Estimates of marine turtle nesting populations in the south-west Indian Ocean indicate the importance of the Chagos Archipelago
JEANNE A. MORTIMER,NIC OL E E STEB AN ANTE N OR NESTOR GUZMAN and GRAEME C. HAY S
Abstract Global marine turtle population assessments high- light the importance of the south-west Indian Ocean region, despite data gaps for the Chagos Archipelago. The archi- pelago hosts nesting hawksbill Eretmochelys imbricata and green turtles Chelonia mydas, both heavily exploited for 2 centuries until protection in 1968–1970. We assessed avail- able 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 sea- sonality 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 annu- ally 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 esti- mates 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 hawks- bill 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 capture- interannual variation in nesting numbers and minimize uncertainty in population estimates.
Keywords Chagos Archipelago, conservation policy, Diego Garcia, global assessments, IUCN Red List, nesting season- ality, regional management unit, threatened species
Supplementary material for this article is available at
doi.org/10.1017/S0030605319001108
Introduction
temporal variability (Sutherland et al., 2013b), knowledge of extinction risk or species loss (Sutherland et al., 2013a), as well as how disturbances are altering species distribution and abundance (Parsons et al., 2014). To provide an overarching view of the conservation status of species, the IUCN Red List of Threatened Species often relies on specialist groups to provide systematically col- lated metrics on population sizes and trends in abundance over time (Barnes et al., 2015). Aspects of the ecology and habitat of a species can make collecting such data difficult (González-Suárez et al., 2012). Population size is usually determined using methods that
T JEANNE A. MORTIMER* (Corresponding author,
orcid.org/0000-0001-6318-
2890) Department of Biology, University of Florida, Gainesville, Florida 32611, USA, and Turtle Action Group of Seychelles, Victoria, Mahé, Seychelles E-mail
jeanne.a.mortimer@
gmail.com
NICOLE ESTEBAN* Department of Biosciences, Swansea University, Swansea, UK
ANTENOR NESTOR GUZMAN Naval Facilities Engineering Command Far East Public Works Department, Environmental Office, Diego Garcia, British Indian Ocean Territory
GRAEME C. HAYS Deakin University, Geelong, Australia *Contributed equally
Received 2 April 2019. Revision requested 16 July 2019. Accepted 30 August 2019. First published online 10 February 2020.
include direct sampling (e.g. mark–recapture through tag- ging) and indirect sampling (e.g. track or clutch observa- tions) but such data may be difficult to obtain for species in remote or inaccessible habitats (e.g. transboundary migrating birds, Bishop et al., 2015; or trans-equatorial migrating basking sharks, Skomal et al., 2009). Direct observation is difficult for marine species that are sub- merged most of the time, range widely or occur at low densities. For some groups, however, aspects of life history provide windows of opportunity to assess their status. For example, species of seabirds and seals may come ashore and congregate to breed, facilitating collection of ex- tended time series of abundance data (Paleczny et al., 2015; Collins et al., 2016; Trillmich et al., 2016). 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, 2004;SWOTReport, 2017). In this manner, global and regional populations of all
marine turtle species have been assessed since 1996. IUCN categorizes green turtles Chelonia mydas as Endangered
Oryx, 2020, 54(3), 332–343 © 2020 Fauna & Flora International doi:10.1017/S0030605319001108
o assess species conservation status, key biological questions focus on population status, trends and spatio-
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