Estimates of marine turtle nesting populations in the south-west Indian Ocean indicate the importance of the Chagos Archipelago

Jeanne A Mortimer,Graeme C Hays,Antenor Nestor Guzman,Nicole Esteban

doi:10.1017/s0030605319001108

Abstract

AbstractGlobal marine turtle population assessments highlight the importance of the south-west Indian Ocean region, despite data gaps for the Chagos Archipelago. The archipelago hosts nesting hawksbill Eretmochelys imbricata and green turtles Chelonia mydas, both heavily exploited for 2 centuries until protection in 1968–1970. We assessed available nesting habitat and spatial distribution of nesting activity during rapid surveys of 90% of the archipelago's coastline in 1996, 1999, 2006 and 2016. We quantified seasonality and mean annual egg clutch production from monthly track counts during 2006–2018 along a 2.8 km index beach on Diego Garcia island. An estimated 56% (132 km) of coastline provided suitable nesting habitat. Diego Garcia and Peros Banhos atolls accounted for 90.4% of hawksbill and 70.4% of green turtle nesting. Hawksbill turtles showed distinct nesting peaks during October–February, and green turtles nested year-round with elevated activity during June–October. Estimates of 6,300 hawksbill and 20,500 green turtle clutches laid annually during 2011–2018 indicate that nesting on the Chagos Archipelago has increased 2–5 times for hawksbill turtles and 4–9 times for green turtles since 1996. Regional estimates indicate green turtles produce 10 times more egg clutches than hawksbill turtles, and the Chagos Archipelago accounts for 39–51% of an estimated 12,500–16,000 hawksbill and 14–20% of an estimated 104,000–143,500 green turtle clutches laid in the south-west Indian Ocean. The improved status may reflect > 40 years without significant exploitation. Long-term monitoring is needed to captureinterannual variation in nesting numbers and minimize uncertainty in population estimates.

Highlights

To assess species conservation status, key biological questions focus on population status, trends and spatiotemporal variability (Sutherland et al, b), knowledge of extinction risk or species loss (Sutherland et al, a), as well as how disturbances are altering species distribution and abundance (Parsons et al, )
We examined patterns of spatial and seasonal distribution, and abundance of hawksbill and green turtle nesting activity, in the Chagos Archipelago during – and compared our estimates of annual egg clutch production to those reported for sites elsewhere in the region
The rapid survey results show that at Diego Garcia atoll (Fig. c) nesting activity was low along the . km western perimeter, hosting only . % of total hawksbill and . % of total green turtle nesting on the atoll, compared to . % of hawksbill and . % of green turtle nesting along the . km eastern perimeter (Table )

Summary

Introduction

To assess species conservation status, key biological questions focus on population status, trends and spatiotemporal variability (Sutherland et al, b), knowledge of extinction risk or species loss (Sutherland et al, a), as well as how disturbances are altering species distribution and abundance (Parsons et al, ). Population size is usually determined using methods that include direct sampling (e.g. mark–recapture through tagging) and indirect sampling (e.g. track or clutch observations) but such data may be difficult to obtain for species in remote or inaccessible habitats (e.g. transboundary migrating birds, Bishop et al, ; or trans-equatorial migrating basking sharks, Skomal et al, ). Species of seabirds and seals may come ashore and congregate to breed, facilitating collection of extended time series of abundance data (Paleczny et al, ; Collins et al, ; Trillmich et al, ). Marine turtles are another group for which population status is often assessed using annual numbers of nesting females or egg clutch production as indicators (Balazs & Chaloupka, ; SWOT Report, )

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