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156 B.J. Godley et al. / Biological Conservation 97 (2001) 151±158 4.2. Magnitude of nesting Our methodology is comparable to that used by Mortimer and Carr (1987), and, therefore, it is evident that 2±3 times more turtles nested in 1998/1999 than in the surveys in the 1970 0 s. This is worthy of cautious optimism regarding the status of the Ascension Island nesting population given that many populations are thought to be depleted or endangered (King, 1982). For some sea turtle populations, such as the green turtles nesting in Pakistan (Asrar, 1999) or leatherback turtles in Malaysia (Chan and Liew, 1996) and Mexico (Sarti et al., 1996), there have been precipitous declines in nesting numbers in recent decades. This is certainly not the case for green turtles on Ascension Island. However, green turtle nesting numbers are prone to high levels of inter-annual variability (Carr et al., 1978; Schulz, 1982; Limpus and Nicholas, 1987), and hence continued monitoring e€orts are necessary in order to identify whether there have been subtle changes in the size of the Ascension Island population. For cheloniids, such changes may be identi®ed by intermittent monitoring over many decades (Hall et al., 1999) and, as such, our results will become an important component in future monitoring studies on Ascension Island. 4.3. Spatial variation in nesting e€ort It is clear that the general present-day distribution of nesting is similar to that recorded in the 1970s, with the same major beaches still holding a large proportion of nesting. 4.4. Temporal distribution of nesting e€ort The timing of the 1998/1999 season as described is broadly similar to that presented by previous authors in preliminary studies (Carr and Hirth, 1962; Carr, 1975) and extremely similar to the detailed ®ndings of Mortimer and Carr (1987). From the shape of the curve of the temporal distribution of nesting (Fig. 2) it appears that although not all nests were recorded by beginning surveying on 1 December a negligible proportion was missed. This was con®rmed by anecdotal accounts by Island residents who suggested there had been occasional nests in November but that nesting in October had not been observed. For most populations of sea turtle, nesting occurs in a distinct season, usually in the warmest months (Miller, 1996). The reasons for this seasonality are rarely considered explicitly. On Ascension, Mortimer and Carr (1987) noted that the turtle nesting season coincided with the time of the year when both highest rainfalls and highest temperatures were recorded. They suggested that the timing was adaptive in that turtles have diculty constructing nests in dry sand, thus by nesting in the wettest part of the year, digging would be facilitated. If this was the case, nesting success might be expected to be higher at times of peak nesting. No evidence was found to support the hypothesis that the seasonality of marine turtle nesting is adaptively linked to higher rainfall to facilitate nesting as suggested by Mortimer and Carr (1987). First, nesting success did not vary during the season. Second, nesting success was not related to rainfall and third, rainfall only explained a small proportion of the variance in nest numbers in our multiple regression. We would therefore suggest that prevailing temperatures are more likely a factor in the explanation of the timing of the nesting season. Mortimer and Carr (1987) presented data illustrating the fact that March and April were the months with most rain. The variance shown in the mean monthly rainfall for these months was very high. This is possibly due to the fact that the magnitude of rainfall in these months is dependent on occasional extreme rainstorms which do not occur every year. We found a strong correlation between the magnitude of nesting and air temperature. By using the close relationship between air temperature and sand temperature at nest depth previously described by Hays et al. (1999), Fig. 4 shows that during the months of negligible or no nesting (July±October), sand temperatures will exist which are close to the lower limit of the thermal tolerance range for marine turtle embryos (25±27 C; Ackerman, 1996). On beach 12 most monthly means of sand temperature during the nesting season at nest depth lie within this range. Turtles may have evolved to nest during the time of highest sand temperatures avoiding the months where, certainly on beach 12 and other beaches of a high albedo (Hays et al., 1995, 1999), nest temperatures may get too low. 4.5. Conclusion In summary, our observations con®rm the regional importance of the green turtles nesting on Ascension Island and suggest that the status of the population is favourable. It is clear that this remains a key rookery for green turtles in the Atlantic. However, additional monitoring is clearly necessary to identify whether long term trends in population size are occurring. With the methodology presented here, accurate estimates of nesting population size can be obtained which will be directly comparable among years and populations. Acknowledgements This work was funded by a grant to GCH from the Darwin Initiative for the Survival of the Species (UK Department of Environment, Transport and the Regions; DETR) and was carried out in conjunction
B.J. Godley et al. / Biological Conservation 97 (2001) 151±158 157 with the Ascension Island Administrator, HH Roger Huxley. The ®eld work would not have been possible without the e€orts of Darwin Initiative Turtle Wardens: Robert Frauenstein, Fiona Glen, Jason Tim, Donald Stevens, Steve Thomas, Deon Yon and the help of the regular volunteer track rakers: Dudley Bowling, Niddy Huxley, Je€ Lowdermilk, Kyla Turton and those many others who helped out on occasion. We are grateful for the additional logistical support of Ascension Island Services, Cable and Wireless, Computer Services Raytheon, First Ascension Scout Group, Johnny Hobson, Merlin Communications Ltd., Dave Rayney, Reed Family, Royal Air Force and the United States Air Force. Many thanks to the Meteorological Oce at the Ascension Island Air®eld for providing data. This manuscript was improved as the result of comments from Matthew Godfrey, Jeanne Mortimer and one anonymous reviewer. References Asrar, F.F., 1999. 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