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

Ra Groom,Ad Griffiths,M Chaloupka

doi:10.3354/esr00795

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.

Highlights

All marine turtle species are exposed to a range of threatening processes including fisheries bycatch, coastal development, pollution, hunting or egg collection, pathogens and climate change (Mast et al 2005, Wallace et al 2011)
To examine variation in recapture probability, we modeled the effects of 2 environmental factors that have been shown to influence recapture of marine turtles: annual rainfall and inter-annual climatic variability
We used the Horvitz-Thompson type estimator (Chaloupka & Limpus 2001, Bjorndal et al 2005) to estimate annual abundance of nesting flatback turtles at Field Island, in which Ni =, where Ni is the number of turtles in the sampling population, ni is the number of turtles captured in the ith year and pi is the recapture probability in the ith year

Summary

INTRODUCTION

All marine turtle species are exposed to a range of threatening processes including fisheries bycatch, coastal development, pollution, hunting or egg collection, pathogens and climate change (Mast et al 2005, Wallace et al 2011). The flatback turtle Natator depressus is the only turtle species endemic to Australia, with foraging grounds generally distributed throughout the Australian continental shelf, into Indonesian and Papua New Guinean waters (Limpus 2007, Whittock et al 2016). It has an extensive nesting distribution from the Pilbara region of Western Australia, along the Northern Territory and Queensland coast, to central Queensland (Bustard et al 1975, Limpus et al 1983a, Parmenter & Limpus 1995, Limpus 2007). There are numerous approaches to long-term monitoring of marine turtle nesting populations. It is often not possible to implement the robust design due to logistic and economic

MATERIALS AND METHODS

RESULTS

DISCUSSION