Sound production during feeding in Icelandic herring-eating killer whales (Orcinus orca)

Killer whale vocal behaviour is intricately connected to dietary preferences. In Iceland, killer whales feeding on herring are vocally active, use tail slaps to debilitate fish and produce herding calls, thought to function to affect prey. However, there is still limited understanding of how sounds are used by the whales or produced during feeding activities. We used acoustic data collected using archival tags deployed in 2009 (n = 4), 2021 (n = 9), 2022 (n = 5) and 2023 (n = 8) to investigate killer whale sound production during feeding on herring. The acoustic record was divided into 5-min bins, and the presence of clicks and tail slaps was marked for each bin. Then, sound production in the 5-min prior and 1-min following each tail slap was examined in detail, marking echolocation clicks, buzzes, herding calls and prey consumption sounds. Most bins including echolocation clicks (81.8%) were found to either contain a tail slap or to immediately precede a tail slap, indicating that echolocation is primarily used during feeding and likely less so for navigation and orientation. Buzzes, which likely function to locate debilitated prey in the immediate vicinity of the whale, followed 73.5% of the tail slaps. Prey consumption sounds might be useful as markers of feeding success and were present in 63% of feeding events, always following buzzes. Herding calls were found in 26% of feeding events in 2021-2023 and absent in 2009, which may suggest that it is a group-specific call or that it is not used in all feeding events. Variations in sound production of herring-eating killer whales during feeding may be driven by environmental characteristics, individual identity or prey characteristics; future studies combining sound with other tag sensor data such as video, and prey sampling will be crucial to explore these possible drivers.