US Navy

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Marine Species Monitoring

Sea turtle tagging in the Mariana Islands Range Complex

Introduction & Objectives

Prior to the start of this project, little was known about sea turtle populations in the Mariana Archipelago.  Although it was recognized that both green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) turtles inhabit the waters around Guam and the Commonwealth of the Northern Mariana Islands, their distribution and habitat use in the region was unclear.  In 2013, under an inter-agency agreement between the United States Navy and the National Oceanic and Atmospheric Administration (NOAA), NOAA began conducting in-water surveys to record and quantify observations of marine turtles around the Mariana Archipelago.  The objective of this long-term study is to develop a better understanding of sea turtle distribution and habitat use in the Mariana Islands, particularly in relation to locations used by the Navy for underwater detonation training.  This will ultimately provide data to evaluate impacts from Navy training and testing activities. 

Technical Approach

Small boat surveys were conducted in the nearshore and coastal waters of Guam, Saipan, and Tinian.  During surveys, the team recorded all observations of turtles along with approximate GPS coordinates.  The team also recorded information on the species, approximate number of individuals, and GPS locations of any observed cetaceans.  If possible, turtles were hand captured for additional data collection. 

All captured turtles were tagged with two metal “flipper tags” and with two Passive Integrated Transponder (PIT) “microchip” tags with unique identification numbers. Skin samples were obtained for DNA and stable isotope analysis. Straight carapace length (SCL), curved carapace length (CCL), and turtle mass were measured (when feasible), and turtles of appropriate SCL were outfitted with a satellite tag.  Two types of satellite tags were used dependent on size of the turtle. The Wildlife Computers SPLASH10-F which recorded and transmitted ARGOS locations, GPS locations, depth, temperature, light level, and wet/dry status.   The Wildlife Computers SPOT-311A tags collected only ARGOS locations and water temperature. 

Tag data were analyzed to determine home ranges and migrations, if any.  Overlap of home ranges with Navy ranges was examined.  Habitat use was examined using temperature, dive depth, maximum depth, and dive duration data. 

Progress & Results

Between 2013 and 2019 (the project is still underway), researchers encountered a total of 517 turtles, 111 of which were captured and equipped with satellite tags, including 97 green turtles and 14 hawksbill turtles. The majority of turtles spent most of their time in shallow coastal waters, with home range estimates revealing limited movements for both species. Ninety−four (89.5%) of the tracked turtles remained within a 1−3 km2 area for the entire life of their tag (average tag retention time = 191 days), demonstrating limited movements and high foraging site fidelity. Notwithstanding this perspective, there were three long-distance movement patterns observed, including shifts in intra-island foraging areas (7 turtles), transitions between inter-island foraging areas (2 turtles), and a long-range migration departure out the Mariana Archipelago (1 turtle). Dive patterns suggest that both green and hawksbill turtles spend most of their time in waters shallower than 25 meters. However, it is possible that habitat partitioning may exist between the two species, with hawksbill turtles spending more time in deeper waters than green turtles, using average depths of 15.3 meters and 10.5 meters, respectively. Spatial analysis of satellite tags has demonstrated limited direct overlap of turtles with Navy detonation sites (i.e., Agat Bay Mine Neutralization Site, Piti Point Mine Neutralization Site, and Outer Apra Harbor Underwater Detonation Site). However, turtles are spending significant amounts of time in and moving through areas within 1−2 km of these sites and additional analyses are needed to better evaluate potential overlap.

The results of this work were used by the Navy to inform the analysis in the Mariana Islands Training and Testing Environmental Impact Statement proposed geographic mitigation areas and Endangered Species Act consultations.

 

Publications

Five decades of marine megafauna surveys from Micronesia.