ISTS: Black turtle (Chelonia mydas) population ecology and fisheries mortality in Bahi­a Magdalena, Mexico

Bahía Magdalena, on the Pacific coast of Mexico, is an important developmental and feeding area for the endangered black turtle. Despite national and international protection, turtles are still being caught and eaten in the area. The objective of this study was to describe population structure, distribution, growth and mortality of black turtles in the bay, to determine how the population is affected by the ongoing exploitation and how sea turtles can be protected more effectively. Black turtles were caught year-round with entanglement nets during four years, measured, tagged and released. During the same period, mortality censuses were conducted in seven communities and on 60km of Pacific beaches. The results show that over 90% of the black turtles caught alive were juveniles (straight carapace length SCL=54.4±9.4, n=217) and that the mangrove channels of Bahía Magdalena are important nursery grounds. As the turtles grow, they generally move towards the open bay and deeper waters. Growth of recaptures was slow (1.7±0.6cm SCL/year) and strictly seasonal, with growth rates being three times higher in summer than in winter. Mortality was high (up to 200 dead black turtles were found per year) and consumption was the most important cause of death (>90% had been eaten). Due to their slow growth, black turtles spend 15 to 20 years in Bahía Magdalena before reaching maturity, a time during which they are being caught and eaten. We conclude that population recovery is unlikely without a more effective protection of the feeding grounds through law enforcement, patrolling, habitat protection and education.