- Email: [email protected]
Nesting of green turtles (Chelonia mydas) at Ascension Island, South Atlantic
Biological Conservation 97 (2001) 151±158
www.elsevier.com/locate/biocon
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 (7 570 S, 14 220 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 19700 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: +44-1792295-447. E-mail address: [email protected] (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 aorded 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
0006-3207/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(00)00107-5
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 (7 570 S, 14 220 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 (7 570 S, 14 220 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 AirField, 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.
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
2.3. Calculation of adult nesting success
1
530
Turtles do not lay eggs during every nesting activity and can abort nesting eorts at a number of dierent 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
337
3
237
4
150
5
160
6
331
7
428
8
178
9
107
10
78
11
267
12
965
14
250
15
48
17
112
21
155
23
87
24
478
25
65
26
50
27
334
28
120
29
201
30
96
31
45
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
Beaches
Total a
Length (m)
5809
N Success 0.33 (0.29±0.38) 0.30 (0.19±0.41) 0.38 (0.26±0.49) 0.32 (0.20±0.43) 0.18 (0.09±0.26) 0.33 (0.23±0.44) 0.41 (0.31±0.51) 0.29 (0.18±0.40) 0.22 (0.03±0.41) 0.13 (0.01±0.24) 0.36 (0.22±0.50) 0.52 (0.46±0.58) 0.32 (0.24±0.41) 0.40 (0.15±0.65) 0.35 (0.21±0.49) 0.38 (0.26±0.49) 0.40 (0.00±0.83) 0.42 (0.14±0.70) 0.31 (0.16±0.45) 0.29 (0.00±0.62) 0.42 (0.37±0.46) 0.31 (0.21±0.41) 0.32 (0.27±0.38) 0.30 (0.17±0.43) 0.38 (0.12±0.65) naa
Total A 4500 1486 1485 1031 1433 1462 1730 711 308 166 222 9651 1917 269 633 992 47 176 553 53 3216 1172 1986 681 159 36036
Nests 1485 (1305±1710) 446 (282±609) 564 (386±728) 330 (206±443) 258 (129±373) 482 (336±643) 709 (536±882) 206 (128±284) 68 (9±126) 21 (2±40) 80 (49±111) 5019 (4439±5598) 613 (147±251) 108 (40±175) 222 (133±310) 377 (258±486) 19 (0±39) 74 (25±123) 171 (88±249) 15 (0±33) 1351 (1190±1500) 363 (246±481) 636 (536±755) 204 (116±293) 60 (19±103) 13881
A/km
Nests/ km
8491
2802
4408
1322
6266
2381
6873
2199
8953
1612
4417
1458
4042
1657
3994
1158
2886
635
2121
265
830
298
10001
5200
7666
2453
5651
2260
5652
1978
6400
2432
534
213
368
155
8547
2649
1060
307
9613
4037
9763
3026
9881
3161
7061
2118
3499
1330
na
na
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 eectively 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=1ÿ 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 eort 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 eort 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 eort 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 (r2=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
1998/1999c
1 2±5 6±11 12 14±20 21±26 27 28 29 30±32 Total
2793 (15.4)a 1821 (10.0) 2453 (13.5) 3118 (17.1) 1341 (7.4) 976 (5.4) 2703 (14.9) 804 (4.4) 1743 (9.6) 440 (2.4) 18,192
2195 (18.2) 1569 (13.0) 1428 (11.8) 1760 (14.6) 783 (6.5) 822 (6.8) 1469 (12.1) 420 (3.5) 1191 (9.8) 456 (3.8) 12,093
(13.1) (11.3) (17.0) (28.1) (1.4) (5.5) (12.5) (1.8) (6.9) (2.6) nab
4500 (12.5) 5434 (15.1) 4599 (12.8) 9651 (26.8) 2819 (7.8) 1820 (5.1) 3216 (8.9) 1172 (3.2) 1986 (5.5) 839 (2.3) 36,036
a b c
Numbers in parentheses denote the percentage of the total for the season. na, Not applicable. 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
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.
