Long-term multitracking reveals contrasting yet highly resident movement ecologies of two sympatric and endangered deep-sea sharks

Studying shark movement ecology is vital for understanding their ecological roles and supporting sustainable management and conservation strategies. However, such information remains scarce for deep-sea sharks. We used biotelemetry to investigate the spatial behaviour and movements of two endangered deep-sea predators, the kitefin ( Dalatias licha ) and bluntnose sixgill ( Hexanchus griseus ) sharks, in the Azores, northern Mid-Atlantic Ridge. We tagged a total of 21 kitefin sharks with acoustic transmitters (some including depth sensors) and seven sixgill sharks with different tag configurations: three were fitted exclusively with acoustic transmitters, two were double-tagged with both acoustic transmitters and pop-up satellite archival tags (PSATs), and two were tagged exclusively with PSATs deployed via a speargun-equipped submersible. Both species exhibited diel vertical migrations and unexpected high site fidelity (up to 4 years), using habitats that inter-connect seamounts, slopes, and island shelves. Sixgill sharks exhibited more extensive and diverse individual home ranges and vertical diel activity patterns than kitefin sharks. Sexual segregation was evident in kitefin sharks, with males and females displaying distinct differences in depth distribution and habitat use, supporting earlier hypotheses based on fisheries data. These behavioural patterns suggest that sixgill sharks function as wide-ranging, deep-sea opportunistic foragers and predators, traversing interconnected habitats in search of prey – including kitefin sharks, which occupy a more slope-associated mesopredator niche. Our novel findings support ecological theory suggesting that deep-sea sharks exhibit far more contrasting spatial ecologies than previously thought, driven by their life histories. These differences may have implications for their high vulnerability to fisheries and climate change-induced habitat degradation. • Long-term tracking reveals strong site fidelity in two endangered deep-sea shark species. • Kitefin and sixgill sharks exhibit contrasting movement ecologies and habitat use. • Sixgill sharks roam widely as apex predators; kitefin sharks show more slope site-attachment. • Sexual segregation in kitefin sharks supports fisheries-based hypotheses. • Findings highlight vulnerability to fisheries and habitat degradation in deep-sea sharks.