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time series documenting marine turtle abundance remain non-significant because they are too short. Our estimates of nesting numbers could be further improved. For example, we assumed that 55%of total turtle tracks resulted in egg lay- ing based on data from Seychelles, but this assumed value, central to our calculations, could be improved by collecting data on rates of egg-laying success across the Chagos Archipelago. Some of the other values we used in our esti- mates are probably more robust. For example, our 95% confidence interval of 0.286–0.345 for the proportion of total observed Chagos green turtle body pits occurring on Diego Garcia enables us to conclude confidently that Diego Garcia hosts a large proportion of all green turtle clutches across the archipelago. Future status assessments may be improved by new methodologies, such as ongoing research on the archipelago deploying cameras to photo- graph daily tracks on beaches. Any inaccuracies our sam- pling imposed on estimates of nesting activity would probably add noise and so dampen any real trends, rather than generate spurious trends. Hence, we are confident that our calculated increase in numbers of green turtle clutches since 1996 is real, a pattern mirrored in other rookeries elsewhere (e.g. Mazaris et al., 2017). Another threat to turtle rookeries is the presence of
non-native rats Rattus spp., which prey on hatchlings (Caut et al., 2008) and embryos (JAM, unpubl. data from Diego Garcia) and disrupt both terrestrial and marine ecosystems (Graham et al., 2018). Ongoing and planned rat eradication in the Chagos Archipelago (Hilton & Cuthbert, 2010)will most likely accelerate turtle recovery. Our study is the first to document nesting seasonality
at Chagos, following earlier snapshot surveys (Frazier, 1975; Mortimer & Day, 1999; Mortimer, 2000, 2007). The October–February nesting peak exhibited by hawksbill turtles on Diego Garcia corresponds to that in Seychelles, where 94 and 98% of annual nesting occurs during October–February in the inner islands (Mortimer & Bresson, 1999) and Amirantes (Mortimer et al., 2011a), respectively, coincident with high north-west monsoon precipitation (Mortimer & Bresson, 1999). Green turtles typically nest year-round throughout the region (Dalleau et al., 2012), with patterns of high intra- and interannual variation (Mortimer et al., 2011a; Mortimer, 2012). The June–October (austral winter) nesting peak on Diego Garcia accords with comparative data from the region, suggesting a tendency for lower latitude nesting to peak in the austral autumn and winter, and higher latitude nesting in the austral summer, a pattern indicating that temperature may be moderating seasonality (Dalleau et al., 2012; Mortimer, 2012). Across species and ocean basins the slope of the rela- tionship between temperature and date of first breeding is steeper at higher latitudes (Mazaris et al., 2013). Variations in environmental parameters (e.g. sea surface temperature) forming part of global climate change (IPCC, 2014) are
associated with shifts in timing of seasonal events for a range of organisms (Walther et al., 2002; Ramp et al., 2015), including earlier onset of nesting in loggerhead turtles Caretta caretta (Hawkes et al., 2007). Further investigation of seasonality in the Chagos Archipelago may also reveal variations in peak nesting amongst the five atolls. Our review of mean annual egg clutch production in
the south-west Indian Ocean indicates that the Chagos Archipelago accounts for 39–51% of hawksbill and 14–20% of green turtle reproduction in the region. The Red List assessment criteria focus on annual numbers of nesting females, but we suggest egg clutch production is a more meaningful statistic given lack of consensus amongst tur- tle researchers regarding within season clutch frequency. Estimates of 3–5 clutches for hawksbill turtles (Mortimer & Donnelly, 2008) accord with data from Seychelles (Mortimer & Bresson, 1999) but an estimate of 3 clutches for green turtles (Seminoff, 2004) is probably an underesti- mate. Esteban et al. (2017) recorded a minimum mean of 6.0 clutches per turtle by satellite tracking inter-nesting female green turtles at Diego Garcia. It follows that although green turtle populations in the region produce almost 10 times more egg clutches than hawksbill turtle populations, there may be only five times as many female green turtles nesting annually. The Seychelles–Chagos hawksbill turtle population, identified by genetics as a separate regionalmanagement unit (Vargas et al., 2016), accounts for 97%of known hawksbill tur- tle nesting in the region. Linkage of the Chagos Archipelago to the region is also supported by migrations of most satel- lite-tracked post-nesting Chagos green turtles to Seychelles, Madagascar and eastern Africa (Hays et al., 2014). The Chagos Archipelago is, however, situated at the interface of the south-west Indian Ocean region and the IUCN Marine Turtle Specialist Group-designated north-west Indian Ocean regional management unit (Wallace et al., 2010). Evidence of a Chagos/north-west Indian Ocean linkage is therefore expected. Three of eight post-nesting Chagos green turtles migrated to the north-west Indian Ocean sites of Maldives and northern Somalia (Hays et al., 2014), and the possibility that some immature hawksbill turtles foraging in Chagos have genetic links with the Arabian Peninsula (Mortimer & Broderick, 1999) warrants further investigation. Our under- standing of marine turtle populations in the wider western Indian Ocean region is improving, and conservation man- agers need to use these findings to work across international boundaries to protect marine turtles at nesting habitats and foraging sites. Our findings demonstrate the importance of the Chagos Archipelago to nesting turtles at both a regional and global scale. The marine protected area can be expected to help ensure long-term protection of these resources.
Acknowledgements This work was supported by the Bertarelli Foundation as part of the Bertarelli Programme in Marine Science. We are grateful to C.R.C. Sheppard and members of the 1996 and
Oryx, 2020, 54(3), 332–343 © 2020 Fauna & Flora International doi:10.1017/S0030605319001108
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