Acoustic tag attachment methods for tracking hatchling sea turtles
The early life history of sea turtles, including unknown movement during their “lost years”, is currently characterized as a passive drifter stage, in which turtles have little impact on their dispersal through the water. This description is affecting modeling efforts of this critical stage as evidence increasingly shows hatchling sea turtles’ active role in their environment. Tagging technology has been used to characterize the dispersal and behavior of many pelagic organisms. However, the small body size of young life stages requires particular consideration that such tagging does not significantly impede their buoyancy or movements. In this study, we tested methods for attaching miniature acoustic transmitters to hatchling sea turtles. We obtained approximately 8-week-old hatchery-reared green sea turtles (Chelonia mydas) (n=12) and examined the effect of attaching Vemco V5 acoustic tags. The animal’s speed, direction, and swimming behavior were first observed for 25 minutes in a 3.5m x 3.5m hexagonal tank as a control. Tag attachment was next completed directly by Velcro and then by harness and their swimming behavior recorded for each treatment to determine if there was any significant change. Vertical distance for dives was denoted by a pole marked with centimeter intervals in the middle of the tank.
The turtles’ swimming speed was significantly different with an acoustic tag attached compared to the control (Repeated measures ANOVA, p<0.05). The harness method had a greater effect than the direct Velcro attachment to the carapace and decreased speed (mean speed reduction 12% versus 5% for harness and Velcro attachment, respectively), an expected effect of flipper obstruction. Some hatchlings negatively reacted to being constrained around their shoulder girdle and completed a distinctive evasive maneuver, a behavior exhibited when any object contacted this point. They also spent more time near the surface (<15 cm depth) when the harness was attached than during the control or direct Velcro attachment. Slower swimming and more surface time could increase a turtle’s risk of predation. No change in behavior was observed when the tag was attached directly with Velcro. The direct Velcro attachment method therefore had the least effect on hatchling movement and behavior and could be used to monitor in-water movements of small, hatchling turtles. Future work will involve quantifying the drag forces induced by the attachment of acoustic tags and field testing these tracking devices to understand dispersal and survival of these vulnerable marine species.