Ecological drivers of the high predation of sea turtle hatchlings during emergence

Abstract

Synchronized emergence of offspring may represent an adaptive strategy to reduce predation risk. This strategy swamps the short-term capacity of predators to consume prey before offspring disperse, inducing a dilution effect, thus improving an individual’s chance of survival. In the case of sea turtles, this occurs during emergence and mass migration to the sea, to avoid potential predators on the beach. In this study, we evaluated the effect of group size on predation rates of loggerhead turtle Caretta caretta hatchlings during the crawl to the sea on Boa Vista Island, Cabo Verde. Our results show that synchronous emergence reduced rates of predation by tufted ghost crabs Ocypode cursor. The mean estimated predation rate overall was 50.3%. Predation was highest (~75%) in the smallest group sizes and decreased to ~25% in larger groups, due to the lower probability of an individual being attacked by a ghost crab. Our observations also indicate significantly higher predation rates at night (55%) than during the day (22%). No relationship between predation rates and the distance between the nest and the surf zone of the sea was identified; however, this is likely due to the behaviour of ghost crabs, i.e. waiting close to the tide line for hatchlings during the night. Our results provide important information for the management and conservation of endangered sea turtle populations in areas with high densities of predatory ghost crabs. Specifically, to reduce predation rates, we recommend that hatchlings are released in large groups at twilight hours and in areas of low ghost crab densities.