Marine turtle nesting populations 339
explain the abundant green turtle nesting. In contrast, Sala- mon, with 10.6% of total nesting habitat, today hosts ,5% of turtle populations despite reports of abundant turtles in 1786 (Horsburgh, 1809, in Mortimer & Day, 1999) and 1813 (Wenban-Smith & Carter, 2016). Salamon comprises a nearly closed island ring, facilitating human access to all nesting beaches regardless of weather. Egmont also has is- lands easily accessible year-round, but its human population was relatively smaller, and habitation ended 37 years earlier. Today its beaches (13.1%of total) host 16.1%of the green and 5.0% of the hawksbill turtles of the archipelago. The interannual variability of the Diego Garcia month-
ly surveys typifies nesting populations globally (Broderick et al., 2001) and within the region (Lauret-Stepler et al., 2007; Mortimer, 2012), driven by varying remigration inter- vals (e.g. Hays, 2000). Individual turtles experience differ- ing environmental conditions year-to-year at their foraging grounds, which modulate the time females take to achieve breeding body condition. So, in some years relatively higher proportions of the population may attain this body con- dition and migrate to breed. Remigration intervals vary between individuals and even over time for the same individual (Miller, 1997). The turtle populations of the Chagos Archipelago show
FIG. 4 The most important nesting populations in the south-west Indian Ocean region of (a) hawksbill and (b) green turtles. Circle sizes indicate estimated mean annual egg clutch production. A question mark indicates inadequately surveyed sites. For source data see Table 4 and Supplementary Table 3. The base map is from Claus et al. (2018). The shapes around each population indicate Exclusive Economic Zones.
human exploitation, with higher nesting densities now re- ported on atolls with relatively less accessible islands. Peros Banhos and Diego Garcia hostmost of the nesting
habitat and the largest populations of nesting turtles in the Chagos Archipelago. Since 1968 and 1970 green and hawks- bill turtles, respectively, have been protected by conser- vation legislation. Since 1973 turtle protection has been reinforced by several Special Nature Reserves that do not have human habitation or artificial lighting. At Peros Ban- hos topography affords protection, with abundant habi- tat on 36 small islands along the rim of the 34 km diameter lagoon. At both Peros Banhos and Great Chagos Bank, rough seas and primitive sail and oar-driven boats would have limited the efficiency of historical turtle hunts (Wenban-Smith & Carter, 2016). At many Great Chagos Bank islands the high energy beaches, which offer ideal habitat for green turtles but restrict human access, may
signs of recovery after 2 centuries of exploitation. Recovery occurs in two phases. Once protection begins, females that would previously have been killed after only a few nesting attempts now survive the season to produce full comple- ments of clutches and return as re-migrant females in sub- sequent seasons. This produces an immediate increase in nesting activity and reproductive output even though absolute numbers of adult females have not yet increased (Mortimer, 1985, 1988; Balazs & Chaloupka, 2004). Then, after a lag of 30–40 years, the time estimated for Indo- Pacific hawksbill (Bell & Pike, 2012) and green turtle (Limpus &Chaloupka, 1997) hatchlings to attain sexual ma- turity, a second phase of recovery commences once the off- spring produced on protected beaches mature and return as reproductive adults (Dutton et al., 2005). In Seychelles a 40-year recovery pattern has been documented for both hawksbill (Allen et al., 2010) and green turtles (Mortimer et al., 2011b), with up to 800% increase in clutches after 4 decades of protection. Such recovery is underway in Chagos and we predict will become more evident with con- tinued protection of the marine protected area (Koldewey et al., 2010) and long-term monitoring (Wallace et al., 2011). The statistically significant increasing trend in the num-
ber of green turtles nesting on Chagos is encouraging, and continued monitoring is needed for hawksbill turtles to determine whether the potentially increasing trend is sig- nificant or simply reflects interannual variability. The need for long time series to confirm statistical significance of apparent trends in abundance was highlighted by Mazaris et al. (2017) who reported that, globally, many
Oryx, 2020, 54(3), 332–343 © 2020 Fauna & Flora International doi:10.1017/S0030605319001108
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