(r2=0.59). Overall, as air temperature increased so did the daily number of nests (Fig. 3). To investigate the possible eect 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 (F1,15=0.03, p=0.87, r2=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; F1,10=498, p<0.001, r2=0.98; Hays et al., 1999) was used in combination with historic temperature data to generate the mean monthly sand temperature on beach 12 (Long Beach) between January 1985 and December 1997 (Fig. 4). There is a marked seasonal cycle
Fig. 3. Relationship between maximum daily air temperature measured at the Ascension Island Air®eld on each day of the nesting season and the number of nests throughout the island on that day (number of nests=32maximum temperature ÿ896; F1,273=367.96, r2=0.58, P<0.001).
155
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).
in nest temperatures, albeit with a limited range (less than 6 C over all years). A peak of sand temperatures is recorded between February and April with consistently low temperatures experienced between August and October. 4. Discussion 4.1. Survey technique It is clearly important to explore the reliability of techniques used to assess the size of populations. For sea turtles, our observations identify several important ways in which errors can be introduced into the population size estimate. First, it is clearly not appropriate to assume that the density of activities is the same on all beaches and so population estimates cannot be generated by simply extrapolating from small sections of beach. On Ascension Island, it has been suggested that the favourability of beaches is in¯uenced, at least partly, by the oshore sea-bed topography, with those beaches that have less oshore rocks having more nests (Mortimer, 1982). Second, the nesting success needs to be measured and, again, cannot be assumed to be constant on dierent beaches or at dierent times of the year. On Ascension, this inter-beach variability in nesting success is thought to be driven by the texture of the sand, with turtles being able to excavate nests more easily when the sand particle size is smaller (Mortimer, 1990). Third, nesting density varies temporally and so without knowledge of the seasonal cycle and its inter-annual variability, it is impossible to extrapolate from observations made over a short period of time. Our surveying overcame all of these problems and hence our estimate for the number of nests laid in the 1998/1999 season should be considered reliable.
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 19700 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 eorts 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 eort 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 eort 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
with the Ascension Island Administrator, HH Roger Huxley. The ®eld work would not have been possible without the eorts 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. Decline of marine turtle nesting populations in Pakistan. Marine Turtle Newsletter 83, 13±14. Ackerman, R.A., 1996. The nest environment and the embryonic development of sea turtles. In: Lutz, P.L., Musick, J.A. (Eds.), The Biology of Sea Turtles. CRC Press, Boca Raton, pp. 83±106. Bailey, N.T.J., 1981. Statistical methods in biology. Hodder and Stoughton, London (216 pp.). Bjorndal, K.A., Whetherall, J.A., Bolten, A., Mortimer, J.A., 1999. Twenty six years of green turtle nesting at Tortuguero Costa Rica: an encouraging trend. Conservation Biology 13, 126±134. Bowen, B.W., Meylan, A.B., Ross, J.P., Limpus, C.J., Balazs, G.H., Avise, J.C., 1992. Global population structure and natural history of the green turtle (Chelonia mydas) in terms of maternal phylogeny. Evolution 46, 865±881. Broderick, A.C., Godley, B.J., 1996. Population and nesting ecology of the green turtle, Chelonia mydas, and loggerhead turtle, Caretta caretta, in northern Cyprus.. Zoology in the Middle East. 13, 27± 46. Carr, A., 1975. The Ascension Island green turtle colony. Copeia 1975, 547±555. Carr, A., Hirth, H., 1962. The ecology and migrations of sea turtles 5:Comparative features of isolated green turtle colonies. American Museum Novitates 2091, 1±41. Carr, A.F., Carr, M.H., Meylan, A.B., 1978. The ecology and migrations of sea turtles 7: the west Caribbean green turtle colony. Bulletin of the American Museum of Natural History. 162, 1±48. Chan, E.H., Liew, H.C., 1996. Decline of the leatherback population in Terengganu, Malaysia, 1956±1995. Chelonian Conservation and Biology 2, 196±203. Encalada, S.E., Lahanas, P.N., Bjorndal, K.A., Bolten, A.B., Miyamoto, M.M., Bowen, B.W., 1996. Phylogeography and population structure of Atlantic and Mediterranean green turtle Chelonia mydas: a mitochondrial DNA control region sequence assessment. Molecular Ecology 5, 473±483. Fortes, O., Pires, A.J., Bellini, C., 1998. Green turtle, Chelonia mydas in the island of Poilao, Bolama-Bijagos Archipelago, Guinea-Bissau, West Africa. Marine Turtle Newsletter 80, 8±10. Groombridge, B., Luxmoore, R., 1989. The Green Turtle and Hawksbill (Reptilia: Cheloniidae): World Status, Exploitation and Trade. CITES Secretariat, Lausanne, Switzerland (601 pp.).
