A review of lethal and non-lethal effects of predators on adult marine turtles

Although predation is recognized as an important factor for sea turtles at early life history stages, the possible influences of predators on adult sea turtles have largely been overlooked. Indeed, because predation rates on adult turtles generally are low, it often is assumed that predators do not influence turtle behavior, affect population sizes, or structure sea turtle impacts on marine communities. Yet, sharks, killer whales and crocodiles may take adult turtles, and no sea turtle species is invulnerable. Furthermore, as long-lived animals, adult sea turtles are predicted to invest in anti-predator behavior even when predation risk is relatively low to avoid the reproductive costs of an early death. Several studies suggest that predators may influence behavior and habitat use of adult turtles. For example, in a pristine ecosystem in Australia green turtles, especially those in good body condition, avoid profitable feeding areas in order to be safe from tiger sharks. Such predation-sensitive foraging could lead to populations being partially regulated by synergistic effects of predation risk and food abundance. Indeed, observed rapid rebounds of sea turtle populations may have been facilitated by reductions in shark populations. Finally, if sea turtles modify their foraging habitat use in response to predators, the spatiotemporal pattern of turtle effects on their communities could be shaped by predators. Historical reconstructions of past turtle populations and conservation strategies aimed at increasing turtle populations must consider the possible effects of predators, including non-lethal ones. Future work should focus on gaining a broader understanding of the effects of predators across a range of sea turtle taxa and habitats.