Marine turtle nesting populations 337


TABLE 4 Sources of data used to estimate the mean annual number of egg clutches produced by hawksbill and green turtles at each site in the south-west Indian Ocean (Fig. 4, Supplementary Table 3).


Country or Territory


Chagos


Archipelago Seychelles


Source(s) This study


Mortimer, 1998; Mortimer, 2004; Allen et al., 2010; Mortimer et al., 2011a, 2011b; Burt et al., 2015; Mortimer, 2017; Alphonse Foundation, Bird Island Lodge, Constance Lémuria Resort, Denis Island Private, Desroches Foundation, Farquhar Foundation, Fregate Island Private, Global Vision International Seychelles, Green Island Foundation, Island Conservation Society, J.A. Mortimer, Marine Conservation Society Seychelles, North Island, Seychelles Ministry of Environment Energy & Climate Change, Seychelles Islands Foundation, Seychelles National Parks Authority, Silhouette Foundation, & WiseOceans, unpubl. data


Mauritius


French Territories


Chapman & Swinnerton, 1996; Mangar & Chapman, 1996


Legall et al., 1986, 1988; Ciccione & Bourjea, 2006, 2010; Bourjea et al., 2007; Lauret-Stepler et al., 2007, 2010; Dalleau et al., 2012, 2015; Jean et al., 2017, cited in SWOT Report, 2017; Quillard & Ballorain, 2017, cited in SWOT Report, 2017


Madagascar


Comoros Somalia Kenya


Tanzania Mozambique


Rakotonirina&Cooke, 1994; Mortimer, 2002; Mortimer & Donnelly, 2008; Bourjea et al., 2006; UNEP-WCMC, 2010, cited in SWOT Report, 2017


Frazier, 1985; Bourjea et al., 2015


Mortimer, 2002; Mortimer & Donnelly, 2008 Okemwa et al., 2004; Olendo et al., 2019


Dunbar, 2011; Joynson Hicks & West, 2017, cited in SWOT Report, 2017


Garnier et al., 2012; Fernandes et al., 2016


1995–1996 to 2017–2018. As a further indication of popula- tion trends, we compared body pit counts made at the same time of year in both 1996 and 2006 along an 80 km section of coastline. We averaged annual egg clutch production recorded at the index beach during five recent seasons between 2011–2012 and 2017–2018 and extrapolated that mean to the entire Diego Garcia atoll using 1999 data indi- cating that 8.6% of hawksbill and 8.5% of green turtle nest- ing activity occurred at the index beach (Table 2). Estimated egg clutch production at the four outer atolls was then extra- polated from calculated spatial distribution of body pits (Table 1). To define seasonality and illustrate intra-annual patterns, track counts calculated for each month at the index beach were graphed for each of the six 12-month (April–March) periods along with a graph of the overall monthly means for all six seasons combined. We assessed


Annual egg clutch production and population trends We estimated mean annual egg clutch production in Chagos for the period 2011–2012 to 2017–2018 as 6,308 for hawksbill and 20,487 for green turtles (Table 1). Estimated nesting ac- tivity at the index beach between 1995–1996 and 2017–2018 showed marked interannual variation but also changes over time (Fig. 2). For green turtles there was amarked, and high- ly significant, increase in numbers of clutches, with an order of magnitude increase in numbers of clutches per season over the 23 year time series. This increase was best described by a logarithmic function (F1,5 = 37.2,P,0.01, r2 = 0.88). Hawksbill turtles showed a non-significant increase in numbers of clutches over the time series (for a linear trend F1,5 = 0.70, r2 = 0.12,P.0.05). Comparison of body pit counts along the 80 km of coastline surveyed rapidly in both 1996 and 2006 suggested an increase of 23% for hawksbill (1996, 91 pits; 2006, 112 pits) and 147% for green turtles (1996, 167 pits; 2006, 413 pits; Mortimer, 2007).


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


the relative contribution of Chagos egg clutch production to the south-west Indian Ocean region based on informa- tion available in the literature, including SWOT Report (2017). We produced maps of the region indicating estima- ted annual egg clutch production at each study site included in our review. Exclusive Economic Zone boundary, country border and coastline data (Claus et al., 2018) were projected with ArcMap 10.5.1 (Esri, Redlands, USA). Data sources for each country (or territory) are presented in Table 4 and es- timated clutch numbers for individual sites within each country (or territory) in Supplementary Table 3.


Results


Available habitat and spatial distribution of nesting activ- ity Suitable turtle nesting habitat occurred along 132 km (58%) of 235 km of oceanic coastline, with 51–76% per atoll (Table 1). For hawksbill turtles per cent of total nesting activity by atoll ranged from 2% on Great Chagos Bank to 48.8% on Diego Garcia, and for green turtles from 3.4% on Salamon to 38.8% on Peros Banhos (Table 1). Per cent total hawksbill and green turtle nesting activity relative to available habitat at each atoll (Table 1) indicated Diego Garcia and Peros Banhos were the most important atolls in all respects. The 1999 rapid survey results show that at Diego Garcia atoll (Fig. 1c) nesting activity was low along the 29.1 km western perimeter, hosting only 8.5% of total hawksbill and 7.8% of total green turtle nesting on the atoll, compared to 89.2% of hawksbill and 88.2%of green turtle nesting along the 31.7 km eastern perimeter (Table 2). No significant nesting was recorded inside the Diego Garcia lagoon, and little, 1.3% of hawksbill and 3.9% of green turtle nesting, at the three small islands.


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