Tidal fronts and complex, submerged topography define movements of flatback sea turtles (Natator depressus)

Sea turtles migrate between nesting beaches and foraging grounds, but the cues that these animals use to direct these migrations are not well known. Here, we used satellite telemetry to follow the movements of 11 flatback sea... [ view full abstract ]

Sea turtles migrate between nesting beaches and foraging grounds, but the cues that these animals use to direct these migrations are not well known. Here, we used satellite telemetry to follow the movements of 11 flatback sea turtles (Natator depressus) after nesting in the waters off the coast and continental shelf of the Kimberley region of northern Australia. State-space models were used to objectively define nesting, migration and foraging behaviour during the 327 ± 315 days that the turtles were tracked. These animals migrated along the coast in water depths averaging 64 ± 5 m to foraging grounds on the raised carbonate bank and terrace systems of the mid-Sahul Shelf in the Timor Sea in average water depths of 74 ± 12 m. The turtles spent >75% of the time they were tracked in these foraging grounds, which were 135 ± 35 km from shore. During migrations, the animals appeared to follow tidal fronts along the Kimberley Coast, which are boundaries between the turbid and well-mixed inshore waters and the clearer, stratified shelf waters. Once on the foraging grounds they focussed their movement at the outside boundary of the tidal fronts, in stratified waters. Oceanic frontal zones of all types are associated with high primary productivity and as a result are the focus of feeding for many species of higher trophic levels. We suggest that the combination of high productivity in the vicinity of tidal fronts and complex topography of the mid Sahul Shelf provide a productive foraging ground for flatback sea turtles and other turtle species within the region. Our study identified both critical habitats for this species and a possible bio-physical cue that these animals may use to navigate while migrating between nesting and foraging grounds. This information is essential to aid spatial planning of conservation for this data-deficient species that is endemic to northern Australia.