Photo-identification and seasonal occurrence of long- finned pilot whales (Globicephala melas) off Vesterålen, northern Norway.

Long-finned pilot whales are an understudied species of the Delphinidae family, that can be found throughout the North Atlantic. This thesis used data collected between 2008 and 2023, to investigating the feasibility of using photo-identification to study long-finned pilot whales (Globicephala melas) in the Vesterålen archipelago in northern Norway, as well as checking if they have a seasonal occurrence and site fidelity. A total of 16,494 images were captured during 37 encounters, resulting in the identification of 178 individuals in the permanent catalogue and 163 individuals in the temporary catalogue. Daily and monthly re-sighting patterns were observed, highlighting their regular occurrence in the study area. The mark rate, which measures the proportion of identifiable individuals among encounters, averaged at 62.4%. Comparison with other studies revealed similar mark rates among different pilot whale groups. However, caution is advised in interpreting the results due to variations in mark rate calculation methods and potential biases in comparisons. Notch markings were the most abundant and consistent mark type, followed by saddle patch, piece protruding, and white scar. Seasonal occurrence analysis showed higher encounter counts in June and July. Overall, the findings demonstrate the usefulness of photo-identification for long-term monitoring of the long-finned pilot whales in Vesterålen and highlight the need for standardized mark rate assessment methods and accurate estimation of group size.