157
Hall, R.J., Henry, P.F.P., Bunck, C.M., 1999. Fifty-year trends in a box turtle population in Maryland. Biological Conservation 88, 165±172. Hays, G.C., Godley, B.J., Broderick, A.C., 1999. Long-term thermal conditions on the nesting beaches of green turtles on Ascension Island. Marine Ecology Progress Series 185, 297±299. Hays, G.C., Adams, C.R., Mortimer, J.A., Speakman, J.R., 1995. Inter- and intra-beach thermal variation for green turtle nests on Ascension Island, South Atlantic. Journal of the Marine Biological Association UK. 75, 405±411. Huxley, R., 1999. Historical overview of marine turtle exploitation on Ascension Island, South Atlantic, 1999. Marine Turtle Newsletter. 84, 7±9. King, W.F., 1982. Historical review of the decline of the green turtle and the hawksbill. In: Bjorndal, K.A. (Ed.), Biology and Conservation of Sea Turtles. Smithsonian Institute Press, Washington DC, pp. 183±188. Lahanas, P.N., Bjorndal, K.A., Bolten, A.B., Encalada, S.E., Miyamoto, M.M., Valverde, R.A., Bowen, B.W., 1998. Genetic composition of a green turtle (Chelonia mydas) feeding ground population: evidence for multiple origins. Marine Biology 130, 345±352. Limpus, C.J., Nicholas, N., 1987. The southern oscillation regulates the annual numbers of green turtles (Chelonia mydas) breeding around northern Australia. Australian Journal of Wildlife Research. 15, 157±161. Luschi, P., Hays, G.C., del Seppia, C., Marsh, R., Papi, F., 1998. The navigational feats of green sea turtles migrating from Ascension Island investigated by satellite telemetry. Proceedings of the Royal Society (B). 265, 2279±2284. Marcovaldi, M.A, Marcovaldi, G.G., 1999. Marine turtles in Brazil:the history and structure of Projecto TAMAR-IBAMA. Biological Conservation. 91, 35±41. Marcovaldi, M.AÃ., Baptistotte, C., De Castilhos, J.C., Gallo, B.M.G., Lima, E.H.S.M., Sanches, T.M. et al., 1998. Activities by Project TAMAR in Brazilian Sea Turtle Feeding Grounds. Marine Turtle Newsletter. 80, 5±7. Miller, J.D., 1996. Reproduction in sea turtles. In: Lutz, P.L., Musick, J.A. (Eds.), Biology of Sea Turtles. CRC Press, Boca Raton, pp. 51±81. Moriera, L., Baptistotte, C., Scalfone, J., Thome, J.C., De Almeida, A.P.L.S., 1995. Occurence of Chelonda mydas on the island of Trinidad, Brazil. Marine Turtle Newsletter 70, 2. Mortimer, J.A., 1982. Factors in¯uencing beach selection by nesting sea turtles. In: Bjorndal, K.A. (Ed.), Biology and Conservation of Sea Turtles. Smithsonian Institute Press, Washington, DC, pp. 45±52. Mortimer, J.A., 1990. The in¯uence of beach sand characteristics on the nesting behaviour and clutch survival of green turtles (Chelonia mydas). Copeia 1990, 802±817. Mortimer, J.A., 1992. An evaluation of the status of the Ascension Island green turtles and their nesting habitat in 1992. Unpublished Report to the Caribbean Conservation Corporation, 22pp. Mortimer, J.A., Carr, A., 1987. Reproduction and migrations of the Ascension Island green turtle (Chelonia mydas). Copeia 1987, 103± 113. Musick, J.A., Limpus, C.J., 1996. Habitat utilisation and migration in juvenile sea turtles. In: Lutz, P.L., Musick, J.A. (Eds.), The Biology of Sea Turtles. CRC Press, Boca Raton, pp. 137±165. Pritchard, P.C.H., 1976. Post-nesting movements of marine turtles (Chelonidae and Dermochelyidae) tagged in the Guianas. Copeia 1976, 749±754. Pritchard, P.C.H., 1996. Evolution, phylogeny and status. In: Lutz, P.L., Musick, J.A. (Eds.), The Biology of Sea Turtles. CRC Press, Boca Raton, pp. 1±28. Sarti, M.L., Eckert, S.A., Garcia, T., N, Barragan, A.R., 1996. Decline of the world's largest nesting assemblage of leatherback turtles. Marine Turtle Newsletter. 74, 2±5. Schroeder, B. Murphy, S., 1999 Population surveys (ground and aerial) on nesting beaches. In: Eckert, K.L., Bjorndal, K.A., Abreu-Grobois,
