The influence of nesting zone on the hatching and emergence success of green turtles (Chelonia mydas) in Tortuguero, Costa Rica
Daveka Boodram’, Catalina Reyes’, Neil Osborne’, German Zapata’, and Juan Rapetti’
‘6 Kalpoo Street, Spring Village, Valsayn, Trinidad, West Indies
* Diagonal 177 No. 65-36 Int. 3, Colombia, South America, Bogota, Cundinamarca 1105, Columbia
*3226 Sovereign Road, Burlington, Ontario L7M 2V8, Canada
“Pablo Ardizzone 7079. B Don Bosco
* Pedro Figari 1668 Montevideo, Carrasco
INTRODUCTION
Favorable nest placement is important to the survival of sea turtle populations. Biotic and abiotic factors may influence offspring survival. The environment of the nest, including proximity to tidal zones and supralittoral vegetation, can influence the variables that affect nest success (Bustard and Greenham 1968). The location of nests in different zones may result in differential levels of mammalian predation or inundation rates or may expose nests to different sunlight levels, thereby affecting sand temperature and consequently sex ratios (Spotila et al. 1987). Consequently, the effects of these variables on the hatching and emergence success in relation to the location of the nests were evaluated
METHODS
Nests were marked between 10° June and 30° September 2001. All nests were classified into zones based on vegetation cover and exposure of the nest to sunlight (Open – complete exposure; Border — Vegetation – partial or no exposure). Data were collected on the northern 5 miles of the 22 mile beach from mile –2/8 to mile 5. Daily surveys of the marked nests were conducted from June to December 2001; poaching, predation, flooding by rainfall or seawater and other disturbances were noted. Nests were excavated after hatchlings had emerged or after the 70° day of the incubation period. The number of empty egg shells, pipped eggs, live and dead hatchlings, unhatched eggs without embryo, unhatched eggs with embryo, unhatched eggs with full embryo, depredated eggs, yolkless eggs, twins, deformed embryos and albinos were determined for each excavated nest. Only eggshells that were more than 50% of an egg were recorded as an empty eggshell. Additionally, sand temperature at 50cm was measured daily at the Caribbean Conservation Corporation field station.
To determine whether nest position on the beach had a significant effect on hatching survival the hatching and emergence success of the disturbed and undisturbed nests between the beach zones were estimated and compared using the MannWhitney U test. The sample size of the individual disturbance was too small to allow statistical analyses. Mann-Whitney U was also used to test the significance of the sand temperature between the zones. All tests were two-tailed and the cut off point for statistical significance was p=0.05.
Hatching success (HS) and Emergence success (ES) was calculated as:
HS=no. of empty shells x 100 Total number of eggs
ES=no. of empty shell – live hatchlinq x 100 Total number of eggs
RESULTS
Statistically there was no significant difference between hatching and emergence success of the disturbed nests in the open and border-vegetation. There was no significant difference between the hatching and emergence success of the undisturbed nest in the open and border- vegetation. Hatching and
emergence success was significantly different in undisturbed and disturbed nests in the open zone, (hatching; W= 1002.5, p < 0.0001; emergence: W=950.0, p<0.0001). Hatching and emergence success was significantly different in the undisturbed and disturbed nests in the border-vegetation, (hatching; W=394.0, p <0.0001; emergence: W=419.5, p<0.0001). The average sand temperature at 50cm was significantly higher in the open than in the border-vegetation, W=52.5, p<0.05 (Table 1). Nests were found to be affected by both biotic and a biotic factors (Fig. 1). Nests in the open were affected more by washing over and washing out, due to high tides caused by heavy rainfall. While nests in the border-vegetation were affected more by biotic factors such as depredation, poaching and digging up by other turtles. However, inundation caused by a combination of flooding and heavy rainfall also affected nests in the border-vegetation. Although statistically there was no significant difference in hatching and emergence success between different zones, both were generally higher in the open (Figs. 2,3). An exception to this finding was found for washed over nests that had a high hatching and emergence success in the border-vegetation.
DISCUSSION
Nest position is important in the egg survivorship of C. mydas. Nests laid in the border-vegetation are disturbed more by biotic factors and inundation than nests in the open where they are greatly disturbed by heavy rains and high tides. In Tortuguero, inundation results from an interaction of the degree of water saturation on the beach and the height of the waves (Horiskoshi 1992). Disturbance reduces the number of eggs in the nest or removes them totally, as a result a reduction in the reproductive success of the turtle is caused. On average, the hatching and emergence success of the disturbed nests was less than one third of undisturbed nests. Although, not proven statistically, this lower hatching and emergence success of the disturbed nests appears to be directly related to the zones. Disturbance occurs in both zones but the type of disturbance varies with the location of the nests.
