Killer whale (Orcinus orca) occurrence in Icelandic waters revealed by passive acoustic monitoring

The distribution and occurrence of killer whales around Iceland are primarily studied in coastal herring grounds, with limited knowledge beyond these areas. This study employed opportunistic passive acoustic monitoring (PAM) data from four regions across Icelandic inshore and offshore waters to investigate killer whale distribution between 2018 and 2023. An automatic detector-and-classifier algorithm identified killer whale burst-pulsed calls, manually confirmed by human observers. Killer whale calls were detected on a total of 59 of 3075 recording days over a broader geographic area than previously documented. Detections were spread throughout the years, and in some cases, potential feeding behavior could be inferred. In the south of Iceland, killer whales were detected throughout the winter period; their presence outside of the summer months was previously undocumented. In the west, daily occurrences differed between two locations, which may be linked to the distribution of herring. In the east and northeast, killer whales were detected in the summer months, aligning with the abundance of herring in these regions. This study highlights the importance of PAM for year-round monitoring of killer whale occurrence in the North Atlantic and contributes to our understanding of how they may respond to potential changes in prey dynamics.