The once and future gray whales: DNA evidence for historic population size and past ecosystem impacts

The restoration of ecosystems may require returning threatened populations of ecologically pivotal species to near their former abundances, a challenge that demands information about historical population sizes. Eastern Pacific gray whales play a key ecological role in their Arctic feeding grounds, and are widely thought to have returned to their pre-whaling abundance, leading to the downgrading of demographic protection. However, independent analyses suggest the population is increasing, and that recent mortality spikes were due to shifting conditions on feeding grounds. To estimate pre-whaling abundance, we measured DNA variability at 10 loci and show that gray whales were typically 3-5 times more numerous than today’s average census size of 22,000 individuals. Coalescent simulations show these estimates could include the entire Pacific meta-population, indicating that our measurement of about 96,000 individuals might have been distributed between the eastern and currently endangered western Pacific populations. Our data suggest the eastern population is at most at 28-56% of its historical abundance and should be considered depleted. Potentially profound ecosystem impacts may have resulted from this decline, including a 72% reduction in annual benthic turnover on whale feeding grounds. At previous levels gray whales may have seasonally resuspended 700 million cubic meters of sediment, as much as twelve Yukon Rivers, and provided food to a million sea birds. These analyses suggest that gray whales once played a larger role in north Pacific ecosystems, and that the single robust population remaining represents only a modest fraction of this species’ former abundance.