Survivors (Image: Mark Carwardine/Solent News/Rex Features)

A SMALL, poorly studied population of non-migrating grey whales may hold the key to how these ocean giants survived the last ice age.

Grey whales feed on the seafloor at depths of up to 50 metres, and rely heavily on the shallow regions in Alaska’s Bering Sea for food. But when Nick Pyenson of the Smithsonian Institution in Washington DC modelled Pacific feeding grounds during peak glaciation, he found that very little of the north Pacific was shallow enough for feeding: sea levels were up to 120 metres lower than today and the Bering Sea was a land bridge. Northern feeding grounds might have supported only a few hundred whales, he told the Society for Vertebrate Paleontology meeting on Monday. Yet genetic studies show no sign of a bottleneck at the time.

Pyenson thinks the whales shifted to open-water feeding to survive. Support for this idea comes from a small group of open-water feeders found living year-round in the Pacific northwest. Pyenson says that after the ice age the non-migratory whales would have been easy targets for early whalers, so the migratory population has come to dominate.