of gray whale flukes by Steven Swartz/NOAA.