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

Biological Conservation 97 (2001) 151±158
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Nesting of green turtles (Chelonia mydas) at Ascension Island,
South Atlantic
Brendan J. Godley *, Annette C. Broderick, Graeme C. Hays
School of Biological Sciences, University of Wales, Swansea SA2 8PP, UK
Received 23 November 1999; received in revised form 7 March 2000; accepted 12 July 2000
Abstract
A detailed survey of the nesting by green turtles (Chelonia mydas) on Ascension Island (7 57 0 S, 14 22 0 W) was conducted
between 1 December 1998 and 1 October 1999. During this period there was a total estimate of 36,036 marine turtle nesting activities,
resulting in the deposition of an estimated 13,881 clutches (95% con®dence limits 13,092±14,660). These data suggest that 2±3
times more turtles nested than when previous detailed surveys were undertaken in the 1970 0 s. The peak of nesting was in March,
with 95% of nesting activity being recorded between 4 January and 18 May. Possible reasons for the evolution of the seasonality of
nesting are discussed. Individual beaches varied greatly in density of nesting activities (range=534±10,001 activities/km;
mean=6204 activities/km), density of nests (range=213±5200 nests/km; mean=2390 nests/km) and the proportion of nesting
activities resulting in nests (nesting success; range=0.13±0.52; mean=0.39). # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Marine turtle; Sea turtle; Population assessment; Seasonal cycle
1. Introduction
For sea turtles, the technique most commonly used to
assess population size is to count the number of clutches
laid in a particular season (Schroeder and Murphy, 1999).
However, although seemingly straightforward, this task
is far from trivial when a population nests on many beaches
and where the nesting season lasts for many months.
In such cases, labour intensive surveys are required, and
for this reason there are surprisingly few assessments of
population size for some of the largest rookeries in the
world. A case in point is the endangered green turtle
(Chelonia mydas) (Groombridge and Luxmoore, 1989).
The green turtle is found circumglobally in the tropics
(Pritchard, 1996), with molecular evidence suggesting a
fundamental split between populations from the Atlantic±Mediterranean
and those from the Indo-Paci®c
(Bowen et al., 1992). Furthermore, even within the
Atlantic±Mediterranean region, populations show a
high level of structuring, which suggests a propensity for
female natal philopatry (Encalada et al., 1996). Consequently,
individual nesting colonies need to be considered
as separate conservation management units.
* Corresponding author. Tel.: +44-1792-205-678; fax: +44-1792-
295-447.
E-mail address: mtn@swan.ac.uk (B.J. Godley).
In the Atlantic, although there are numerous areas
where small numbers of green turtles still nest, evidence
suggests that the major nesting colonies for the green
turtle are: Tortuguero, Costa Rica (Bjorndal et al., 1999);
Ascension Island, UK (Mortimer and Carr, 1987); Suriname
(Schulz, 1975); Aves Island, Venezuela (Sole and
Medina, 1989); Poilao, Guinea Bissau (Fortes et al., 1998);
and Trinidade, Brazil (Moriera et al., 1995). Although
these are arranged in an approximate order of decreasing
magnitude (as per Bowen et al., 1992), quantitative
status surveys are often lacking, making rigorous intercolony
comparisons dicult. For example, even though
Ascension Island is thought to be one of the largest
green turtle rookeries in the Atlantic, surprisingly little
monitoring of the population has been undertaken. In
fact, the only comprehensive surveys were conducted in
1976/1977 and 1977/1978 (Mortimer and Carr, 1987)
where the number of nests was estimated as 7910±10,764
and 5257±7154, respectively. The need for the size of
this population to be re-assessed is, therefore, acute.
Although there is a long history of exploitation of the
turtle population on Ascension for meat, both by seafarers
and island residents, since the 1930s, the population has
been a€orded almost full protection on the nesting beaches,
and few, if any, turtles have been killed by man
since 1957 (Huxley, 1999). However, turtles only spend
a small proportion of their lives at Ascension, and hence
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152 B.J. Godley et al. / Biological Conservation 97 (2001) 151±158
mortality occurring away from the island may have
important impacts on the population size.
The life cycle of marine turtles involves movements
over great spatial scales (see Musick and Limpus, 1996
