Inferring vertical and horizontal movements of juvenile marine turtles from time-depth recorders

Acquiring knowledge of the spatial ecology of many aquatic species, particularly their juvenile life stages, is challenging when they are small in size; this limits the attachment of tracking devices that routinely provide spatially explicit information on larger juveniles and adults. Using depth utilisation data gathered by time-depth recorders, we inferred patterns of habitat utilisation of juvenile hawksbill turtles Eretmochelys imbricata occupying a shallow reef system in the Caribbean. Hawksbill turtles (n = 9, curved carapace length = 42.3 ± 9.7 cm, range = 27.7 to 60.0 cm) demon- strated diel patterns of behaviour, with greater activity and shorter dives during the day (mean dura- tion = 7.5 ± 5.0 min, range = 2.6 to 34.4 min) and longer, less active dives during the night (mean dura- tion = 16.6 ± 12.0 min, range = 2.5 to 61.5 min). A level of fidelity to sites facilitated recaptures (mean minimum straight-line distance from release of TDR-equipped turtles to their subsequent recapture locations = 0.9 ± 0.9 km, range = 0.01 to 2.3 km), but recapture success after 130 h, over a 43 d period, was relatively low: of 19 individuals instrumented, 9 were recaptured. From depth records, i.e. max- imum depths achieved, it was clear that some recaptured individuals moved considerably during the monitoring period; 3 individuals moved to deeper waters, with minimum possible straight-line move- ment distances from their capture sites of 3.7 to 9.2 km. These findings emphasize the likely influence of habitat structure on site fidelity and further highlight that data from traditional capture-mark- recapture studies could lead to an underestimation of home-range size.