The results of aerial surveys of the Pacific Common Eider () are presented herein, covering the coastal region of the Sea of Okhotsk from the mouth of the Okhota River in Khabarovsk Krai to the estuary of the Penzhina River in Kamchatka. Surveys of breeding and molting habitats were conducted from 19 June to 15 July 2024. A total of 7,918 individuals were recorded, comprising 1,603 females, 222 immature males, and 153 ducklings. Extrapolating eider density to the unsurveyed coastal segment and accounting for the proportion of females still incubating eggs during the survey period, the estimated population of Pacific Common Eiders in the Sea of Okhotsk is approximately 9,550 individuals. Among the counted birds, males predominated, with a sex ratio of approximately 79:21 (males to females). This observed sex ratio may either reflect a low reproductive potential and a depressed population status or indicate a possible influx of males from the Bering Sea during the molting period. To investigate this, we analysed the sex ratios of eiders in adjacent regions of the Bering Sea based on aerial surveys conducted in July and August of 2023 and 2024. Over these two years, the average sex ratio was approximately 29% males to 71% females (n = 6,574 individuals). The pronounced predominance of females in the Bering Sea suggests a migration of males to other waters for molting. The eider population of the Sea of Okhotsk is therefore not isolated during the summer season; males, and potentially females, from neighbouring waters of the Bering Sea congregate there during the molting period. Recommendations for further research and conservation measures for the eiders of the Sea of Okhotsk are also provided.
Solovyeva et al. (2025) studied this question.