Winter records of killer whales (Orcinus orca) in the waters of the Wilhelm archipelago, West Antarctica

Abstract

Killer whales (Orcinus orca (Linnaeus, 1758); KWs) are apex marine predators. In Antarctica, they have diverse trophic interactions and belong to five known ecotypes that have substantial differences in morphology, ecology, and acoustic repertoire. Around the Antarctic Peninsula, two distinct sympatric forms of Type B KWs are the most common. Both ecotypes have similar pigmentation patterns, but the larger form has been observed to feed mainly on seals (B1), and a smaller form feed on Pygoscelis penguins (B2). The waters of the West Antarctic Peninsula are experiencing marine ecosystem transformations caused by climate change at one of the fastest rates on Earth. Little is known about the austral winter distribution and ecology of KWs in this changing environment. The purpose of the present study was to reveal the winter distribution and ecology patterns of KWs in the waters of the Wilhelm Archipelago. Boat-based visual and photo-identification observations were performed. The boat cruises were conducted within 14 nautical miles of the Ukrainian Akademik Vernadsky Research Station, Galindez Island. In 2019, 10 surveys of a total of 194 nm were conducted: 4 trips in June and 6 in July. In 2021, 37 surveys of a total of 605 nm were conducted: 12 trips in June, 14 in July, and 11 in August. Surveys were performed when the ice conditions were suitable. During both winters, we managed to record only three groups of KWs. A group of 7 KWs was encountered on 27 June 2019 near Hovgaard and Pleneau islands. The group consisted of adults of both sexes and juvenile individuals. In the same area, on 13 July 2021, another group of KWs was observed, consisting of at least 5 individuals (up to 7). This group included 1 adult male, 2 adult females, a calf, and a juvenile of unknown sex. A comparison of the photos showed different individuals in the two groups. According to the external morphology patterns, it was determined that both groups belong to Type B, most likely of a smaller form (B1). Large gatherings of Pygoscelis penguins were observed in this area during the winter in both years. On 22 July 2021, a group of KWs consisting of 2 adult males was observed in the Penola Strait. Detailed observations of their joint feeding on a crabeater seal (Lobodon carcinophaga Hombron & Jacquinot, 1842) were collected. According to external morphology patterns and behaviour, they belong to a large form of Type B ecotype (B2). As Antarctic krill (Euphausia superba Dana, 1850) move inshore during winter, the Pygoscelis penguins and crabeater seals likely took advantage of ice-free areas to feed on it, and their gatherings could attract KWs of both Type B ecotypes. Future work should include collecting photogrammetry data, skin biopsy samples, feeding behaviour observations, and acoustic recordings to clearly differentiate between KW ecotypes and better understand their winter distribution and ecological patterns in this rapidly changing marine ecosystem.