158
B.J. Godley et al. / Biological Conservation 97 (2001) 151±158
F.A., Donnelly, M. (Eds.), Research and Management Techniques for the Conservation of Sea Turtles. IUCN/SSC Marine Turtle Specialist Group, Washington, DC, Publication No 4, pp. 45±55. Schulz, J.P., 1975. Sea turtles nesting in Suriname. Zoologische Verhandelingen 143, 1±143.
Sole, G., Medina, G., 1989. The green turtles of Aves Island. In: Eckert, S.A., Eckert, K.A., Richardson, T.H. (Compilers). Proceedings of the Ninth Annual Workshop on Sea Turtle Conservation and Biology. NOAA Technical Memorandum NMFS-SEFC-232, pp. 171±173.
www.elsevier.com/locate/biocon
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 (7 570 S, 14 220 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 19700 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: +44-1792295-447. E-mail address: [email protected] (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 aorded 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
0006-3207/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(00)00107-5
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 (7 570 S, 14 220 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 (7 570 S, 14 220 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 AirField, 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.
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
2.3. Calculation of adult nesting success
1
530
Turtles do not lay eggs during every nesting activity and can abort nesting eorts at a number of dierent 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
337
3
237
4
150
5
160
6
331
7
428
8
178
9
107
10
78
11
267
12
965
14
250
15
48
17
112
21
155
23
87
24
478
25
65
26
50
27
334
28
120
29
201
30
96
31
45
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
Beaches
Total a
Length (m)
5809
N Success 0.33 (0.29±0.38) 0.30 (0.19±0.41) 0.38 (0.26±0.49) 0.32 (0.20±0.43) 0.18 (0.09±0.26) 0.33 (0.23±0.44) 0.41 (0.31±0.51) 0.29 (0.18±0.40) 0.22 (0.03±0.41) 0.13 (0.01±0.24) 0.36 (0.22±0.50) 0.52 (0.46±0.58) 0.32 (0.24±0.41) 0.40 (0.15±0.65) 0.35 (0.21±0.49) 0.38 (0.26±0.49) 0.40 (0.00±0.83) 0.42 (0.14±0.70) 0.31 (0.16±0.45) 0.29 (0.00±0.62) 0.42 (0.37±0.46) 0.31 (0.21±0.41) 0.32 (0.27±0.38) 0.30 (0.17±0.43) 0.38 (0.12±0.65) naa
Total A 4500 1486 1485 1031 1433 1462 1730 711 308 166 222 9651 1917 269 633 992 47 176 553 53 3216 1172 1986 681 159 36036
Nests 1485 (1305±1710) 446 (282±609) 564 (386±728) 330 (206±443) 258 (129±373) 482 (336±643) 709 (536±882) 206 (128±284) 68 (9±126) 21 (2±40) 80 (49±111) 5019 (4439±5598) 613 (147±251) 108 (40±175) 222 (133±310) 377 (258±486) 19 (0±39) 74 (25±123) 171 (88±249) 15 (0±33) 1351 (1190±1500) 363 (246±481) 636 (536±755) 204 (116±293) 60 (19±103) 13881
A/km
Nests/ km
8491
2802
4408
1322
6266
2381
6873
2199
8953
1612
4417
1458
4042
1657
3994
1158
2886
635
2121
265
830
298
10001
5200
7666
2453
5651
2260
5652
1978
6400
2432
534
213
368
155
8547
2649
1060
307
9613
4037
9763
3026
9881
3161
7061
2118
3499
1330
na
na
