WHAT HAPPENED TO GRAY WHALES DURING THE PLEISTOCENE? EVALUATING THE IMPACT OF SEA-LEVEL CHANGES ON FEEDING HABITAT IN THE EASTERN NORTH PACIFIC OCEAN

Gray whales (Eschrichtius robustus) are renowned for their annual migrations from the coasts of Baja California to Alaska, where they capitalize on the seasonally productive benthos of the Bering Sea using a suction-feeding mode that is unique among living mysticetes. The in- and epifaunal invertebrates that form the primary prey of gray whales are restricted to shallow benthic habitats, but global sea-level changes during the Pleistocene glacial maxima have eliminated this habitat multiple times. Because the fossil record of gray whales is coincidental with the onset of widespread glaciation in the Northern Hemisphere, they survived these massive changes to their critical feeding habitat, but how? Using habitat area defined by bathymetry, we quantified the extent of these changes in benthic area as it relates to sea-level changes during the Pleistocene. We used two different estimates of pre-whaling population size (census estimates and molecular estimates) as initial values in reconstructing population size changes over the past 0.5 Ma. Using census estimates, which assume that gray whales have currently recovered to pre-whaling population size, our results indicate that glacial episodes would have forced gray whales into low enough numbers to cause multiple genetic bottlenecks. Such genetic patterns, however, are absent in studies of gray whale mitochondrial haplotype data. Using molecular estimates, which place pre-whaling population sizes 2-3 times greater than now, our results suggest that gray whales likely survived glacial episodes in sufficient numbers to maintain genetic diversity. Given the lack of benthic feeding habitat during glacial maxima, we propose that gray whales survived by employing generalist filter-feeding modes, similar to those by non-migrating sub-populations off the coast of British Columbia today. Our study emphasizes importance of historical approaches that rely on independent lines of evidence to constrain hypotheses of ecological history.