TABLE 1 Physical features of each of the five islanded atolls of the Chagos Archipelago (Fig. 1, Supplementary Fig. 1), with numbers of islands, total oceanic coastline and oceanic coastline suitable for nesting. For hawksbill and green turtles, indices of nesting activity were calculated from body pit counts conducted during rapid surveys of all five atolls in 1996 and 2006 and at Diego Garcia in 1999. These indices were used to calculate per cent contribution of each atoll to total Chagos nesting activity. Monthly track surveys on the index beach at Diego Garcia facilitated an estimate of mean annual egg clutches produced at that atoll, and from that to estimate mean annual clutch production at all five atolls.


Hawksbill turtles Atoll


Peros Banhos Salamon


No. of islands


36 11


Great Chagos Bank 8 Egmont


Total outer islands 63 Diego Garcia Total Chagos


8 4


67


Total oceanic coastline (km)


80.7 26.3 32.9 22.8


162.7 72.1


234.8


Coastline suitable for nesting, km (% of total)


Index of nesting density1


Relative levels of nesting activity


1996 2006 Mean


41.2 (51) 9.0 13.9 (53) 1.2 18.8 (57) 1.0 17.3 (76) 1.3 91.2 (56)


Estimated mean total body pits


5.9 7.5 307 1.5 1.4 19 0.6 0.8 15 3.0 2.2 37


40.5 (56) 6.6 11.2 8.9 360 131.7 (56)


738


% of total for Chagos (95% CI)2


Estimated annual mean egg clutches4


41.6 (38.0–45.2) 2,627 2.5 (1.4–3.7) 2.0 (1.0–3.1) 5.0 (3.4–6.6)


100


158 126 316


48.8 (45.1–52.5) 3,0813 6,308


Green turtles


Index of nesting density1


Relative levels of nesting activity


1996 2006 Mean


Estimated mean total body pits


5.8 12.8 9.3 383 0.1


4.5 12.4 8.5 159 6.9


6.7


4.7 2.4 33 4.8 5.9 101 8.7 7.7 312


988


% of total for Chagos (95% CI)2


Estimated annual mean egg clutches4


38.8 (35.7–41.9) 7,941 3.4 (2.2–4.5)


16.1 (13.8–18.4) 3,295 10.2 (8.3–12.2) 2,088


100


31.6 (28.6–34.5) 6,4673 20,487


1Mean numbers of body pits counted per km of suitable coastline surveyed in 1996 and 2006. 2Percentages of total egg clutch production contributed per atoll, calculated by averaging the 1996 and 2006 indices for each atoll and using 1999 survey data to extrapolate to total suitable coastline. 3Mean estimated numbers of egg clutches produced annually at Diego Garcia atoll were calculated from monthly track surveys of the Diego Garcia index beach conducted during the 2011–2012 to 2017–2018 nesting


seasons. 4Estimated egg clutch production by each species at each of the four outer-island atolls were extrapolated based on calculated spatial distribution of nesting activity.


696


Marine turtle nesting populations 335


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


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