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 eectively 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=1ÿ 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 eort 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 eort 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 eort 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 (r2=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
1998/1999c
1 2±5 6±11 12 14±20 21±26 27 28 29 30±32 Total
2793 (15.4)a 1821 (10.0) 2453 (13.5) 3118 (17.1) 1341 (7.4) 976 (5.4) 2703 (14.9) 804 (4.4) 1743 (9.6) 440 (2.4) 18,192
2195 (18.2) 1569 (13.0) 1428 (11.8) 1760 (14.6) 783 (6.5) 822 (6.8) 1469 (12.1) 420 (3.5) 1191 (9.8) 456 (3.8) 12,093
(13.1) (11.3) (17.0) (28.1) (1.4) (5.5) (12.5) (1.8) (6.9) (2.6) nab
4500 (12.5) 5434 (15.1) 4599 (12.8) 9651 (26.8) 2819 (7.8) 1820 (5.1) 3216 (8.9) 1172 (3.2) 1986 (5.5) 839 (2.3) 36,036
a b c
Numbers in parentheses denote the percentage of the total for the season. na, Not applicable. 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
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.
(r2=0.59). Overall, as air temperature increased so did the daily number of nests (Fig. 3). To investigate the possible eect 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 (F1,15=0.03, p=0.87, r2=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; F1,10=498, p<0.001, r2=0.98; Hays et al., 1999) was used in combination with historic temperature data to generate the mean monthly sand temperature on beach 12 (Long Beach) between January 1985 and December 1997 (Fig. 4). There is a marked seasonal cycle
Fig. 3. Relationship between maximum daily air temperature measured at the Ascension Island Air®eld on each day of the nesting season and the number of nests throughout the island on that day (number of nests=32maximum temperature ÿ896; F1,273=367.96, r2=0.58, P<0.001).
155
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).
in nest temperatures, albeit with a limited range (less than 6 C over all years). A peak of sand temperatures is recorded between February and April with consistently low temperatures experienced between August and October. 4. Discussion 4.1. Survey technique It is clearly important to explore the reliability of techniques used to assess the size of populations. For sea turtles, our observations identify several important ways in which errors can be introduced into the population size estimate. First, it is clearly not appropriate to assume that the density of activities is the same on all beaches and so population estimates cannot be generated by simply extrapolating from small sections of beach. On Ascension Island, it has been suggested that the favourability of beaches is in¯uenced, at least partly, by the oshore sea-bed topography, with those beaches that have less oshore rocks having more nests (Mortimer, 1982). Second, the nesting success needs to be measured and, again, cannot be assumed to be constant on dierent beaches or at dierent times of the year. On Ascension, this inter-beach variability in nesting success is thought to be driven by the texture of the sand, with turtles being able to excavate nests more easily when the sand particle size is smaller (Mortimer, 1990). Third, nesting density varies temporally and so without knowledge of the seasonal cycle and its inter-annual variability, it is impossible to extrapolate from observations made over a short period of time. Our surveying overcame all of these problems and hence our estimate for the number of nests laid in the 1998/1999 season should be considered reliable.
