Internesting diving behavior and population structure of hawksbill sea turtles (Eretmochelys imbricata) on St. Croix, USVI

Hawksbill sea turtles (Eretmochelys imbricata) are distributed circumtropically and populations in many locations have been severely depleted. Developing management plans for this species is hindered by major gaps in knowledge concerning habitat use, behavior, and population structure. This study addresses these knowledge gaps for hawksbill sea turtles nesting at Sandy Point National Wildlife Refuge, St. Croix, US Virgin Islands. Specifically, I will focus on research priorities identified by the US Fish and Wildlife Service in the Recovery Plan for Hawksbill Sea Turtles in the US Caribbean Sea. The first chapter addresses Recovery Plan Objective 111: Identify Important Nesting Beaches. Three seasons of nighttime monitoring were conducted on Sandy Point to tag and identify nesting hawksbills and to document all nesting activities on the beach. A total of 78 individual hawksbills were identified, with an average of 26 nesting females per season and a range of 100-500 nesting activities per year, which puts Sandy Point in the top 5% of hawksbill beaches in the Wider Caribbean based on these annual numbers of turtle activities. These data establish Sandy Point as a major rookery in the Eastern Caribbean and support its designation as an index beach for future monitoring of hawksbill nesting trends. In the second chapter, Objective 2112 is addressed: Determine adult internesting movements. Time-depth recorders were deployed on nesting hawksbills to examine internesting diving behavior. Hawksbill turtles spent the majority of the internesting interval relatively inactive, with long dives to a constant depth and short surface intervals. This behavior suggests individuals are resting on the seafloor in a localized internesting residence area. The depth utilized during this time showed pronounced individual variations, with some turtles remaining in shallow water less than 5 meters deep, while others consistently resided in waters 30 meters in depth. In the few days before returning to nest, dive depth for all internesting turtles became much deeper, with two turtles attaining maximum dive depths of 84.5 and 94.6 meters, which are the deepest recorded dives for hawksbills during the internesting interval. These extremely deep dives were possible because the water column is very deep close to Sandy Point due to the narrow continental shelf, and in addition the water temperature does not fall below 24.5°C in the top 100 meters of the water column in these tropical waters. Such conditions permit long bouts of deep diving that are not possible at other locations for the relatively smaller hawksbills. These results demonstrate that when water temperature is not a limiting factor, internesting hawksbill sea turtles can dive up to 100 meters to locate the seafloor just prior to an emergence and adjacent to the nesting beach. Lastly, in Chapter 3 I address Objective 217: Determine the genetic relationships among Caribbean hawksbill nesting populations. Tissue samples were collected from nesting hawksbills to sequence a control region of mitochondrial DNA. Haplotype profiles from Sandy Point were then compared to previously published haplotype data from other rookeries across the Caribbean. This mitochondrial DNA analysis showed that Sandy Point hawksbills are genetically distinct from every other rookery, including Buck Island Reef National Monument, which is also part of the St. Croix complex of islands and within 40 kilometers of Sandy Point. This genetic differentiation is supported by mark-recapture data, as none of the 78 nesting turtles identified had ever been encountered on nearby Buck Island. These population demographic findings demonstrate that St. Croix has two genetically distinct nesting populations of hawksbill sea turtles. As a result, management plans should consider these beaches separately in order to assess the unique threats facing each site. Overall, this study provides much needed insight into the biology and population structure of hawksbill sea turtles in the US Virgin Islands. These results provide a foundation on which to develop management plans for hawksbills, while identifying areas of research that should be a priority in the future