Movements and distribution of hawksbill turtles in the Eastern Indian Ocean

Abstract

Western Australia (WA) is one of the largest remaining hotspots for hawksbill turtles. Providing data from this region is a total of 42 satellite transmitter deployments on nesting hawksbill turtles at six rookeries (five located in WA and one in Timor-Leste) between 2000 and 2017. We used this data to quantify inter-nesting, migratory and foraging habitat use and overlap with protected areas. Turtles from WA rookeries (n = 40) remained in WA waters during their inter-nesting, migration and foraging phases, whereas those from Timor-Leste (n = 2) migrated to and foraged in WA. While inter-nesting, turtles remained in relatively small areas with an average 50% occupancy core of 39 ± 28 km2. Apart from one rookery, most of their inter-nesting distribution was contained within existing protected areas. Turtles migrating from WA rookeries remained in shallow continental-shelf waters (< 200 m), mainly following the coastline while dispersing in a north-easterly direction, whereas turtles from Timor-Leste crossed international waters. We calculated high overlap of individual turtles (56%) in a migratory corridor along the Pilbara coast, an area previously identified as critical for other migratory megafauna species. Turtles used at least 13 distinct foraging grounds, extending over ~900 km and in waters ranging from 1.5 to 84 m deep. Only 33% of this area was encompassed by either designated Biologically Important Areas for foraging hawksbill turtles (10%), or Australian and State-protected areas (23%). Unusually for this species, 17% of the turtles foraged in relatively deep waters (> 20 m), revealing new foraging habitats in need of further research and management. Our study provides the first quantitative stock-wide distribution maps for WA adult female hawksbill turtles during the different phases of their life cycle. These distributions can be used to directly refine critical areas for this species, particularly for foraging as currently only around a third of the foraging distribution we calculated overlapped with designated protected areas. It appears that Australia-focussed management strategies are appropriate for adult female hawksbill turtles from WA rookeries as they do not leave Australian waters. However, for Timor-Leste adult females, evidence of international migrations, unknown stock affiliation and limited samples sizes show that further investigation is required to understand how domestic and regional management is structured to ensure their future conservation.