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 19700 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 eorts 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 eort 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 eort 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
with the Ascension Island Administrator, HH Roger Huxley. The ®eld work would not have been possible without the eorts 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. Decline of marine turtle nesting populations in Pakistan. Marine Turtle Newsletter 83, 13±14. Ackerman, R.A., 1996. The nest environment and the embryonic development of sea turtles. In: Lutz, P.L., Musick, J.A. (Eds.), The Biology of Sea Turtles. CRC Press, Boca Raton, pp. 83±106. Bailey, N.T.J., 1981. Statistical methods in biology. Hodder and Stoughton, London (216 pp.). Bjorndal, K.A., Whetherall, J.A., Bolten, A., Mortimer, J.A., 1999. Twenty six years of green turtle nesting at Tortuguero Costa Rica: an encouraging trend. Conservation Biology 13, 126±134. Bowen, B.W., Meylan, A.B., Ross, J.P., Limpus, C.J., Balazs, G.H., Avise, J.C., 1992. Global population structure and natural history of the green turtle (Chelonia mydas) in terms of maternal phylogeny. Evolution 46, 865±881. Broderick, A.C., Godley, B.J., 1996. Population and nesting ecology of the green turtle, Chelonia mydas, and loggerhead turtle, Caretta caretta, in northern Cyprus.. Zoology in the Middle East. 13, 27± 46. Carr, A., 1975. The Ascension Island green turtle colony. Copeia 1975, 547±555. Carr, A., Hirth, H., 1962. The ecology and migrations of sea turtles 5:Comparative features of isolated green turtle colonies. American Museum Novitates 2091, 1±41. Carr, A.F., Carr, M.H., Meylan, A.B., 1978. The ecology and migrations of sea turtles 7: the west Caribbean green turtle colony. Bulletin of the American Museum of Natural History. 162, 1±48. Chan, E.H., Liew, H.C., 1996. Decline of the leatherback population in Terengganu, Malaysia, 1956±1995. Chelonian Conservation and Biology 2, 196±203. Encalada, S.E., Lahanas, P.N., Bjorndal, K.A., Bolten, A.B., Miyamoto, M.M., Bowen, B.W., 1996. Phylogeography and population structure of Atlantic and Mediterranean green turtle Chelonia mydas: a mitochondrial DNA control region sequence assessment. Molecular Ecology 5, 473±483. Fortes, O., Pires, A.J., Bellini, C., 1998. Green turtle, Chelonia mydas in the island of Poilao, Bolama-Bijagos Archipelago, Guinea-Bissau, West Africa. Marine Turtle Newsletter 80, 8±10. Groombridge, B., Luxmoore, R., 1989. The Green Turtle and Hawksbill (Reptilia: Cheloniidae): World Status, Exploitation and Trade. CITES Secretariat, Lausanne, Switzerland (601 pp.).
157
Hall, R.J., Henry, P.F.P., Bunck, C.M., 1999. Fifty-year trends in a box turtle population in Maryland. Biological Conservation 88, 165±172. Hays, G.C., Godley, B.J., Broderick, A.C., 1999. Long-term thermal conditions on the nesting beaches of green turtles on Ascension Island. Marine Ecology Progress Series 185, 297±299. Hays, G.C., Adams, C.R., Mortimer, J.A., Speakman, J.R., 1995. Inter- and intra-beach thermal variation for green turtle nests on Ascension Island, South Atlantic. Journal of the Marine Biological Association UK. 75, 405±411. Huxley, R., 1999. Historical overview of marine turtle exploitation on Ascension Island, South Atlantic, 1999. Marine Turtle Newsletter. 