336 J. A. Mortimer et al.


TABLE 2 Distribution of suitable nesting habitat and levels of turtle nesting activity recorded during the 1999 rapid survey of 72.1 km of oceanic coastline, by section, on Diego Garcia (Fig. 1c).


Total Section of coastline1


oceanic coastline (km)


Diego Garcia island AB (north-west coast) 16.1 BC (south-west coast) 13.1 9.8


CD (South Point to Horsburgh Point)


DE (Horsburgh Point to Cust Point)


EF (Cust Point to Barton Point)


FG (Inside lagoon: Barton to Cust Point)


Island total


Index beach (on Diego Garcia, within CD)


Three small islands West, Middle, East


12.8 9.1 8.2


69.1 2.8


3.0


Coastline suitable for nesting,


km (% of total)


% of total nesting activity2


Hawksbill turtle


9.2 (56) 2.4 3.0 (23) 6.1 9.4 (95) 44.5


8.6 (67) 27.0 5.9 (65) 17.7 4.1 (50) 0.9 40.2 (58) 98.6


2.8 (100) 8.6 0.3 (9) 1.3


Green turtle


3.9 3.9


41.2 41.8 5.2 0.0


96.1 8.5


3.9


129.1 km along the outer west coast (AB, BC), 31.8 km along the outer east coast (CD, DE, EF; including the 2.8 km Diego Garcia index beach within CD), 8.2 km inside the east coast of the lagoon (FG), and the coastlines of


the three small islands at the mouth of the lagoon (Fig. 1c). 2Nesting activity recorded in each section of coastline (based on numbers of body pits) are presented as per cent of total nesting activity on Diego Garcia.


laid and describe seasonality of nesting activity. For each species mean monthly track counts were calculated. For five unsurveyed monthswe averaged data fromthemonths before and after. Estimated total numbers of nesting emergences (i.e. turtle tracks) were extrapolated for each month using estimates of track longevity calculated in November–De- cember 2018: for hawksbill turtles 2.8 days (mean 2.79 ± SE 0.2621, range = 1–7,n = 45)and forgreen turtles 3.9 days (mean 3.94 ± SE 0.4950, range = 1–9,n = 23). To calculate numbers of egg clutches laid each month at the index beach, we then assumed that 55% of total turtle tracks resul- ted in egg laying, based on data from similar habitats in the Seychelles (Mortimer & Bresson, 1999; Mortimer et al., 2011a). Estimated egg clutches laid annually by hawksbill and green turtles at the index beach were then calculated for each of the following six 12-month (April–March) peri- ods: 2006–2007, 2011–2012, 2012–2013, 2015–2016, 2016– 2017, 2017–2018. These were graphed over time along with an estimate of egg clutches laid annually at the index beach in 1995–1996 based on the mid points of the bracketed estimates derived in 1996 by Mortimer & Day (1999) and an assumption of consistent spatial distribution of nest- ing activity over time. Least squares regressions were calcu- lated to provide indications of population trends during


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


TABLE 3 Dates and coverage of the four rapid surveys, conducted in 1996, 1999, 2006 and 2016. Year


Dates


1996 1999 2006 2016


10 February–18 March 27 January–21 February 4 February–23 March 21 March–14 April


Survey coverage No. of atolls 5


1 5 4


No. of islands 49


4


45 39


Coastline length (km) 109.2


72.1


121.7 105.0


% of coastline surveyed Four outer-island atolls 60


0


73 64


Diego Garcia atoll 16


100 4 0


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