With respect to the undisturbed nest the hatching success is about the same in both zones but the emergence success is slightly higher in the open zone. The lower emergence success in the border-vegetation zone is partly due to hatchlings being trapped by the roots of vegetation thereby preventing them from emerging. Furthermore ants and other animals present in vegetation sometimes attacked hatchlings in the nests before they emerged. Hatching success may be affected by many factors, such as gas exchange with the incubation medium (sand) and between eggs within the nest, bacterial growth, sand particle size, sand and atmospheric temperature, as well as sand moisture. Though sand temperature was the only variable measured, the temperature in the open zone was significantly higher than in the border-vegetation in each month. This factor may partly explain the higher hatching success in the open. However, more factors have to be measured in order to prove this.
It appears that no particular zone is completely ideal for maximizing hatching and emergence success. Nests can be disturbed in both zones. Nevertheless, the type of disturbance differs with the zones. Also the severity of disturbance differs in the zones. Washed out nests have 0% hatching and emergence success, and most of the washed out nests occurred in the open zone which is closest to the high tide mark. It is therefore bene
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LITERATURE CITED
Bustard, H.R. and P. Greenham. 1968. Physical and Chemical factors affecting hatchlings in the green sea turtle, Chelonia mydas. Ecology 49:269-276. Horikoshi, K. 1992. Egg survivorship and primary sex ratio of green turtles, Chelonia mydas, at Tortuguero, Costa Rica. Ph.D dissertation. University of Florida, Gainesville, Florida. Lahanas, P.N., A. Bjorndal, A.B. Bolten, S. Encalada, M.M. Miyamoto, R.A. Valverde, and B.W. Bowen. 1998. Genetic composition of a green turtle (Chelonia mydas) feeding ground population: evidence for multiple origins. Marine Biology 130:345-352. Spotila, J.R. E.A. Standora, S.J. Morreale and G.J. Ruiz. 1987. Temperature dependent sex determination in the green turtle (Chelonia mydas). Effect on the sex ratio on a natural nesting beach. Herpetologica 43:74-81.
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Status of green turtle (Chelonia mydas) populations nesting in the French West Indies
Johan Chevalier’ and participants to the Marine Turtle Conservation Program in the FWI’
‘ONCFS *AEVA, DIREN de Martinique, DIREN de Guadeloupe, PNR de Martinique, ONF, RN de Petite Terre, RN de St Barth, Brigade Vertes de Terre de Haut des Saintes
The French West Indies includes numerous islands located in the Lesser Antilles : Martinique, Guadeloupean Archipelago (Guadeloupe, Marie-Galante, Les Saintes), St Barthelemy and half of St Martin. A marine turtle conservation programs has recently started in all those islands. As in the initial stages of most new programs, one of the first objectives has been to review the population status of each nesting species. This work has brought attention to the case of the green turtle nesting
populations. While most of the large nesting populations in the region (Aves, Tortuguero, Ascension) show an encouraging trend, the situation of this species on the beaches of the FWI is very critical. This poster presents a summary of the current and past status of the green turtle nesting populations of the French West Indies and emphasizes the importance of the nesting populations of this species in the marine turtle conservation programs of the Lesser Antilles.
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Nesting of the hawksbill turtle in the southern Caribbean coast of Costa Rica
Didiher Chacon
Asociacion ANAI, Apdo. 170-2070, Sabanilla, San Jose, Costa Rica
INTRODUCTION
Palmer (1986), mentions that during the second half of the 18th Century, Afro-Caribbeans came to the Caribbean coast of Costa Rica from Bocas del Toro and the Nicaraguan coast, in search of turtles to catch, using harpoons. They arrived in March and left in September. They sold the carapaces in Bocas delToro, Panamá; to be exported to Germany and be used in the manufacturing of combs and buttons. Although there is little historical information before year 1950 and data is limited, Parsons (1962 mentioned in Groombridge and Luxmoore 1989) mentions that the annual catch of hawksbills around 1923 was 750 individuals. Groombridge and Luxmoore (1989) indicate that the area between Tortuguero and Matina, as well as the section between Cahuita and the Sixaola river host the greatest nesting of the species. There are three well-defined coral areas developed along the 212 km of coast between the borders to Nicaragua and Panamá: Uvita Island, Cahuita Point and the section Old Harbor-Monkey Point. Between the coral zones there are sandy beaches with varying stability (constantly eroding to very stable), rocky shores and mangroves (Fig. 1). The hawksbill sea turtle comes to shore for nesting and lay multiple clutches per season. Hawksbills turtles in the Caribbean typically produce 4 to 5 clutches at two weeks intervals. Males may breed annually, but females return to the nesting beach only after periods of 2 to 5 years. They begin feeding on benthic organisms and rare likely to remain sedentary for years or perhaps decades until reaching maturity an carapace lengths of 60 to 75 cm (van Dam 1997).
