A study was undertaken on the temperature profiles of sea turtle nesting beaches in Western Australia.
Four species of sea turtles - Loggerhead turtle Caretta caretta, Green turtle Chelonia mydas, Hawksbill turtle Eretmochelys imbricata, and Flatback turtle Natator depressus - have nesting populations of global significance in Western Australia. So far sea turtle research in Western Australia has focused mainly on the tagging of nesting females to estimate the size of nesting populations and to generate data on migration and on monitoring individual nests. Virtually nothing is known on the demography of sea turtle populations, including sex ratios of adults, juveniles and hatchlings.
Sea turtles have temperature dependent sex determination, with warmer egg incubation temperatures producing females and cooler temperatures males. Sex ratios of nests are influenced by climatic conditions of nesting beaches and may show substantial seasonal variations. Sea turtles are known to nest along the Western Australian mainland coast and on many islands from the Shark Bay to
the Kimberley. Several species have extended nesting periods (peak nesting along the west coast generally occurs between October and March) and nest along a gradient of latitudes. This suggests that nest temperatures and hatchling sex ratios of sea turtles in Western Australia may vary spatially and temporally.
We measure sand temperatures at nest depth (standardised at 50 cm depth) at most major sea turtle nesting beaches on the Western Australian mainland from Carnarvon to the Dampier Peninsula and on the Dirk Hartog, Muiron, Barrow, Rosemary, Lowendal, Montebello, and Lacepede Islands for up to 2 1/2 years. Thermochron iButtons log temperatures in four hour intervals. Depending on beach profiles and the area used by sea turtles for nesting we deploy one logger at 50 cm depth just above the high water line where the lowest nests are found and a second 59 cm deep logger plus one at 10 cm depth near the highest nests at the dune crests. Data collection started in September 2004 and will be completed in 2007.
Southern beaches are generally cooler than beaches further north. Sand temperatures increase during the start of the peak nesting season and highest sand temperatures (> 30°C) are generally measured in February and March. Among year variability includes the passage of cyclones. No major cyclone passed through the study area in 2004/05, but several in 2005/06 and 2006/07. During and after cyclones sand temperatures drop down significantly.
This study will provide an overview of temperature variations at sea turtle nesting beaches and will only allow rough estimates of sex ratio variations according to published male and female producing temperatures for the different species. The data will indicate potential geographic and seasonal differences in the production of male and female hatchlings at nesting beaches and provide a baseline for more targeted research into hatchling sex ratios at particular beaches.
