Insights into size, seasonality and biology of a nesting population of the Olive Ridley turtle in northern Australia

The Olive Ridley turtle (Lepidochelys olivacea), classed as endangered in Australia, is one of Australia’s least studied marine turtles and is little known in the south-east Asian region. This is the first detailed study of the nesting biology and ecology of L. olivacea in Australia or south-east Asia, which adds to the regional knowledge of the species and will aid management locally. Daytime surveys of nesting tracks at 14-day intervals in 2004 and irregular surveys in 2005 indicated that the nesting season extended from February to November with peak nesting in April and May. Daily track counts over a 14-day period in April 2004 during peak nesting showed that nesting abundance varied between nights and along the beach. Nightly numbers ranged from 2 to 59 turtles per night over the 10-km beach while, spatially, nesting densities (0.1–6.9 tracks km–1 night–1) varied between sectors. Nesting in this population was solitary, as opposed to the mass nesting behaviour of L. olivacea observed elsewhere in its range, such as in India, Mexico and Costa Rica. The size of nesting L. olivacea was normally distributed with a mean curved carapace length of 69.6 ± 2.3 (s.d.) cm (range = 65.0–75.2, n = 85). During the peak of the nesting season dingoes (Canis lupus dingo) were responsible for the highest egg mortality (over 14%), followed by varanids (Varanus spp., 4.5%) and humans (1.7%). Cyclone Ingrid caused significant egg loss in 2004. Saltwater crocodiles (Crocodylus porosus) were a significant predator of adult nesting turtles.