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

154 B.J. Godley et al. / Biological Conservation 97 (2001) 151±158
nesting success was 99.5 (S.D.= 122; range= 5±444)
while the mean number of surveys to collect these data
was 14.4 (S.D.= 7.5; range= 10±37). The mean values
for nesting success for each beach varied 4-fold, from
0.13 on beach 10 to 0.52 on beach 12 (Table 1), with a
mean of 0.33 for all beaches (S.D.= 0.08; n=25 beaches).
The overall nesting success for Ascension Island
calculated as total nests divided by total activities was
0.39. When nesting success for the three main beaches
(1, 12, 27) were plotted against time, no trends were
detected.
By multiplying the nest success (p) for each beach by
the respective number of activities (a), the number of
nests on Ascension Island over the season was estimated
at 13,881. The fate of a nesting activity is e€ectively a
binomial problem, with outcomes of either a ``nest'' or
``no nest''. We used a standard equation to calculate the
variance in the estimate of the mean nesting success
value for each beach, i.e. variance=pq/a where q=1 p
(Bailey, 1981), and then from this variance 95% con-
®dence limits (95% CL) were determined. Since the
variance of the sum of a series of estimates is simply the
sum of their individual variances, the 95% con®dence
limits on the estimate for the total number of nests was
readily determined as 13,092±14,660.
3.2. Magnitude of nesting
A total of 36,036 marine turtle nesting activities was
estimated over the entire season, with the number of
activities per beach ranging from 47 to 9651. Density of
nesting activities varied widely from 534 to 10,001
activities/km, as did nest density which ranged from 213
to 5200 nests/km. Overall density of activities (total
activities/total beach surveyed) was 6204 activities/km
and total nest density (total nests/total beach surveyed)
was 2390 nests/km.
3.3. Spatial distribution of nesting e€ort
Included for comparison are data from the surveys by
Mortimer and Carr (1987; Table 2), and Mortimer
(1992; from a partial survey carried out in April 1992).
There were many more activities in the 1998/1999 season
than in the two seasons recorded by Mortimer and
Carr (1987), with the total being greater by an order of
two and three than in 1976/1977 and 1977/1978 seasons,
respectively. To compare the spatial variation in nesting
e€ort of the 1998/1999 season with that of previous
surveys (Mortimer and Carr, 1987; Mortimer, 1992),
data from individual beaches were combined into the
clusters de®ned by Mortimer and Carr (1987; Table 1).
In general, among beach spatial variation was similar in
all seasons. The only marked change since the studies in
1976±1978 appears to be the proportion of nesting
occurring on beach number 12.
3.4. Temporal distribution of nesting e€ort
The temporal distribution of nesting activities is
shown in Fig. 2. Nests were ®rst recorded on 2 December
1998 and continued until the last clutch was laid on
31 August, 1999. The peak of nesting was in March with
90% of nesting occurring between 13 January and 7
May 1999 and 95% of nesting recorded between 4 January
and 18 May 1999.
We used stepwise multiple regression to investigate
the relationship between the available weather parameters
(maximum temperature ( C), minimum temperature
( C), rainfall (mm)) recorded on each day (2
December 1998 to 31 August 1999) and the estimated
number of nests for that day on the whole island. The
most signi®cant variable was maximum daily temperature
(r 2 =0.58) with a model including temperature and
rainfall explaining slightly more of the variance
Table 2
Estimated number of activities recorded in the 1998/1999 season in comparison with data collected by Mortimer and Carr (1987) in the 1976/1977
and 1977/1978 seasons. In addition, data from the partial survey carried out by Mortimer (1992) are included
Beaches 1976/1977 1977/1978 1991/1992 1998/1999 c
1 2793 (15.4) a 2195 (18.2) (13.1) 4500 (12.5)
2±5 1821 (10.0) 1569 (13.0) (11.3) 5434 (15.1)
6±11 2453 (13.5) 1428 (11.8) (17.0) 4599 (12.8)
12 3118 (17.1) 1760 (14.6) (28.1) 9651 (26.8)
14±20 1341 (7.4) 783 (6.5) (1.4) 2819 (7.8)
21±26 976 (5.4) 822 (6.8) (5.5) 1820 (5.1)
27 2703 (14.9) 1469 (12.1) (12.5) 3216 (8.9)
28 804 (4.4) 420 (3.5) (1.8) 1172 (3.2)
29 1743 (9.6) 1191 (9.8) (6.9) 1986 (5.5)
30±32 440 (2.4) 456 (3.8) (2.6) 839 (2.3)
Total 18,192 12,093 na b 36,036
a Numbers in parentheses denote the percentage of the total for the season.
b na, Not applicable.
c It should be noted that in 1998/1999, data were not included from beaches 13, 18, 19, 20 and 32 (see Section 2).