Satellite Tracking Kemp's Ridley, Loggerhead, and Green Sea Turtles
Kemp's ridley sea turtle returning to the Gulf of Mexico with a satellite transmitter attached.
NPS photo
Researchers at Padre Island National Seashore use satellite telemetry to learn more about migratory patterns, distributions, and habitat use of adult sea turtles. Biologists at the park deployed over 150 satellite transmitters on foraging, nesting, and stranded sea turtles from 1995–2019 and have led collaborative efforts in Mexico and other USA states.
Once they’ve nested, adult female Kemp's ridley sea turtles migrate to foraging areas to feed (Morreale et al. 2007, Shaver et al. 2013b). Most post-nesting female Kemp’s ridley sea turtles equipped with satellite transmitters migrated to the northern Gulf of Mexico, mainly off the coast of Louisiana in waters less than 200 ft deep and approximately 20 miles from shore (Figure 1; Shaver et al. 2013b, Hart et al. 2018a, Gredzens and Shaver, 2020). Several tracked females stopped at multiple foraging areas while traveling northward; however, most migrated directly to their final foraging site (Shaver et al. 2013b). Many turtles display foraging area fidelity, meaning, they returned to the same areas in different years (Shaver et al. 2013b, Gredzens and Shaver, 2020).
Satellite tracking research gives us an in-depth look into the movements of females during their seasonal nesting period and post-nesting migration (Shaver et al. 2016, 2017). The waters off Padre Island National Seashore are an important migration route for the Kemp’s ridley sea turtle population, as many females migrate northward after nesting in Mexico (Shaver and Rubio 2008, Shaver et al. 2016, Gredzens and Shaver, 2020). The existence of this migratory corridor in nearshore waters of both the USA and Mexico demonstrates that international cooperation is necessary to protect essential migratory habitat for this imperiled species (Shaver et al. 2017, Gredzens and Shaver 2020). Additionally, knowledge of Kemp's ridley foraging hotspots is important in determining where to direct further research and protection.
Studies by Division staff of the movements of adult male Kemp's ridley turtles captured in waters off Rancho Nuevo, Mexico, indicate that, unlike females, most adult male Kemp's ridley sea turtles remain close to the nesting beach year-round (Shaver et al. 2005, Morreale et al. 2007). Prior to this work, little was known about the movements, residency, and habitat use of adult male Kemp’s ridley sea turtles. Opportunities to outfit male Kemp’s ridley sea turtles with satellite transmitters are infrequent compared with female Kemp’s ridleys who nest seasonally at the National Seashore. However, when males are incidentally captured or stranded, researchers can use the occasion to attach a transmitter and learn more. Since 2006, researchers at Padre Island National Seashore have tracked the movements of seven adult male Kemp’s ridleys that were incidentally captured or found stranded in the local area. Similar to the males tracked in Mexico, most of these males remained near the nesting beaches of the Texas coast for the duration of their tracking. However, two of the males tracked (one in 2006 and one in 2012) traveled to waters off the coast of Louisiana after being released, similar to post-nesting female Kemp’s ridleys. These differences indicate that there is some level of plasticity in the movements of male Kemp’s ridleys, but the percentage of the male population that undertakes large-scale migrations like those seen in females is unknown. While additional research is needed to determine whether adult males remain resident year-round in waters off south Texas nesting beaches, our research clearly indicates recovery programs and conservation measures for Kemp's ridley sea turtles should incorporate considerations regarding year-round residency of adult males.
Satellite telemetry has also been used in conjunction with computer modeling to compare habitat use between Kemp’s ridley and loggerhead sea turtles (Hart et al. 2018b, Fujisaki et al. 2020). In one study, switching state-space modeling (SSM) was used to determine foraging behavior of Kemp’s ridley and loggerhead sea turtles in the Gulf of Mexico (Hart et al. 2018b). The second study used ensemble ecological niche modeling (EENM) to predict suitable foraging habitat for these same sea turtle species (Fujisaki et al. 2020). Results from both studies indicate these species do not overlap greatly during foraging. The predicted suitable habitats for both species spanned the entire Gulf of Mexico, but most of the predicted foraging areas for Kemp’s ridleys and loggerheads do not overlap. Both studies tracked only female turtles, therefore, determining habitat use and whether similar drivers affect the distribution for males and juveniles will be important for the conservation of these species. Research using satellite telemetry combined with computer modeling can be a powerful tool for species management. Our research identified areas for multi-species marine habitat conservation within a large geographic extent and has important implications for other marine species studies in the Gulf of Mexico and elsewhere.
Satellite tracking studies by researchers at the park are not limited to Kemp’s ridley and loggerhead sea turtles. The park also provides important nesting, foraging, developmental, and migratory habitat for green turtles. In 2018 and 2019, researchers at the park deployed three satellite transmitters on adult female green turtles found nesting at the National Seashore. Two of these transmitters provided enough information to identify foraging areas off the Yucatán Peninsula in Mexico. These deployments represent the first satellite tracks of post-nesting green turtles along the Texas coast, providing evidence that at least some of the green turtles nesting at the park are a shared resource with Mexico. These results highlight the need for continued and increased international collaboration for the protection and management of other sea turtle species nesting at the park, on top of those already in place for Kemp’s ridleys. In addition, the shallow Laguna Madre west of the National Seashore and the Mansfield Channel to the south are important habitats for foraging, developing, resting, and migrating juvenile green sea turtles. In the late 1990’s and early 2000’s, juvenile green sea turtles were studied from the Laguna Madre and Mansfield Channel using satellite transmitters to determine residency, seasonality, distribution, movements, and migration patterns in Texas (Shaver 1994, 1998, 2000, 2005, Shaver et al. 2013a, Shamblin 2017, Shaver et al. 2020, Meylan et al. 2020). Satellite tracking data support that the Mansfield Channel and Laguna Madre are important developmental habitats for juvenile green turtles. Some tracked green turtles remained at the Mansfield Channel jetty area, some used the Mansfield Channel as a passageway between the Laguna Madre and Gulf of Mexico, and some moved southward along the Gulf of Mexico coast to waters off the coast of Mexico (Shaver 1998, 2000, 2005, Shaver et al. 2013a).
Results from these satellite tracking studies have been and will continue to be used by managers to develop and evaluate regulations and protection measures and improve restoration programs undertaken for sea turtle species. These studies also underscore the importance of continued international partnership for the recovery of sea turtle populations in the Gulf of Mexico.
To view maps of the satellite tracking research conducted by Padre Island National Seashore visit: www.seaturtle.org
Figure 1. Switching state-space model-derived foraging days for post-nesting female Kemp’s ridley sea turtles summarized in 25 km hex-bin cells: PAIS = Padre Island National Seashore, Texas; RNMX = Rancho Nuevo, Tamaulipas (Tamps) Mexico; VCMX = Tecolutla, Veracruz (Ver), Mexico. The Exclusive Economic Zone (EEZ) is the division between the northern and southern Gulf of Mexico (nGoM, sGoM). Figured reproduced from Gredzens and Shaver (2020).
NPS Image.
Literature Cited:
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