Life in (and out of) the lagoon: fine-scale movements of green turtles tracked using time-depth recorders

Tracking fine-scale movements in relation to threats is fundamental to the management of exploited marine species, yet there is considerable difficulty associated with gathering such data at sea. By combining a capture-recapture study with deployment of time-depth recorders (TDRs) and ultrasonic tags, we elucidated distribution and daily movements of juvenile green turtles Chelonia mydas exposed to a legal marine turtle fishery in the Cayman Islands. For instrumented turtles, distinct diel activity patterns were observed: dives during the day were shorter and more active than those at night, implying diurnal foraging and nocturnal resting. Spatially, while capture and recapture locations suggested fidelity to a shallow lagoon, when turtles were fitted with TDRs and ultrasonic tags we demonstrated that they regularly moved out of the lagoon and onto the reef, where they could legally be captured in the marine turtle fishery. Our results are thus novel and valuable in a management context in that we demonstrated that seemingly protected aggregations of juvenile green turtles within a lagoon were, in fact, exposed to legal capture on a near-daily basis. This emphasizes the importance of assessing diel activity patterns of juvenile marine turtles, particularly with respect to directed take and other threats.