1CEDTM-Kélonia, 97436 St Leu, La Réunion (France)
2ADSEI Itsamia, Moheli (Union of Comoros)
Mohéli is the most important site of the Comoros Archipelago for the reproduction of Chelonia mydas (Frazier, 1985). Created in 2001, the Mohéli Marine Park (MMP) includes all the southern area from Miringoni Itsamia covering more than 40000 Ha. The fringing reef surrounding the MMP is well developed and between 250-1300m wide. Itsamia is the Eastern village of the MMP. Since 1994, the Association pour le Developpement Socio- Economique d’Itsamia (ADSEI) has been preserving the importance of the beaches of Itsamia for green turtles through protective actions. Currently, the beaches of Itsamia are one of the more important nesting sites for Chelonia mydas in the South West of the Indian Ocean. About 3000 females nest per year on the five beaches monitored daily by the ADSEI/MMP field guards (ADSEI, unpublished data) as part of a monitoring program that has been running since 1998. A few Eretmochelys imbricata are observed too. The present study describes data collected between April and June 2009.
Each female that laid was measured (curved carapace length – CCL) and tagged with a monel tag. Sixty nests were monitored to determine clutch size and emergence success (Miller, 1999). During the ovoposition, 10 eggs were measured and weighed from each nest. A Wemco temperature logger was positioned in the centre of the clutch for 30 nests. The nest location was indicated by a net. These nets were examined twice per day to monitor if the emergence had occurred. Fifteen emerged individuals per nest were measured (straight carapace length – SCL) and weighed before being released on the beach. Once 72 hours had passed since the last emergence, nests were excavated to categorize nest contents and determine hatchling and emergence success (Miller, 1999).
Means are presented with associated standard deviations. During the study 742 females were measured, and the mean CCL was 108.12 ± 5.29cm (range: 92-129). The average clutch size was 116 ± 24 eggs. The mean egg size and weight were respectively 42.38 ± 1.32 mm and 42.5 0± 3.88g. The mean length and weight of hatchlings was 47.43 ± 1.51mm and 21.42 ± 1.85g. Three of the 60 nests monitored were destroyed by females during the study, and two nests laid by the same female did not hatch. The hatching success was 75.3 ± 33.37 % (range 0-146; n=57). The emergence success is 64.80 ± 29.00% (range 0-100; n=57).
Nesting females dug up four of the 30 temperature loggers during the study. Table 1 and Figure 2 suggest that nest temperature may be different between the two beaches studied, although the sample size at Itsamia is small.
Figure 2: Temperature profile for two nests (It 33 and Ts 57) and three control loggers: Air temperature (Temp. Ext), sand temperature at 70 cm deep on Itsamia beach (Temp. It), and sand temperatureMtsanga nyamba beach (Temp. Ts). The vertical bars indicate the thirds of the incubation. Itsamia (Mohéli – Union of Comoros).
The mean temperature during the 2nd third of the incubation determines the sex ratio of the clutch (Yntema & Mrosovsky, 1980; Davenport, 1997; Godley et al., 2002). Even if the precise pivotal temperature is unknown for the species in Mohéli, the temperature during the 2nd third of the incubation period for the nests monitored was higher than 30°C during the study (Figure 2). So the sex ratio may be biased in favour of the females. Additionally, analysis of the temperature profiles shows that for 11 of the 26 nests the critical temperature of 35°C was exceeded during the incubation. The mean duration spent beyond this temperature was 11.55 ± 7.81 days (range=1.4-23.9, n=11).
This study confirms the importance of Itsamia as a site for green turtle reproduction, with 742 females tagged during four months on two of the five beaches of Itsamia. Clutch size was close to the mean number of eggs for Chelonia mydas (113 ± 4 eggs, Miller, 1997). Egg size and weight in Itsamia was slightly lower than the mean values for the species (44.9 ± 0.7 mm and 46.1 ± 1.6 g, Miller, 1997). On the other hand, the size and weight of hatchlings was similar to the mean values for the species (46 – 57 mm and 20 – 25 g, Pritchard & Mortimer, 1999). The fact than the temperature in some nests exceeded 35°C can explain the relative low hatching and emergence rate observed during the study, and may have a significant impact on marine turtle reproduction in the context of global warning, though more data are needed to confirm this hypothesis.
Thanks to the ADSEI members for their involvement in the monitoring and the conservation of sea turtles in a difficult context, and the Regional Council of La Reunion and European Union for their financial support.
Davenport, J. 1997. Temperature and the life-history strategies of sea turtles. Journal of Biology 22: 479-488.
Frazier, J. 1985. Marine Turtles in the Comoro Archipelago. North-Holland Publishing Company, Amsterdam, 177 p.
Godley, B.J., A.C. Broderick, F. Glen & G.C. Hays. 2002. Temperature-dependent sex determination of Ascension Island green turtles. Marine Ecology Progress Series 226: 115-124.
Miller, J.D. 1997. Reproduction in sea turtles. In: Biology of sea turtles (eds. Lutz P.L. & J.A. Musick), pp 51-81. CRC Marine Science Series.
Miller, J.D. 1999. Determining clutch size and hatching success. In: Research and Management Technique for Conservation of Sea Turtles (eds. Eckert K.L., K.A. Bjorndal, F.A. Abreu-Grobois & M. Donnelly), pp 124-129. IUCN/SSC Marine Turtle Specialist Group Publication.
Pritchard, P.C.H. & J.A. Mortimer. 1999. Taxonomy, external morphology and species identification. In: Research and Management Technique for Conservation of Sea Turtles (eds. Eckert K.L., K.A. Bjorndal, F.A. Abreu-Grobois & M. Donnelly), pp 21-40. IUCN/SSC Marine Turtle Specialist Group Publication.
Yntema, C.L. & N. Mrosovsky. 1980. Sexual differentiation in hatchling loggerheads (Caretta caretta) incubated at different controlled temperatures. Herpetologica 36: 33-36.
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