Acoustic behaviour of northern bottlenose whales (Hyperoodon ampullatus) in Icelandic inshore waters

Northern bottlenose whales (Hyperoodon ampullatus) are among the deepest diving cetaceans, regularly foraging at depths >800 m. They are primarily found in offshore habitats, but occasionally they are also sighted within coastal waters. The drivers for these inshore movements remain unknown. Northern bottlenose whales use regular echolocation clicks and terminal ëbuzzes’ to find and capture prey, but they likely produce clicks for other functions like maintaining group cohesion. Between August and October 2022, a group of 3 northern bottlenose whales spent multiple weeks inshore within Eyjafjöròur in northern Iceland. Here we quantify the acoustic signals attributed to the whales and describe their usage. Acoustic recordings were conducted on three days in late August and revealed the production of regular clicking and buzz-like rapid click trains. Click rates of rapid click trains were slower than those documented for northern bottlenose whale foraging buzzes and may instead have served a communication function. One animal among the group showed consistent unusual behaviour with prolonged logging at the surface; this animal washed ashore dead nine days later without fresh prey in its stomach. Thus, the group likely moved inshore seeking shelter and used echolocation for communication, though a navigational function cannot be ruled out.