Distribution of eastern Bering Sea belugas from aerial line-transect surveys in 2024 (original) (raw)

2024, AFSC Processed Report

The Eastern Bering Sea (EBS) beluga (Delphinapterus leucas) stock inhabits the waters of Norton Sound and the Yukon River Delta, Alaska, during the ice-free period from sea-ice breakup in spring to the beginning of freeze-up in autumn. During June, July, and August, belugas aggregate near the Yukon River Delta, where they feed on seasonally abundant anadromous fish such as Pacific herring (Clupea pallasii) and salmon (Oncorhynchus spp.). EBS belugas are an important nutritional and cultural resource for Alaska Natives, and are harvested by more than 20 communities near Norton Sound and the Yukon River. To collect data for an updated abundance estimate of EBS belugas, aerial line-transect surveys were conducted in Norton Sound and near the Yukon River Delta from 18 June through 1 July 2024. Flights were conducted on 7 days (18-21, 24, 30 June and 1 July) covering 6,934 km of transect effort. Within the study area, a high-density stratum, where belugas have been observed in large numbers during previous surveys in 2017 and 2022, was sampled multiple times. A digital camera mounted in the belly port of the aircraft collected images primarily within the high-density stratum at 3-second intervals. A total of 29,031 images were collected. Images were reviewed to determine if an observer correction factor for belugas missed along the trackline (g(0)) could be estimated. Images will also be reviewed for age class based on skin color and size (calf, juvenile (light gray), adult (white)) and group composition. Caveats to developing a g(0) correction factor based on a review of sightings within the camera and observer fields of view are discussed. Throughout the study area, 338 beluga groups totaling 1,716 belugas were sighted (328 sightings, 1,195 belugas on transect). This included 24 cow-calf pairs (23 observed on transect). Five beluga carcasses were also observed, photographed, and reported to the Alaska Marine Mammal Stranding Network. The highest densities of belugas extended along the Yukon River Delta north of Scammon Bay within the 5 m depth contour, then east into Pastol Bay, broadening northward to Unalakleet within the 5-15 m depth contours. Other marine mammal sightings included a minke whale (Balaenoptera acutorostrata), a pod of six killer whales (Orcinus orca), harbor porpoises (Phocoena phocoena, 2 sightings, 4 total), 5 small, unidentified pinnipeds, and one unidentified marine mammal. Compared to the 2017 and 2022 surveys, the encounter rate of belugas (whales/km) observed on transect in 2024 was slightly lower, though this was largely due to diverting from the trackline north of Scammon Bay, where a group of approximately 500 belugas were sighted while circling, and therefore not included in the encounter rate.