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

More documents

Recommendations

Info

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
Page 1: Biological Conservation 97 (2001) 1
Page 5 and 6: B.J. Godley et al. / Biological Con
Page 7 and 8: B.J. Godley et al. / Biological Con

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

Create successful ePaper yourself

Delete template?

Save as template?