Acoustic behaviour and spatiotemporal occurrence of northern bottlenose whales in the Nordic Seas

Northern bottlenose whales (Hyperoodon ampullatus) are echolocating beaked whales found in arctic and subarctic waters of the North Atlantic. Their offshore distribution and deep-diving behaviour make them challenging to study visually, thus knowledge gaps remain on the species’ behaviour and distribution. This thesis advanced our understanding of northern bottlenose whales’ acoustic behaviour and their occurrence in the Nordic Seas. It combined animal-attached sound and movement tag data and passive acoustic monitoring (PAM), together with visual observations and oceanographic data. Specifically, chapter two describes the acoustic characteristics and contextual production of two types of rapid click trains, buzzes and rasps. The results revealed that these click trains are acoustically and contextually distinct, with buzzes associated with foraging and rasps with communication. Classification of buzzes and rasps can also be applied to PAM data to infer animal behaviour, which was shown in a case study on northern bottlenose whale inshore acoustic behaviour in chapter three. The identification of rasps but not buzzes in inshore recordings, in conjunction with behavioural observations and further inshore sightings, pointed towards a shelter driven movement of bottlenose whales into coastal waters. Occurrence and distribution drivers within offshore waters are then addressed in chapter four using long-term bottom-moored hydrophone data. The results showed a year-round occurrence of northern bottlenose whales in the Nordic Seas, and seasonal variations in distribution were linked to oceanographic variables indicative of prey availability. Acoustic detections were only temporarily low from July into September, discounting the long-standing seasonal north-south migration hypothesis for overwintering but instead supporting potential short-term skin maintenance migrations. Chapter five investigates the species’ acoustic detection probability and discusses the impacts of click characteristics, animal behaviour and duty-cycle choice. The estimated detection functions have important implications for PAM studies and constitute a first step towards acoustic density estimation of northern bottlenose whales.