Basking sharks of the Arctic Circle: year-long, high-resolution tracking data reveal wide thermal range and prey-driven vertical movements across habitats

Abstract Background Understanding the movement ecology of marine megaplanktivores is essential for conserving these ecologically significant species and managing their responses to environmental change. While telemetry has advanced our knowledge of filter-feeding mammal migrations, the annual movement patterns of large filter-feeding sharks, such as basking sharks ( Cetorhinus maximus ), remain poorly understood. This is particularly the case near their high latitude range limits where climate impacts are intensifying. In this study, we deployed pop-up satellite archival tags (PSATs) on C. maximus in northern Norway to investigate individual movement patterns and possible environmental drivers over an entire annual cycle. Results Geolocated tracks from two females revealed contrasting migration strategies: one shark performed a return migration spending boreal winter close to the Azores, while the other resided north of the Arctic Circle until January before moving to the North Sea in spring. Across these diverse habitats, both sharks utilized a wide thermal range. This included previously unrecorded short-term exposures to sub-zero temperatures, extending the known thermal tolerance of the species. High-resolution time series data from recovered PSATs enabled the use of advanced signal processing and gradient-based filtering techniques to investigate vertical movement patterns in relation to the physical and biological environment. In oceanic habitats, elevated use of the mesopelagic was observed together with diel vertical migration, whereas in shelf areas depth-use patterns were confined by topography and more variable, reflective of more dynamic hydrographic conditions and prey distributions. With zooplankton distributions being structured by ambient light, density gradients, and local topography, the alignment of frequented depths with isolumes, mixed layer depths, bathymetric contours, and bioluminescence events suggests these sharks actively track prey layers across diverse habitats. Conclusions Recorded eurythermy and behavioural plasticity suggest C. maximus to be well-adapted to dynamic ocean conditions. These traits may be critical for responding to the rapid climate-driven changes in the abiotic and biotic environments in high-latitudes, providing insights into how these endangered filter-feeders might navigate shifting ecosystems. Graphical Abstract