for review), probably taking decades for turtles to reach
adulthood. Although adults from the Ascension population
are thought largely to forage in Brazilian coastal
waters (Carr, 1975; Luschi et al., 1998), the life cycle of
juveniles involves extended periods in pelagic ocean
current systems (Musick and Limpus, 1996) and they
probably share coastal foraging areas with juveniles
from other populations (Lahanas et al., 1998). This life
cycle may expose this population to a number of ®sheries.
Although small-scale traditional ®sheries for marine turtles
once existed in Brazil (Pritchard, 1976), they are no
longer in operation (Marcovaldi and Marcovaldi, 1999),
and all marine turtle species are legally protected. Turtles
are, however, still incidentally caught in ®shing
gear. A national programme of marine turtle conservation
now exists in the Brazilian feeding grounds where
the mortality resulting from catch by artisanal ®sheries
has been reduced (Marcovaldi et al., 1998).
The fundamental goal of our study was to update
information on the number of green turtles nesting on
Ascension Island and hence to identify whether there
have been any dramatic changes in nesting numbers
over recent decades. We describe the design and implementation
of a rigorous survey regime which has produced
a robust estimate for the number of clutches laid
in a season and which will provide a template for future
monitoring work.
2. Methods
2.1. Study site
Ascension Island (7 57 0 S, 14 22 0 W) is an isolated
volcanic peak on the mid-Atlantic ridge that has 32
beaches and coves (Mortimer and Carr, 1987). The
location of beaches around the Island are shown in Fig.
1. Many of the beaches are backed by uninhabited
beach huts (maximum of one per beach) which are
used occasionally by island residents for recreational
purposes.
2.2. Enumeration of marine turtle nesting activities
Surveys of nesting beaches are often used to assess the
status of marine turtle populations (Schroeder and
Murphy, 1999). These utilise the fact that each time a
female turtle emerges from the water to attempt nesting,
(a ``nesting activity'') it creates a distinctive set of tracks
on the sand: with a track ascending to any aborted digging
attempts or successful nest, and a further track
descending to the sea. By counting tracks to and from
Fig. 1. Map of Ascension Island (7 57 0 S, 14 22 0 W) illustrating marine
turtle nesting beaches. Numbering system follows that of Mortimer
and Carr (1987). Key to common names: (1) South West Bay; (2)
Turtleshell; (3) Clarke's; (4) Payne Point; (5) Mitchell's Cove; (6)
Blowhole; (7) POL South; (8) POL North; (9) Deadman's; (10) Fort
Hayes; (11) Georgetown; (12) Long Beach; (13) Comfortless Cove;
(14) English Bay (EB); (15) EB Cove 1; (16) EB Cove 2; (17) EB Cove
3; (18) EB Cove 4; (19) Ladies Loo West; (20) Ladies Loo East; (21)
Porpoise Point (PP) Cove 1; (22) PP Cove 2; (23) PP Cove 3; (24) PP
Cove 4; (25) PP Cove 5; (26) PP Cove 6; (27) Northeast Bay; (28)
Beach Hut; (29) Hannay; (30) Pebbly West; (31) Pebbly East; (32)
Spire. In addition, the major settlement, Georgetown, and the Air-
Field, where meteorological observations were made, are marked.
the sea (and dividing by two) it is possible to infer how
many nesting activities have occurred.
Based upon the work of Mortimer and Carr (1987),
beaches on Ascension Island were divided into two
classes: those of major importance (beaches 1, 12, 27
and 29; Fig. 1); and those of minor importance (all
other beaches). From 1 December 1998 until 1 October,
1999, major beaches were visited on 3 successive days
each week. On day 1, all visible tracks were destroyed
using large rakes. On the morning of day 2 all new
activities were counted and the associated tracks
destroyed. On day 3, again all new activities were
counted. Minor beaches were visited every 1±3 weeks,
but only on 2 successive days, with tracks being raked
on day 1 and new activities being counted on day 2.
An estimate of the number of turtle activities on each
beach for each inter-survey day was generated by interpolation.
For major beaches, the mean daily count of
two successive 3-day survey bouts was attributed to
each intervening day. For minor beaches, a mean of the
two successive counts was used to generate an interpolated
value for intervening days. Thus an estimated
number of activities was attributed to every beach for
each day.
Track raking and counting was undertaken by a diverse
group of workers including the authors, local turtle wardens
and volunteers. Several beaches were removed
from the surveying regime as a result of both hosting

