Spatiotemporal occurrence of northern bottlenose whales (Hyperoodon ampullatus) within the Nordic Seas based upon passive acoustic monitoring

Northern bottlenose whales (Hyperoodon ampullatus) are echolocating, deep-diving beaked whales found primarily in arctic and sub-arctic offshore waters. In the eastern North Atlantic, the species has been suggested to undergo seasonal north–south migrations, however, previous whaling data and more recent sighting surveys insufficiently covered the winter months. To address this data gap, bottom-moored hydrophone deployments (n = 8) were conducted at three locations in the Nordic Seas: off Jan Mayen (Norway; 2015–2017), north-east Iceland (2020–2022) and east Iceland (2020–2023). Automated click detection allowed identification of species-specific clicks. Detector precision and recall were manually evaluated using subsets of the data and precision was used to correct the weekly proportion of snapshots that contained clicks. Generalized additive mixed models were used to investigate whether environmental variables associated with prey availability explained occurrence patterns. Results revealed near year-round presence of northern bottlenose whales in the Nordic Seas with a gradual northward shift in spring between Iceland and Jan Mayen. The lowest numbers of detections occurred from July into September, contradicting the long-standing hypothesis of north–south migrations to enable overwintering at lower latitudes. The observed seasonal occurrence patterns were stable across years and associated with higher sea surface height variation, indicative of eddy activity. We interpreted this as support of a prey-driven distribution, as eddies characterize the spawning grounds of northern bottlenose whales’ main squid prey in the Nordic Seas, Gonatus fabricii. The observed occurrence patterns can inform planning of future anthropogenic activities in these waters to avoid habitat degradation and reduce stressors on this species.