338 J. A. Mortimer et al.


FIG. 2 Estimated annual clutch numbers laid by (a) hawksbill turtles Eretmochelys imbricata and (b) green turtles Chelonia mydas at the 2.8 km Diego Garcia index beach (Fig. 1c) during 1995–2017. Closed circles represent estimated clutch numbers derived from monthly track surveys conducted in April–March in six periods (2006–2007, 2011–2012, 2012–2013, 2015–2016, 2016–2017 and 2017–2018). Open circles are derived from midpoints of bracketed estimates for 1995–1996 (Mortimer & Day, 1999). Calculations of least squares regressions showed significant order of magnitude increase in green turtle clutches, as shown by the dashed line in (b).


Seasonality Mean nesting seasonality of Chagos hawksbill turtles indicated 86% occurred in October–February, with a peak of 28% in December. Green turtle nesting was more variable, occurring year-round, with 64% of nesting in June–October, a peak of 19%in August, and 3–7%of annual nesting emergences in other months (Fig. 3).


Relative importance of the Chagos Archipelago in the south- west Indian Ocean region Total estimated mean annual reproductive output in the region, at 20 hawksbill and 26 green turtle sites, are 12,466–16,047 hawksbill and 103,944–143,466 green turtle clutches (Supplementary Table 3). Five sites, in Seychelles (Inner Islands, Amirantes), Madagascar (general), and Chagos (Peros Banhos, Diego Garcia), each produce annually 1,001–5,000 hawksbill clutches. Five sites, in Seychelles (Cosmoledo group, Aldabra group), French islands (Europa, Mayotte), and Comoros, each produce .10,000 green turtle clutches annually (Fig. 4). The Chagos Archipelago has available


FIG. 3 Monthly distribution of track counts along the 2.8 km Diego Garcia index beach (Fig. 1c). Dashed lines indicate individual seasons, and solid lines overall mean figures for all seasons combined. (a) Hawksbill turtle nesting peaked in October–February. (b) Green turtles nest year-round with a peak during June–October.


nesting habitat (132 km) comparable to that of Seychelles (193 km; Supplementary Table 3).


Discussion


Current estimates of 6,300 hawksbill and 20,500 green turtle clutches laid annually in Chagos represent an increase of 225–525% for hawksbill and 465–930% for green turtles since the 1996 surveys, which estimated 1,200–2,800 and 2,200–4,400 clutches, respectively (Mortimer & Day, 1999). The relatively higher increase for green turtles accords with trends documented at the Diego Garcia index beach during 2006–2018. Differing patterns of nesting density between atolls


probably reflect a combination of ecological factors and historical human impact. The Chagos Archipelago was in- habited by 650–1,200 coconut plantation labourers who collected wood and caught fish and turtles, beginning in 1776 at Diego Garcia, 1808 at Egmont, and 1813 at Peros Banhos, Salamon and Great Chagos Bank, and ending in 1935 at Egmont and Great Chagos Bank, and in 1971– 1973 at Diego Garcia, Salamon and Peros Banhos (Wenban-Smith & Carter, 2016; Supplementary Table 2). Current nesting densities may therefore reflect historical


Oryx, 2020, 54(3), 332–343 © 2020 Fauna & Flora International doi:10.1017/S0030605319001108


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