Using acoustic monitoring to reveal nearly year-round presence of humpback whales (Megaptera novaeangliae) in the waters of southern Iceland

Abstract

Icelandic waters are a crucial feeding ground for North Atlantic humpback whales, yet their occurrence in these subarctic waters remains underexplored. This study examined seasonal and diel patterns in humpback whale occurrence off the Vestmannaeyjar archipelago, southern Iceland, from June 2018 to May 2019, using automated acoustic detections from bottom-moored hydrophones. Vocalizations were manually categorized into social calls, song fragments, or songs. Further classification identified seven social sound types and 25 unique song units. Humpback whales were detected on 126 out of 329 recording days and were present in nearly all months except April and May. Social calls were most common in summer and fall, while song fragments and songs were prevalent in winter. No diel singing pattern was observed. These findings suggest southern Iceland serves as a vital habitat for humpback whales, acting as a feeding ground in summer and a migration stopover or overwintering site in winter. The study underscores the potential of acoustic monitoring to uncover important habitats year-round, especially when field observations are scarce, and emphasizes the need for ongoing monitoring of habitat use. Continued acoustic monitoring could provide further insights into the whales' behavioral patterns and preferences, essential for their conservation.