In the northwestern Atlantic Ocean, tiger sharks are fierce ambush predators with serrated teeth and darkly-hued backs that help them to stay incognito as they hunt surfacing sea turtles from below. Researchers studying this predator-prey relationship were surprised to find that the turtles don’t seem to alter their surfacing behavior to avoid shark attacks when the risk is high. The sharks, however, do modify their behavior: They know when to move toward turtle-abundant shores. The findings were published in Ecology.
Predators are really good at creating what are called "landscapes of fear." They force prey animals to weigh the need to eat against the need to avoid being eaten. Tasmanian devils, for example, keep brushtail possums from spending too much time looking for fruits on the ground. According to this model, as an animal's landscape changes from low to high risk of predation, prey species will alter their behavior. But does this hold for interactions in vast, open marine systems with wide-ranging, highly mobile species?
To investigate, a team led by University of Miami’s Neil Hammerschlag bolted satellite tags to the dorsal fins of large tiger sharks (Galeocerdo cuvier) and glued the tags to the shells of adult female loggerhead sea turtles (Caretta caretta) after they finished laying eggs on the beach. During winter months in the Atlantic, there’s little overlap between the two species. The cold water lowers the metabolism of turtles so that they stay closer to the sea floor where they’re less vulnerable; meanwhile, sharks surface more to take advantage of warmer, shallower water. But in the summertime, it’s flipped: Turtles increase their surfacing activity to nest and forage, while stealthy sharks lurk below. In addition to their darkly-pigmented backs and shell-breaking teeth, tiger sharks also have specialized eyes for easy detection of turtles resting at the surface.
Using satellite tracking to study the movement patterns of the two species relative to each other would allow the team to see if turtles modify their behaviors accordingly. Based on the "landscape of fear" model, turtles should be reducing their exposure in times of increased habitat overlap with sharks. But as it turns out, the sea turtles didn’t alter their surfacing behaviors when shark–turtle overlap was high in the summer. The tiger sharks, however, modified their behaviors, moving into areas where the turtles are the most abundant: nesting sites off the Carolinas. The sharks also reduced their surfacing to better ambush attack turtles.
While turtles influence shark distribution, other life history trade-offs seem to play a bigger role in turtle habitat use. "Sharks may not be an important factor influencing the movements of turtles in the study region," Hammerschlag says in a statement. "In addition to the unpredictability of a shark attack over such a large area, it is possible that fishing of tiger sharks has reduced their populations to levels that no longer pose a significant threat to turtles, with other factors becoming more important, such as the need to avoid boat strikes."