METHODS
Work began during the second week of February 1995. The Gandoca Beach, from the north side of Monkey Point to the mouth of the Sixaola River, was divided into 50 meter segments to facilitate mapping of the nests. The Black Beach, from the Cahuita Point to the Black Beach town, was divided into 90 meter segments. Each female encountered on the beach was tagged after oviposition, with a metal tag (monel 49). Tags were placed over the second axilar scale in the front flipper, following the methodology of Chacón et al. (1996). For each adult encountered on the beach some biometric parameters were taken according to the methodology in Eckert et al. (2000). When the monitors saw females, the nests encountered were examined directly digging the nest chamber and counting the eggs. Observations about the nests and nesting behavior were annotated. The monitors observed some nests around the season and check them after 60 days of incubation period.
RESULTS AND DISCUSSION
The hawksbill is rare in comparison with the abundance of green and leatherback turtles in Caribbean Costa Rica. At Gandoca Beach only over a hundred female nesters were counted during the period between 1995-2000 whilst on the same beach and period 1678 individual leatherback turtle were counted. It can be stated that the species nests inside, as well as outside of protected areas, that it likes to nest under vegetation and that nesting occurs from March to November. Since 1995 in Gandoca Beach and during the 2001 nesting season in Black Beach were collected the information of 148 nesting females, this data are presented in the Table 1. The CCL for Hawksbill Sea Turtles in Bay Islands and Cochinos Keys was 88,95 cm and the CCW was 79.28 (M. Arrone, pers, comm.); in Bocas del Toro region the Hawksbill Sea Turtles shows a CCL of 88,37 cm and a CCW of 73,43 cm (C. Ordoñez and I. Alvendas, pers, comm.).
The nesting peak was placed in July. (Fig. 1). Meylan (1983) establishes the nesting season as May to November with a peak in May-June. Bjorndal et al. (1985) and Troéng et al. (2001) found that the species may show a nesting pattern with two reproductive peaks. The first in May to late July and the second around October.
The Hawksbill Sea Turtles in the Caribbean of Honduras (Bay Island and Cochinos Keys) shows a mean of 161 eggs/nest, in the case of Punta Manabique in Guatemala that data were 143 egg/nest and in Bocas del Toro region in Panama the clutch shows 121 eggs/nest. (M. Arrone, W. Katzs, C. Ordoñez and I. Alvendas, pers, comm.). Bjorndal et al. (1985) determined a mean of 158 eggs per nest (n=93, S.D.-29) with a range of 86206 eggs, while Bravo (1983) determined the mean to 161 eggs/nest with a range of 56-206 eggs/nest. The range for nest size in the Caribbean is 101-161 eggs/nest (Meylan 1983). Hatching success has been documented by Chacón et al. (2001) as 91% for nests protected by REGAMA, whilst the success for natural nests from which hatchlings emerged was 58.3% with 91.6% of the nests producing hatchlings (Bjorndal et al. 1985). The nests in Utila Island, Honduras shows 81% of success rate (G. Pedersenn pers, comm.)
Tag returns from tags attached to nesting females in Gandoca shows a renesting interval of approximately 15 days for a maximum of three times, although the statistic mode is two times per season. The remigration range is between 2 and 3 years. On the other hand, Bjorndal et al. (1985) determined the period to be between 16 and 17 days for females nesting on the Tortuguero Beach. The last authors estimated that the mean number of days of incubation for the species is 58.5 days while this period was of 66.6 days for Gandoca Beach. Hawksbill nesting at REGAMA occurs with a mean of 5.5 nests per season for the period 1994-2000 (Chacón et al. 2001). Hawksbill nesting has been documented at Tortuguero National Park with a mean of 7.82 nests/season for the time period 1972-2000 (Meylan et al. 1997, S. Troéng, pers. comm.). Other areas such as Cahuita National Park, the beaches between Old Harbor and Monkey Point are areas where there exists nesting but where there has been no systematic or reliable nest data collection. Using data from Carr and Stancyk (1975), Bjorndal et al. (1993) and Unpublished Data from the Caribbean Conservation Corporation, Troéng (2001) concluded that the decline of the hawksbills along the north Caribbean coast of Costa Rica is -3.9%/year. With the information presented here the southern coast of Costa Rica confirmed that is the better region for the nesting Hawksbill Sea Turtle in the country, factor that is coincident with the kind of habitats present in the zone.
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