Marine turtle nesting populations 333
based on global population declines of 37–61% over the pre- vious three turtle generations (Seminoff, 2004), and hawksbill turtles Eretmochelys imbricata as Critically Endangered based on a decline of .80% using the same criteria (Mortimer & Donnelly, 2008). Fortunately, marine turtle populations respond well to extended periods of protection at nesting beaches, and population recovery has been documented at multiple sites for both hawksbill and green turtles (Mazaris et al., 2017) and several subpopulations of green turtles have been downlisted by the IUCN Species Survival Commission Marine Turtle Specialist Group. On a global scale, the south-west Indian Ocean, which includes the Chagos Archipelago (hereafter occasionally referred to as Chagos), hosts some of the most important national popu- lations of hawksbill (Mortimer & Donnelly, 2008) and green turtles (Seminoff, 2004). Genetic studies of both nesting and foraging hawksbill turtles (Mortimer & Broderick, 1999; Vargas et al., 2016) and nesting green turtles (Bourjea et al., 2015) demonstrate linkages between Chagos and else- where in the south-west Indian Ocean, especially Seychelles. After almost 2 centuries of permanent human settlement
and associated exploitation and trade in green turtle meat, hawksbill shell, oil and eggs (Mortimer, 2009; Wenban- Smith & Carter, 2016), the Chagos islands have been unin- habited since 1973 (except Diego Garcia, site of a joint UK/ USA military base). In 2010 one of the largest (640,000km2) permanent no-take marine protected areas was created within the British Indian Ocean Territory (Koldewey et al., 2010). This presented an opportunity to track the status of remnant marine turtle populations that are no longer ex- ploited. In 1970 J. Frazier visited a few of the islands, in- terviewed inhabitants, and concluded only a few hundred hawksbill and green turtles remained (,1,000 clutches laid annually by each species; Frazier, 1975). In 1996 the first systematic, territory-wide snapshot survey of turtle nest- ing in Chagos was conducted (49 islands over 6 weeks; Mortimer & Day, 1999), estimating 300–700 nesting hawksbill (1,200–2,800 clutches) and 400–800 green turtles (2,200–4,400 clutches) annually by using seasonality data from Seychelles (Mortimer, 1988;Mortimer&Bresson, 1999) to extrapolate from the 1996 rapid-survey data. Long- term monitoring was recommended, to define critical habi- tats, nesting seasonality and long-term population trends. Recently updated estimates of turtle nesting activity
in the region were summarized in SWOT Report (2017) but did not include information about the Chagos Archipelago. Our study aims to remedy this dearth of knowledge relative to elsewhere in the south-west Indian Ocean region.Weexamined patterns of spatial and season- al distribution, and abundance of hawksbill and green turtle nesting activity, in the Chagos Archipelago during 1996–2018 and compared our estimates of annual egg clutch production to those reported for sites elsewhere in the region.
Study area
The Chagos Archipelago comprises c. 67 islands and 235 km of oceanic coastline distributed across five atolls (Fig. 1, Supplementary Fig. 1;Mortimer&Day, 1999). These include four groups of outer islands (Table 1): Peros Banhos atoll (36 islands, 80.7 km of coastline), Salomon atoll (11 islands, 26.3 km), Great Chagos Bank (8 islands, 32.9 km), and Egmont atoll (5–8 dynamic sand cay islands, c. 22.8 km). The main atoll, Diego Garcia, comprises 4 islands with 72.1 km of coastline (96% on Diego Garcia island). The spatial boundaries of the south-west Indian Ocean
regional management units for both green and hawksbill turtles have been defined by the IUCN Marine Turtle Spe- cialist Group based on genetic linkages and documented migratory patterns. They include territorial waters of main- land countries from southern Somalia to South Africa plus the islands of Comoros, Madagascar, Mauritius, Mayotte, Réunion and its scattered islands, Seychelles, and the Chagos Archipelago (Wallace et al., 2010).
Methods
Data collection Habitat assessment JAM conducted rapid surveys of the five islanded atolls in 1996, 1999 and 2006, and scored all stretches of surveyed coastline in terms of suitability for nesting based on accessibility of adequate beach sand platform to turtles. Four features of the shoreline were each rated on a scale of 1–4, with 1–2 considered accessible, and 3–4 inaccessible: offshore approach, foreshore, high tide line (erosion cliff and associated barriers), and beach plat- form (see Supplementary Material 1 for detailed criteria). Where any of the four features were scored as 3 or 4, turtles were unlikely to either successfully emerge onto the beach or to lay eggs. The lengths of suitable (i.e. accessible) oceanic coastline were calculated for each island surveyed, including Diego Garcia (Tables 1 & 2, Fig. 1).
Spatial distribution of nesting activity amongst atolls Four sets of rapid surveys were conducted: by JAM in 1996, 1999 and 2006, and by NE in 2016. To calculate spatial distri- bution, JAM conducted rapid surveys in February and March of both 1996 and 2006 across the entire archipela- go (Mortimer & Day, 1999; Mortimer, 2007), and in late January and February of 1999 along the entire oceanic coastline of Diego Garcia atoll (Mortimer, 2000; Table 3, Supplementary Table 1). Data were collected by walking along the perimeter of each island high on the beach plat- form and recording the locations and characteristics of all turtle tracks and body pits. A track is defined as the imprint a turtle makes when crawling on the sand, and a body pit is
Oryx, 2020, 54(3), 332–343 © 2020 Fauna & Flora International doi:10.1017/S0030605319001108
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