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Satellite Tracking of Endangered Kemp's Ridley Sea Turtles

Padre Island NS
Darrell Echols
A cooperative study is being undertaken using satellite telemetry and GIS mapping to investigate the inter-nesting and post-nesting movements of endangered Kemp's ridley sea turtles (Lepidochelys kempii) that nested at or near Padre Island National Seashore (PAIS), Texas, U.S.A. The U.S. Geological Survey leads this investigation in close partnership with the National Park Service. The National Park Foundation, University of Alabama, Unilever, U.S. Fish and Wildlife Service, and others have also partnered in this work.

From 1978-1988, a multi-agency project was conducted to re-establish a nesting colony of Kemp's ridley turtles at PAIS through experimental imprinting and head-starting (rearing in captivity for approximately the first year of life). Efforts to detect Kemp's ridley turtles nesting at PAIS were initiated in 1986 and have continued since that time, with the goals of fostering re-establishment of the nesting colony, determining results of the experimental procedures used in this project, and collecting other biological data important to the protection and management of this species. During recent years, nesting has increased at PAIS and over half of the Kemp's ridley nests located in the U.S.A have been found there.

The objectives of this satellite telemetry study were to (1) predict where and when Kemp's ridley turtles would lay successive clutches (Kemp's ridley turtles nest one to three times per year, every one to three years), (2) investigate movements and habitat utilization of Kemp's ridley turtles during and after the nesting season, and (3) compare movements and habitat utilization of wild and head-started Kemp's ridley turtles. Data were also gathered to evaluate the vulnerability of adult Kemp's ridleys to threats in waters offshore from PAIS and in other Gulf of Mexico waters, for use by resource agencies in the evaluation and development of regulations and protection strategies. This was the first study of movements and habitat utilization by Kemp's ridley turtles that nested in the U.S.A. and the first study comparing the movements of wild and head-started adult sea turtles.

Twenty-three Kemp's ridley turtles that nested at or near PAIS were outfitted with satellite (UHF) radio transmitters between 1997 and 2003. Three of these individuals received a second transmitter after they returned to nest but were missing their initial transmitters. These Telonics ST-18, ST-6, and ST-14 Platform Transmitter Terminals (PTTs) were configured in a backpack style and attached to the anterior of each turtle’s carapace using thin layers of fiberglass cloth and polyester resin (Figure 1). PTTs were programmed with a transmission (duty) cycle of 6 h on/6 h off to extend the battery life. The turtles selected for this study were the first individuals encountered during a nesting season that were deemed suitable for tracking; turtles with obvious injuries or unsuitable carapace characteristics were excluded.

Turtle movements were monitored via satellite until transmissions ceased due to transmitter detachment or failure. Transmission information was downloaded from the ARGOS system at least once every seven days. Turtle locations (latitudes and longitudes) were calculated by the ARGOS Data Collection and Location System from Doppler-shift in transmission frequency. All data were extensively screened to eliminate land locations, duplicate points, points where the rate of movement of a turtle between two consecutive locations exceeded 6 km/hour, and locations that were obviously inaccurate. After data screening, remaining locations from ARGOS class codes 3, 2, 1, 0, A, and B were mapped for each transmitter using ESRI GIS applications ARCVIEW and Tracking Analyst (Figure 2).

Data were received from individual PTTs for 1-15 months. Information collected through the satellite tracking of nesting females has already been, and should continue to be, of direct value to a variety of entities. Tracking data were successfully used to predict where and when four of the turtles laid successive clutches within a nesting season, thereby improving our ability to document reproduction, protect the nesting turtles and their eggs, and evaluate the experimental effort to establish a secondary nesting colony.

Most of the turtles tracked left south Texas and traveled northward, parallel to the coastline, after they completed nesting for the season, with their last identified locations in the northern or eastern Gulf of Mexico. However, one traveled northward after nesting, turned and went southward to waters off the coast of Mexico, and later traveled northward again. Additionally, two turtles remained in south Texas after the nesting season was completed. Both of these turtles were head-started individuals tracked during 1999. Since the differences noted in movements and habitat utilization of wild and head-started turtles could be due to low sample sizes and year-to-year variations, further study is needed.

For turtles tracked from 1997-2000, 42.9% of their identified positions in Gulf waters off the south Texas coast were in 10 fathoms depth or less, within about 7.4 nautical km from shore, and 82.9% of their identified positions in Gulf waters off the south Texas coast were in 20 fathoms depth or less, within about 27.4 nautical km from shore. Texas Parks and Wildlife Department used these data during their development of a regulation that recently established a seasonal closure of south Texas Gulf waters to shrimp trawling. This regulation should help ensure enhanced protection and survival of adult Kemp's ridley and other sea turtles inhabiting waters offshore from PAIS and elsewhere in south Texas.

Data are continuing to be collected for the four transmitters deployed during April and May 2003 and occasional transmissions are still being received for two of the four transmitters deployed during 2002. Additional turtles may be outfitted with transmitters and studied in the future, depending on funding availability.

April 08, 2004