Estimating the abundance of polar bears on Wrangel Island during late summer using high-resolution satellite imagery: a pilot study

Abstract

Wrangel Island, Russia is well-known as a critical denning location in the Chukchi Sea polar bear (Ursus maritimus) population, but little is known about abundance and distribution of bears there. Our objective was to apply high-resolution satellite imagery (VHR; 0.3–0.5 m resolution) methods to estimate the density and abundance of bears on Wrangel Island. We used image-differencing to search 9% of the island and count polar bears on WorldView-3 imagery. We calculated potential densities of polar bears under three stratification scenarios: (1) one stratum, assuming constant density across the search area; (2) two strata, delineated into a higher-density area within 8 km around the coast and a lower density in the Wrangel Island interior; and (3) three strata such that the higher-density, coastal zone was bisected between the north and south coasts, since polar bears may be more likely to be present on the northern coast given the northward recession of sea-ice throughout summer. We counted 26 bears on this single image, resulting in density estimates ranging from 1 to 18 polar bears/100 km². Assuming constant densities within each strata across the island, abundance estimates may range from 290 to 331 independent polar bears during August 2015. These estimates of density and abundance are comparable to previous research on Wrangel Island, although our estimates require ground validation and should be interpreted with caution. Using WV-03 imagery provides increased spatial resolution that is expanding applications and increasing our ability to detect polar bears.