Satellite Tracking of Head-Started Juvenile Green Turtles (Chelonia mydas) Reveals Release Effects and an Ontogenetic Shift

SIMPLE SUMMARY: Tracking juvenile animals is often difficult due to their small body sizes. We put miniature satellite tags on 40 captive-raised (“head-started”) green sea turtles (Chelonia mydas) and released them during two different times (January and July) off the Cayman Islands to determine whether there were differences in their movements. We used a statistical model and plots of turtle swimming directions versus ocean currents to find out what influenced their swimming direction and speed. We found that turtles released in January had very different movement patterns from those in July. The January turtles all moved in random directions from the islands around the Caribbean, whereas the July turtles all moved north of the islands. Additionally, we found that both the January and the July turtles swam harder and with more direction the closer they got to coastal areas. Turtles also appeared to mostly swim in the opposite direction to ocean currents to keep their desired path. These results show that captive-raised juvenile green turtles can likely move through deeper ocean waters after being released to coastal habitats where resources (such as food and shelter) are available. These results are important for future releases of captive-born turtles and for the conservation of wild Caymanian green turtles. ABSTRACT: Juveniles of marine species, such as sea turtles, are often understudied in movement ecology. To determine dispersal patterns and release effects, we released 40 satellite-tagged juvenile head-started green turtles (Chelonia mydas, 1–4 years) from two separate locations (January and July 2023) off the coast of the Cayman Islands. A statistical model and vector plots were used to determine drivers of turtle directional swimming persistence and the role of ocean current direction. More than half (N = 22) effectively dispersed in 6–22 days from the islands to surrounding areas. The January turtles radiated out (185–1138 km) in distinct directions in contrast to the northward dispersal of the July turtles (27–396 km). Statistical results and vector plots supported that daily swimming persistence increased towards the end of tracks and near coastal regions, with turtles largely swimming in opposition to ocean currents. These results demonstrate that captive-reared juvenile greens have the ability to successfully navigate towards key coastal developmental habitats. Differences in dispersal (January vs. July) further support the importance of release timing and location. Our results inform conservation of the recovering Caymanian green turtles and we advise on how our methods can be improved and modified for future sea turtle and juvenile movement ecology studies.