Sea turtles of the Cocos (Keeling) Islands, Indian Ocean

The Cocos (Keeling) Islands support high density resident green and hawksbill turtles and low to moderate density nesting green turtles. Dedicated studies were conducted on resident foraging turtles of the southern atoll between 1999 and 2012 and opportunistic observations were conducted on nesting turtles on both atolls between 1999 and 2009. In-water capture surveys resulted in a species composition of 51% green and 49% hawksbill turtles while counts during boat-based strip transect surveys resulted in a composition of 93% and 7% respectively. Captured green turtles in the foraging grounds had a bimodal distribution with modal size classes at 45–50 and 105–110 cm curved carapace length (ccl) (mean size = 64.7 cm, sd = 20.0, range = 33.5–115.6 cm, n = 984) while hawksbill turtles had a modal size class of 50–60 cm ccl (mean size= 57.6 cm, sd = 13, range = 24.8–86.7 cm, n = 950). New recruits to the foraging grounds were observed annually. Green turtle diet was dominated by seagrass and algae while hawksbill turtle diet was dominated by algae and sponge. Blood chemistry values of both species captured on the foraging grounds were within the published reference ranges. Opportunistic beach surveys conducted on five islands between 1999 and 2012 revealed low density nesting by green turtles (highest: 10.2 tracks km –1 night–1) with no other species recorded. Nesting success was low because of dry sand and natural and anthropogenic debris on the beaches. The mean size of nesting turtles was 107.2 cm ccl (sd = 3.7, range = 96.6–115.9 cm, n = 16). Sand temperatures at nest depth (50 cm) ranged between 25.0 and 29.1°C between January and April.