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tracks were least likely to be washed away by high tides. Surveys were during March 2006–April 2007 (14 months), April 2011–May 2013 (26 months) and November 2014– March 2018 (41 months). Monthly survey frequencies were 10 months with 3–4 surveys (12.3%), 45 with 2 (55.6%), 21 with 1 (25.9%), and 5 months without surveys (6.2%). Dur- ing each survey all tracks were counted, and their widths measured to confirm the species (hawksbill turtle tracks are typically ,95 cm wide and green turtle tracks .100 cm wide; Pritchard & Mortimer, 1999). To estimate longevity of turtle tracks, fresh tracks of both species were marked and monitored during 27 November–9 December 2018.
Data analysis
FIG. 1 (a) The location of the Chagos Archipelago in the south-west Indian Ocean with the boundary of the British Indian Ocean Territory, (b) bathymetry of the Archipelago in 100 m contours (source: GEBCO Compilation Group, 2018), indicating delineation of atolls separated by deep water, and (c) location of 2.8 km index beach and three small islands on Diego Garcia. Six sections of coastline (AB, BC, CD, DE, EF, FG) were surveyed to assess suitable nesting habitat and relative per cent nesting activity by species (Table 2).
the large bowl-shaped depression a turtle leaves when dig- ging a nest (Mortimer & Day, 1999). Tracks can easily be erased during periods of high tide, but body pits, usually dug above the high tide line, can remain visible for weeks, providing a reliable indicator of spatial habitat use, especially when nesting is sparse. All rapid surveys were conducted outside the peak nesting seasons of both species as defined by the results of the present study. A subsample of the coastline (comprising 34 islands and 80 km) was surveyed in 1996 and again in 2006 (Mortimer, 2007). During the three rapid surveys conducted in 1996, 1999, and 2006, 211.8 km (90% of the Chagos oceanic coast- line) were surveyed at least once, and .80 km repeatedly (Supplementary Table 1). In 2016 NE surveyed 39 islands (105 km) by foot and helicopter, to confirm general patterns of spatial distribution.
Assessment of annual egg clutch production and nesting seasonality Along the south-east coast of Diego Garcia island a 2.8 km long index beach was selected that hosted some of the highest densities of nesting activity identified during the 1999 rapid survey (Mortimer, 2000). Located partially within the Diego Garcia Ramsar Site and adjacent to a paved road, it is easily accessible. Monthly track surveys were conducted by environmental personnel of the US Navy Support Facility and US and UK base personnel volunteers, at 2-week intervals towards the end of neap tides when
Suitability of nesting habitats Using the habitat accessibil- ity data collected during the rapid surveys of 1996, 1999 and 2006, the amount of suitable habitat for nesting turtles was calculated for each island surveyed, and a mean per cent estimate of suitable habitat was calculated for each atoll (Table 1). For atolls where survey coverage was not 100% (Peros Banhos and Egmont) mean figures for each atoll were used to estimate suitable habitat along the 9%(21 km) of oceanic coastline not surveyed.
Spatial distribution of nesting activity amongst atolls Indices of nesting density were calculated for each species by averaging numbers of body pits per km of suitable coast- line surveyed at each of the four outer-island atolls (Peros Banhos, Salomon, Great Chagos Bank, Egmont) during the 1996 and 2006 surveys, which took place on approximately the same dates in both seasons (Table 3). For each atoll, the mean of the 1996 and 2006 indices were then calculated and multiplied by estimated total km of suitable habitat to pro- duce indices of relative levels of nesting activity (Table 1). For Diego Garcia, the 1996 and 2006 surveys alone were not adequate to estimate mean total body pits as only 16 and 4%, respectively, of the 72.1 km coastline were surveyed. In 1999, however, the entire Diego Garcia oceanic coastline was surveyed, and in both 1996 and 2006 the 2.8 km coastline that became theDiego Garcia index beach had been surveyed. Assuming relatively constant spatial distribution of nesting activity from year to year, we used 1999 data to extrapolate total body pits at Diego Garcia atoll in 1996 and 2006 based on body pit counts recorded at the index beach. For both species, using indices of body pit numbers at all five atolls (Table 1), we calculated per cent contribution of each atoll to total Chagos nesting activity (Table 1).
Annual egg clutch production and nesting seasonality Data from monthly track surveys at the index beach were used to estimatemonthly total nesting emergences and egg clutches
Oryx, 2020, 54(3), 332–343 © 2020 Fauna & Flora International doi:10.1017/S0030605319001108
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