A multi-year analysis of acoustic occurrence and habitat use of blue and fin whales in eastern and central Fram Strait.

Climate change-induced habitat alterations in the Arctic Ocean are expected to affect spatial and temporal occurrence patterns of seasonally migrating baleen whale species, leading to poleward range shifts and prolonged stays in Arctic waters. The aim of this study was to investigate occurrence patterns of blue (Balaenoptera musculus musculus) and fin (B. physalus) whales in Fram Strait, a summering habitat and historic whaling ground for both species. Passive acoustic monitoring data were collected between 2012 and 2021 at eight different mooring positions in eastern and central Fram Strait. These data were initially analyzed for the acoustic presence of blue and fin whales. Afterwards, the environmental impact on acoustic occurrences and the potential temporal expansion of acoustic presence periods were investigated. Blue whale acoustic presence showed a clear seasonality, with most calls being detected from July to October. Additionally, sporadic blue whale calls were detected in winter in three years on one or a few consecutive days. Fin whale acoustic presence varied between years, with peak presence from mid-summer through autumn, moderate presence in winter, and sporadic presence in spring. Random forest models suggested that zooplankton mass content, sea surface temperature and day of the year influenced blue whale acoustic presence, while fin whale acoustic presence was impacted by day of the year. For both species, no temporal trend in the onset and offset of acoustic presence periods was found. Our findings highlight eastern Fram Strait as key marine mammal habitat, probably serving as a feeding ground. Occasional (blue whales) and regular (fin whales) acoustic presence during the winter months further supports the hypothesis of complex migration behaviors in both species.