Wallace J. Nichols, Louise Brooks, Melania Lopez, Jeffrey A. Seminoff, Record of pelagic east Pacific green turtles associated with macrocystis mats near Baja California Sur, Mexico, Marine Turtle Newsletter, 2001, Volume 93, Pages 10-11, http://www.seaturtle.org/mtn/archives/mtn93/mtn93p10.shtml?nocount
Juvenile East Pacific green turtles (Chelonia mydas), also known commonly as black turtles, encountered 20 miles off the coast of Baja California Sur (BCS), Mexico, may help answer questions about the pelagic stage of this population. While pelagic habits are documented in some Atlantic sea turtle populations (Carr 1986; Carr & Meylan 1980; Witherington 1994), the location and habits of Mexican East Pacific green turtles are generally unknown from the time they leave their natal beaches in Michoacán, Mexico, until they reach benthic feeding grounds such as those along the Baja California, Mexico, coast. While almost half of the green turtles on western Atlantic feeding grounds measure less than 40 cm straight carapace length (SCL), measured notch to tip (Bresette & Gorham 2001; Ehrhart et al. 2000; Schroeder et al. 1998), nearly all of the green turtles that we have encountered on Baja California’s benthic foraging grounds are larger than 40 cm (SCL). Less than 10% of turtles in this group are in the 40 to 45 cm SCL class. This suggests that the East Pacific green turtle may recruit to shallow coastal feeding areas at larger sizes than Atlantic green turtles.
Of 2,742 pelagic sea turtle sightings in the Eastern Tropical Pacific Ocean (ETP), Pitman (1990) reported only 10 Chelonia. None were recorded near the Baja California peninsula. Arenas and Hall (1991) found mostly olive ridley turtles (Lepidochelys olivacea) associated with floating objects in their survey of pelagic fauna in the ETP. These earlier reports are consistent with our previous pelagic sighting of logs along the Baja California coast where olive ridley and loggerhead turtles (Caretta caretta) were the only two species encountered (Nichols et al. 2000a).
During the summer of 1999, pelagic surveys were conducted, mainly for basking loggerhead and olive ridley turtles. On July 14, 1999 five juvenile green turtles were found near 24.313° N, 112.237° W, approximately 20 miles offshore of Bahía Magdalena, BCS. All of the turtles were initially sighted basking near or on top of floating mats of kelp, Macrocystis sp. Kelp mats were up to 300m2 in area in water of depths well over 100m. We were able to catch one of the turtles by hand. Its SCL was 43.7 cm. Four other turtles were sighted at Macrocystis mats in the vicinity and were slightly larger and smaller in size, estimated between 35 and 45 cm (SCL). All of the turtles swam quickly and actively, making them difficult to capture by hand. Those eluding capture dove to approximate depths of 5 to 10 m, remaining within sight and exhibiting fidelity to the area around the kelp mat. No external signs of injury or poor health were noticed. Barnacles (presumed Chelonibia sp.) were present on the carapaces of all five turtles.
The main green turtle nesting beaches in the region occur on the mainland Mexican coast in Colola and Maruata, Michoacán. The turtles associated with the kelp mats may have dispersed from these beaches, as most green turtles found in Baja California are apparently part of the Michoacán stock (Alvarado & Figueroa 1992; Nichols et al. 2000b). Alternatively, these turtles may have been associated with these mats prior to the drifting of the kelp into the pelagic environment.
Juvenile East Pacific green turtles may actively or passively associate with pelagic kelp mats, residing in them as they drift along frontal regions. These mats may provide safe developmental and foraging habitat for turtles during their pelagic phase, prior to recruitment to benthic habitats—algae and seagrass beds—along the Baja California coast. Future research on the pelagic stage of this population will involve satellite telemetry and diet analysis.
Acknowledgements: Thanks to Alberto Ojeda Aripez, Dr. Luis Calderon, the staff and students of the Centro para Estudios Costeros, and Kristen Bird. Funding for this work was provided by the School for Field Studies, and permits were provided by SEMARNAP.
Alvarado, J. & A. Figueroa. 1992. Post-nesting recaptures of black marine turtles (Cheloniaagassizii) tagged in Michoacán, Mexico. Biotropica 24: 560-566.
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Bresette, M. & J. Gorham. 2001. Growth rates of juvenile green turtles (Chelonia mydas) from the Atlantic coastal waters of St. Lucie County, Florida, USA. Marine Turtle Newsletter 91:5-6.
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