Researchers have discovered tiger sharks are a little “lazy” when it comes to their food sources with an international team finding evidence they prefer to opportunistically scavenge on dead or weakened green turtles rather than actively hunting healthy individuals despite more opportunities to do so.

The study, conducted on Australia’s Great Barrier Reef, also suggests healthy green turtles don’t perceive tiger sharks as a major threat during their nesting season.

The study satellite tagged tiger sharks and green turtles in the waters around Raine Island, located 620 km north-north-west of Cairns, during a five-year period. The location is opportune as in a peak nesting season up to 100,000 green sea turtles aggregate around Raine Island to lay eggs. This mass aggregation of turtles offers scientists a natural laboratory to compare the movements and behaviours of the turtles and tiger sharks.

“After long nesting periods onshore, many green turtles become weakened from exhaustion. In years when nesting numbers are high, at the extreme, up to as many as 80 individuals a night may die, and some of these remains eventually get washed into the water during high tide,” said study co-investigator Adam Barnett, a researcher from Biopixel & James Cook University.

The study found surfacing of both tiger sharks and green turtles was highest where they overlapped in core home range, closest to the island. Surfacing also increased for both animals with increasing proximity to the shoreline. In other studies, where green sea turtles and tiger shark home range overlap have been analyzed, scientists have observed turtles avoid isolating themselves at the water surface when they are vulnerable to ambush from tiger sharks. Likewise, when tiger sharks are actively hunting turtles, they stalk their prey from deep below to launch a stealthy attack.

“From analyzing the behavioral data from tracked tiger sharks and green turtles, it appears tiger sharks are patrolling the shores of Raine Island for opportunities to scavenge on the few dead turtles washed into the water or the weakened individuals making their way in the water, instead of actively hunting the thousands of healthy green turtles that they are encountering daily during the turtle nesting season,” said the study’s lead author Neil Hammerschlag, a research assistant professor at the UM Rosenstiel School and UM Abess Center for Ecosystem Science and Policy.

“The sharks are probably having to go out of their way to avoid hundreds of live turtles to find the dead and weakened ones. It is energetically more advantageous and also safer for sharks to scavenge on carcasses when the opportunity arises rather than have to chase down live turtles.”

“Raine Island is the most ecologically significant island on the Great Barrier Reef. It is the largest breeding site in the world for green turtles and is home to the most important seabird rookery in the Great Barrier Reef World Heritage area,” said study co-investigator Richard Fitzpatrick, a researcher at Biopixel & James Cook University.

“To date much of the research has been done on the turtles and birds. Our research is increasing the understanding of the apex predators that frequent the Raine Island region, and in turn enhancing our understanding of the whole Raine Island ecosystem.”

The study, titled “Behavioral evidence suggests facultative scavenging by a marine apex predator during a food pulse” was published on 8 August 2016 in the journal Behavioral Ecology and Sociobiology. DOI 10.1007/s00265-016-2183-2. The study’s authors include: Hammerschlag and Austin J. Gallagher from the UM Rosenstiel School and Abess Center; Ian Bell from the Queensland Department of Environment and Heritage Protection in Australia; Richard Fitzpatrick and Adam Barnett from Biopixel & James Cook University; Lucy A. Hawkes and Matthew J. Witt from the University of Exeter; Mark G. Meekan and Michele Thums from the Australian Institute of Marine Science; and John D. Stevens from CSIRO Marine and Atmospheric Research.

A new major project studying this globally significant ecosystem is now underway, with BHP Billiton, the Queensland Government, the Great Barrier Reef Marine Park Authority, Wuthathi and Kemer Kemer Meriam Nation (Ugar, Mer, Erub) Traditional Owners and the Great Barrier Reef Foundation collaborating to fund and deliver the five-year Raine Island Recovery Project to protect and restore the island’s critical habitat to ensure the future of key marine species including green turtles, seabirds and apex predators.