Shark scavenging and predation on sea turtles in northeastern Brazil

Large sharks have the potential to help structure ecosystem dynamics through top-down impacts on their prey, including sea turtles. Studies of interactions between large sharks and sea turtles, however, are practically nonexistent along the Brazilian coast. Between September 2002 and May 2011 we examined 655 sea turtles – including green turtles Chelonia mydas (n = 607), olive ridleys Lepidochelys olivacea (n = 10), hawksbills Eretmochelys imbricata (n = 33), and loggerheads Caretta caretta (n = 5) – that stranded on Paraíba coast, northeastern Brazil. A total of 63 green turtles (10.4%), two olive ridleys (20.0%) and one hawksbill (3.0%) had shark-inflicted bites. Most bites could not be definitively attributed to scavenging or attacks on living turtles, but the presence of healed shark bites and freshly bleeding bites suggests that some attacks occurred pre-mortem. Bite characteristics suggest that tiger sharks Galeocerdo cuvier were responsible for most bites that could be identified to a particular species. Within green turtles, the only species with sufficient sample size, the probability of carcasses having been bitten increased with carapace length but did not vary across seasons and years. However, there was spatial variation in the probability of a carcass having been bitten by sharks. Our estimates of the minimum proportion of turtles attacked while alive (∼4%) and bitten overall are similar to other areas where shark-turtle interactions have been studied. Turtles likely are an important food for tiger sharks in northeastern Brazil, but further studies are needed to determine the relative frequencies of scavenging and predation.