84, 7±9. King, W.F., 1982. Historical review of the decline of the green turtle and the hawksbill. In: Bjorndal, K.A. (Ed.), Biology and Conservation of Sea Turtles. Smithsonian Institute Press, Washington DC, pp. 183±188. Lahanas, P.N., Bjorndal, K.A., Bolten, A.B., Encalada, S.E., Miyamoto, M.M., Valverde, R.A., Bowen, B.W., 1998. Genetic composition of a green turtle (Chelonia mydas) feeding ground population: evidence for multiple origins. Marine Biology 130, 345±352. Limpus, C.J., Nicholas, N., 1987. The southern oscillation regulates the annual numbers of green turtles (Chelonia mydas) breeding around northern Australia. Australian Journal of Wildlife Research. 15, 157±161. Luschi, P., Hays, G.C., del Seppia, C., Marsh, R., Papi, F., 1998. The navigational feats of green sea turtles migrating from Ascension Island investigated by satellite telemetry. Proceedings of the Royal Society (B). 265, 2279±2284. Marcovaldi, M.A, Marcovaldi, G.G., 1999. Marine turtles in Brazil:the history and structure of Projecto TAMAR-IBAMA. Biological Conservation. 91, 35±41. Marcovaldi, M.AÃ., Baptistotte, C., De Castilhos, J.C., Gallo, B.M.G., Lima, E.H.S.M., Sanches, T.M. et al., 1998. Activities by Project TAMAR in Brazilian Sea Turtle Feeding Grounds. Marine Turtle Newsletter. 80, 5±7. Miller, J.D., 1996. Reproduction in sea turtles. In: Lutz, P.L., Musick, J.A. (Eds.), Biology of Sea Turtles. CRC Press, Boca Raton, pp. 51±81. Moriera, L., Baptistotte, C., Scalfone, J., Thome, J.C., De Almeida, A.P.L.S., 1995. Occurence of Chelonda mydas on the island of Trinidad, Brazil. Marine Turtle Newsletter 70, 2. Mortimer, J.A., 1982. Factors in¯uencing beach selection by nesting sea turtles. In: Bjorndal, K.A. (Ed.), Biology and Conservation of Sea Turtles. Smithsonian Institute Press, Washington, DC, pp. 45±52. Mortimer, J.A., 1990. The in¯uence of beach sand characteristics on the nesting behaviour and clutch survival of green turtles (Chelonia mydas). Copeia 1990, 802±817. Mortimer, J.A., 1992. An evaluation of the status of the Ascension Island green turtles and their nesting habitat in 1992. Unpublished Report to the Caribbean Conservation Corporation, 22pp. Mortimer, J.A., Carr, A., 1987. Reproduction and migrations of the Ascension Island green turtle (Chelonia mydas). Copeia 1987, 103± 113. Musick, J.A., Limpus, C.J., 1996. Habitat utilisation and migration in juvenile sea turtles. In: Lutz, P.L., Musick, J.A. (Eds.), The Biology of Sea Turtles. CRC Press, Boca Raton, pp. 137±165. Pritchard, P.C.H., 1976. Post-nesting movements of marine turtles (Chelonidae and Dermochelyidae) tagged in the Guianas. Copeia 1976, 749±754. Pritchard, P.C.H., 1996. Evolution, phylogeny and status. In: Lutz, P.L., Musick, J.A. (Eds.), The Biology of Sea Turtles. CRC Press, Boca Raton, pp. 1±28. Sarti, M.L., Eckert, S.A., Garcia, T., N, Barragan, A.R., 1996. Decline of the world's largest nesting assemblage of leatherback turtles. Marine Turtle Newsletter. 74, 2±5. Schroeder, B. Murphy, S., 1999 Population surveys (ground and aerial) on nesting beaches. In: Eckert, K.L., Bjorndal, K.A., Abreu-Grobois,
158
B.J. Godley et al. / Biological Conservation 97 (2001) 151±158
F.A., Donnelly, M. (Eds.), Research and Management Techniques for the Conservation of Sea Turtles. IUCN/SSC Marine Turtle Specialist Group, Washington, DC, Publication No 4, pp. 45±55. Schulz, J.P., 1975. Sea turtles nesting in Suriname. Zoologische Verhandelingen 143, 1±143.
Sole, G., Medina, G., 1989. The green turtles of Aves Island. In: Eckert, S.A., Eckert, K.A., Richardson, T.H. (Compilers). Proceedings of the Ninth Annual Workshop on Sea Turtle Conservation and Biology. NOAA Technical Memorandum NMFS-SEFC-232, pp. 171±173.