Abundance and distribution of ice-associated seals in the western Bering Sea and Sea of Okhotsk, 2012-2013

Abstract

In the springs of 2012-2013, Russian Federation scientists conducted aerial surveys of ice-associated seals (including bearded, ribbon, ringed, and spotted seals) in the Sea of Okhotsk and in Russian waters of the western Bering Sea. These instrument-based surveys used a combination of thermal imaging and digital photography to detect and photograph seals basking on sea ice. Initial estimates of seal distribution and abundance from these surveys have been published in Russian journals, but did not make adjustments for observation error. In this paper, we reanalyze these data with the goal of producing spatially explicit estimates of seal abundance that properly account for incomplete detection and species classification errors. Our analysis used estimated relationships between seal abundance and habitat variables to predict abundance in unsurveyed locations, and auxiliary data on detectability to model the observation process. Under this framework, we estimated that in 2012 there were approximately 185,330 bearded seals, 68,564 ribbon seals, 272,609 ringed seals, and 226,003 spotted seals in the western Bering Sea; in 2013, these numbers were 144,105, 39,435, 203,130, and 90,501, respectively. In the Sea of Okhotsk, estimated abundance was 218,740 bearded seals, 221,228 ribbon seals, 179,893 ringed seals, and 169,870 spotted seals (2013 only). These are the first credible estimates of ice-associated seals in the Okhotsk and western Bering Seas, and should be useful for population management and for measuring how density and distribution change as a function of decreasing Arctic sea ice. Declines in seal estimates in the western Bering Sea from 2012 to 2013 mirror patterns observed in the eastern Bering Sea during concurrent U.S. surveys and could represent a decrease in abundance or a substantial decrease in pupping and molting activity.