During late summer and autumn, Pacific walruses are resting on shore north of the Bering Strait more often and in larger numbers in both the United States and Russia. Historically, walruses rested primarily on floating sea ice over their offshore foraging grounds in this region, but climate warming has reduced availability of sea ice. With greater numbers of walruses gathering on shore, USGS scientists and collaborators in Russia are capitalizing on the opportunity to improve walrus population monitoring. USGS has evaluated optical and radar satellite imagery ranging in pixel resolutions from 40 m to ~1 m for the detection and quantification of large walrus groups resting on shore. USGS flew UAS orthoimagery surveys and was able to use these images to validate satellite imagery interpretation. The debut study site at Point Lay, Alaska, was optimal for remote sensing because the beach is fine-grained with negligible topography. Range-wide walrus population monitoring efforts, however, would require reconnaissance of haulouts, which are anticipated to be more challenging to imagery interpretations due to variable beach substrates and adjacent bluffs.
Example satellite imagery types evaluated for detecting Pacific walruses resting on shore near Point Lay, Alaska. The top row shows optical imagery types displayed with a true-color red/green/blue (RGB) enhancement, in which walrus aggregations are more reddish-brown compared to dark-brown soiled beach or lightly hued untrampled beach: (1) Landsat 8 (30-meter resolution), (2) Sentinel-2 (10-meter), (3) PlanetScope (4-meter), and (4) WorldView-2 (1.5-meter). The bottom row shows synthetic aperture radar (SAR) images of normalized backscatter, in which walrus aggregations return a strong backscatter signal compared to the surrounding flat landscape comprised of fine-grained sediments: (5) Sentinel-1 EW (extra wide) swath mode with VV polarization (40 m), and (6) Sentinel-1 IW (interferometic wide) swath mode with HV polarization (10 m), (7) TeraSAR-X stripmap (dual polarization 3.5 m), and (8) TeraSAR-X staring spotlight with VV polarization (1.4 m).