Biology of the East Pacific green turtle, Chelonia mydas agassizii, at a warm temperature feeding area in the Gulf of California, Mexico

Abstract

I studied the East Pacific green turtle, Chelonia mydas agassizii , at the Bahia de los Angeles foraging grounds of the Gulf of California from July 1995 to October 1999. Entanglement nets were used to capture 153 green turtles among 16 capture sites. Pooled straight carapace length (SCL; mean ± SE) was 75.1 ± 0.79 cm. Though there was no significant variation in mean SCL among years, there was evidence of a difference in mean SCL among capture sites. Most of the population (58%) consisted of immature turtles but did not differ significantly from a 1:1 immature:adult ratio. Pooled adult sex ratio (F:M) was biased toward probable females (2.70:1.00) and differed substantially from a 1:1 ratio. Approximately 5% of the population showed signs of anthropogenic-derived injuries ranging from missing flippers to boat impact scars. I collected gastric lavage samples from 101 green turtles and fecal samples from a subset of 45 turtles. Digestive tract contents were analyzed from seven stranded carcasses. The primary alga recovered in diet samples was Gracilariopsis lemaneiformis. Other major diet items included the green algae Codium sp. and Ova lactuca. Animal matter was found in all sample types. Tube worms (Sabellidae), sponges (Porifera), sea hares (Aplysia vaccaria), and sea pens ( Ptilosarcus undulatus) were the most commonly ingested invertebrates. I studied home range sizes and movements of 23 C. m. agassizii . Mean minimum convex polygon home range area was 1,801 ha. Kernel density estimates (95%) of home range area had a mean of 1,545 ha. Home range length averaged 7,548 m. Mean 24-h vagility was 9,535 m. Diurnal and nocturnal movements were evident. Turtles exhibited a bimodal pattern in which they foraged in nearshore algae pastures during evening and night hours and moved to offshore and/or insular resting sites by midmorning. This study indicates that Bahia de los Angeles is an important foraging and developmental habitat for C. m. agassizii. Management efforts should focus on reducing human disturbance and the use of entanglement nets in the entire bay. In such efforts it is essential that local inhabitants are provided with alternatives that encourage sustainable marine resource use and economic progress.