In April 2003, we conducted an operational test of an airborne multispectral scanner (AMS) over pack ice in the Bering Sea to evaluate the potential of this system as a survey tool for Pacific walruses. We scanned a total of 28,875 km2 of sea ice habitat at a spatial resolution of 4 m and collected high resolution photographs from a subset of the thermally detected walrus groups. We found a significant positive relationship between walrus group size and the amount of heat measured by the AMS and used this relationship to estimate total walrus numbers in the survey area. The number of walruses hauled out onto sea ice in our study area was estimated at 4,785 animals with a 95% confidence interval of 2,499–7,111. We believe that the AMS system as configured for this study would be a highly effective tool for surveying large areas of sea ice habitat for walrus groups. With a 6 km swath width, it should be possible to sample more 10,000 km2 in an 8-hr flight. Although walrus groups > 4 animals were easily detected and enumerated in the 4 m thermal data, the system was unable to detect individual walruses or seals (Phoca spp. and Erignathus barbatus). We found that most (94.6%) of the walruses photographed in our survey area occurred in groups > 6 animals, therefore we expect the magnitude of any bias due to undetected groups of hauled out animals would be relatively small.
