Estimating long-term trends in abundance and survival for nesting flatback turtles in Kakadu National Park, Australia

Abstract

Flatback turtles Natator depressus are endemic to Australia and Papua New Guinea's tropical oceans and, although the species has an extensive distribution around northern Australia, there are few published long-term abundance trends of nesting populations. We conducted a longterm capture-mark-recapture program on nesting flatback turtles on Field Island in Kakadu National Park, a World Heritage Area that is jointly managed by Aboriginal landowners and the Australian Government, from 2002 to 2013 for between 12 and 20 monitoring days per year. We used a Cormack-Jolly-Seber (CJS) model that accounted for transience and recapture heterogeneity to estimate apparent survival and recapture probability, and estimated abundance using a Horvitz-Thompson type estimator. A total of 257 flatback turtles attempted nesting during that period, averaging 3.68 ± 0.28 (mean ± SE) nesting attempts per night of monitoring. Annual apparent survival of nesting flatback turtles was 0.97 (95% CI = 0.94 to 0.98) and increased relative to body size. Recapture probability averaged 0.38 (95% CI = 0.34 to 0.42) and was influenced by inter-annual climatic variability. The size of the Field Island nesting flatback turtle population ranged from 97 (95% CI = 87 to 106) to 183 (95% CI = 165 to 200) and there was a non-significant trend over 12 yr of monitoring. Understanding long-term population trends of nesting marine turtles is fundamental for management and recovery of these at-risk species.