Nesting of green turtles (Chelonia mydas) at ... - Seaturtle.org

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