B.J. Godley et al. / Biological Conservation 97 (2001) 151±158 153
negligible nesting activity and being logistically dicult
to survey (beach nos. 13, 18, 19, 20 and 32; Fig. 1). In
addition, although surveyed throughout the season,
beaches 16 and 22 were subsequently deleted from analysis
after having had no clutches deposited throughout
the entire survey period.
2.3. Calculation of adult nesting success
Turtles do not lay eggs during every nesting activity
and can abort nesting e€orts at a number of di€erent
stages of the nesting process, returning to the sea,
usually to emerge later on the same or subsequent night.
Experienced observers can assess with a high degree of
con®dence whether or not an activity has resulted in the
laying of a clutch, i.e. ``a nest'' (cf. Bjorndal et al., 1999).
This is comparable with the technique used previously
on Ascension Island (Mortimer and Carr, 1987).
All beaches were visited throughout the season as an
integral part of track counting to enumerate ``adult
nesting success'' i.e. the proportion of activities which
resulted in a nest. For consistency, this was always carried
out by the authors (BJG and ACB) who were
experienced in this technique (Broderick and Godley,
1996). On a given survey date, a portion of the activities
was assessed. On beaches with low levels of nesting this
was all the activities, whereas on those with high levels
of nesting a sample of 10±20 activities were assessed.
2.4. Calculation of number of clutches laid
For each beach, the number of clutches laid on each
day was calculated by multiplying the interpolated or
measured number of nesting activities for that day by
the overall nesting success for the season for that beach.
2.5. Additional information
The length of each beach was measured along the
high water mark using a 50 m ®breglass surveying tape
measure. In addition, meteorological observations
(daily maximum and minimum air temperature and
rainfall) taken throughout the study were obtained from
the Meteorological Oce at the Ascension Island Air-
®eld. Historic meteorological air temperature from 1
January 1985±31 December 1997 were also provided.
3. Results
3.1. Nesting success
Nesting was recorded on 25 beaches and these were
surveyed throughout the season. Nesting beaches totalled
5809 m in length, with these individual beaches ranging
from 45 to 965 m (Table 1). In order to produce an
Table 1
Length, nesting success (N success) (95% CL), number of activities
(total A), number of nests (95% CL), density of activities (A/km) and
density of nests (nests/km) on each of the 25 beaches which had nesting
and were surveyed throughout the season
Beaches
Length
(m)
N
Success
Total
A
Nests A/km Nests/
km
1 530 0.33 4500 1485 8491 2802
(0.29±0.38) (1305±1710)
2 337 0.30 1486 446 4408 1322
(0.19±0.41) (282±609)
3 237 0.38 1485 564 6266 2381
(0.26±0.49) (386±728)
4 150 0.32 1031 330 6873 2199
(0.20±0.43) (206±443)
5 160 0.18 1433 258 8953 1612
(0.09±0.26) (129±373)
6 331 0.33 1462 482 4417 1458
(0.23±0.44) (336±643)
7 428 0.41 1730 709 4042 1657
(0.31±0.51) (536±882)
8 178 0.29 711 206 3994 1158
(0.18±0.40) (128±284)
9 107 0.22 308 68 2886 635
(0.03±0.41) (9±126)
10 78 0.13 166 21 2121 265
(0.01±0.24) (2±40)
11 267 0.36 222 80 830 298
(0.22±0.50) (49±111)
12 965 0.52 9651 5019 10001 5200
(0.46±0.58) (4439±5598)
14 250 0.32 1917 613 7666 2453
(0.24±0.41) (147±251)
15 48 0.40 269 108 5651 2260
(0.15±0.65) (40±175)
17 112 0.35 633 222 5652 1978
(0.21±0.49) (133±310)
21 155 0.38 992 377 6400 2432
(0.26±0.49) (258±486)
23 87 0.40 47 19 534 213
(0.00±0.83) (0±39)
24 478 0.42 176 74 368 155
(0.14±0.70) (25±123)
25 65 0.31 553 171 8547 2649
(0.16±0.45) (88±249)
26 50 0.29 53 15 1060 307
(0.00±0.62) (0±33)
27 334 0.42 3216 1351 9613 4037
(0.37±0.46) (1190±1500)
28 120 0.31 1172 363 9763 3026
(0.21±0.41) (246±481)
29 201 0.32 1986 636 9881 3161
(0.27±0.38) (536±755)
30 96 0.30 681 204 7061 2118
(0.17±0.43) (116±293)
31 45 0.38 159 60 3499 1330
(0.12±0.65) (19±103)
Total 5809 na a 36036 13881 na na
a na, Not applicable.
overall value for the number of nests in the entire season,
both the number of activities and the nesting success
need to be quanti®ed. For individual beaches the
mean number of activities used in the assessment of

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).

B.J. Godley et al. / Biological Conservation 97 (2001) 151±158 155
Fig. 2. Temporal distribution of nesting activities of green turtles on
Ascension Island during the 1998/1999 season. For each beach, the
number of nests was subsequently calculated by multiplying the actual
or interpolated number of nesting activities for that day and the overall
nesting success for the season on that beach.
Fig. 4. Monthly mean sand temperature on beach 12 (January 1985±
December 1997) predicted using the relationship between monthly
mean maximum air temperature measured at the Ascension Island
Air®eld and the sand temperature at nest depth on that beach (Hays et
al., 1999).
(r 2 =0.59). Overall, as air temperature increased so did
the daily number of nests (Fig. 3). To investigate the
possible e€ect of rainfall on nesting success, for each 10
day period, mean rainfall and mean nesting success were
compared using regression analysis and no signi®cant
relationship was found (F 1,15 =0.03, p=0.87, r 2 =0.02).
The close link between air temperature and sand
temperature at nest depth has previously been described
for Ascension Island (Hays et al., 1999). This relationship
(mean monthly sand temperature ( C) on Long
Beach =1.6+0.908 mean monthly daily maximum
temperature; F 1,10 =498, p

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.
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