Long-term monitoring of black turtles Chelonia mydas at coastal foraging areas off the Baja California Peninsula

Abstract

Monthly in-water monitoring was conducted at 5 foraging areas off the Baja California Peninsula, Mexico, from 2001 to 2006 to assess black turtle population status. Turtles were captured with entanglement nets, size (straight carapace length, SCL) and weight of each individual were recorded and turtles were tagged on both rear flippers. A total of 1238 turtles were captured, involving 6309 h of effort. Of this total, 937 ind. were marked and 155 were recaptured. SCL ranged from 30 to 95.5 cm; immature turtles accounted for > 94% of the total catch and were dominant at all sites. However, size class distribution varied among sites, and turtles from the Gulf of California were significantly larger than those caught in the Pacific. Catch per unit of effort (defined as the number of turtles caught per 100 m of net soaking for 24 h) varied from 1.79 at Baha Magdalena to 17.35 at Punta Abreojos. Mean ( SD) annual growth rate for black turtles was 2.27 0.71 cm yr -1 , with the lowest and highest growth rates in Baha Magdalena (1.37 0.71 cm yr -1 ) and Laguna Ojo de Liebre (3.05 2.23 cm yr -1 ), respectively. Lack of recaptures from adjacent sites and multiple recaptures of specimens at the site of origin over several years indicate a low connectivity among foraging areas and a high site fidelity. Hence, strengthening local protection is imperative to the recovery of black sea turtle populations.