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As historically abundant spongivores, hawksbill turtles Eretmochelys imbricata likely played a key ecological role on coral reefs. However, coral reefs are now experiencing global declines and many hawksbill populations are critically reduced. For endangered species, tracking movement has been recognized as fundamental to management. Since movements in marine vertebrates encompass three dimensions, evaluation of diving behavior and range is required to characterize marine turtle habitat. In this study, habitat use of hawksbill turtles on a Caribbean coral reef was elucidated by quantifying diel depth utilization and movements in relation to the boundaries of marine protected areas. Time depth recorders (TDRs) and ultrasonic tags were deployed on 21 Cayman Islands hawksbills, ranging in size from 26.4 to 58.4 cm straight carapace length. Study animals displayed pronounced diel patterns of diurnal activity and nocturnal resting, where diurnal dives were signiWcantly shorter, deeper, and more active. Mean diurnal dive depth (±SD) was 8 ± 5 m, range 2-20 m, mean nocturnal dive depth was 5 ± 5 m, range 1-14 m, and maximum diurnal dive depth was 43 ± 27 m, range 7- 91 m. Larger individuals performed signiWcantly longer dives. Body mass was signiWcantly correlated with mean dive depth for nocturnal but not diurnal dives. However, maximum diurnal dive depth was signiWcantly correlated with body mass, suggesting partitioning of vertical habitat by size. Thus, variable dive capacity may reduce intraspeciWc competition and provide resistance to degradation in shallow habitats. Larger hawksbills may also represent important predators on deep reefs, creating a broad ecological footprint over a range of depths. [ABSTRACT FROM AUTHOR]