Although the presence of four sea turtle species (Chelonia mydas, Dermochelys coriacea, Lepidochelys olivacea and Eretmochelys imbricata) nesting in Bioko Island, Equatorial Guinea (E.G.), has been known for a decade (Butynski & Koster 1989), critical population monitoring was not carried out until recently. In October 1996, the Spanish NGO Asociación Amigos de Doñana, in collaboration with the University of Valencia, started a tagging programme on the beaches of the south of Bioko (between 8°23’E-3°16’N and 8°40’E-3°13’N). This programme continued for two nesting seasons, until March 1998. Tagging focused mainly on the green turtle, the most abundant of the four species (Tomas et al. 1999; 2000). Analysis of recapture data within nesting seasons resulted in a preliminary estimation of a population size of 400-600 female green turtles per season (Tomas et al. 1999; 2000). Based on this estimate, South Bioko should be classified as one of the most important nesting areas for the green sea turtle in central Africa, and surely as the most important in the Gulf of Guinea (Tomas et al. 1999).
Materials and Methods
During the nesting season of 1996/1997, we tagged 196 green turtles, and in 1997/98, we tagged 15 more. Of these 211, 168 were marked with two tags in both front flippers, and the rest with one tag in one front flipper. We used yellow plastic cattle-ear tags, with the inscription: BIOKO-SUR APTDO. 2182, 41080 SEVILLA-SPAIN. In the entire Gulf of Guinea, this type of tag was used only in Bioko.
We also measured curved carapace length from notch to tip (CCLn-t) (Bolten 1999). Recaptures were reported by one of the authors (A.F.), by other researchers and NGOs working in the area, or directly by fishermen. The minimum distance between the tagging site and the recapture site was calculated (error = ±10 km).
Results and Discussion
Twelve green turtles were recaptured away from the nesting grounds in the 3 years since tagging began (Table 1). With respect to the tag recoveries from Cap Esterias and Cameroon, we were not notified of the total number of captured tagged individuals, so we include the minimum estimate of one recapture per site. In addition, we know of 4 tags, corresponding to 2-4 Bioko turtles, which were captured by fishermen in Corisco waters for sale in Bata (E.G.) or in Libreville (Gabón), but we were unable to examine these tags to confirm their numbers. Four of the 12 turtles migrated westward, one remained close to Bioko and the others migrated to the south: 6 to Corisco Bay and the north of Gabon, and one further away, reaching southern Gabon. Minimum migration distances ranged from 130 km to 1250 km (Table 1, Figure 1). The turtle size was positively correlated to the distance from the place of tagging to the place of recapture (n= 10, r= 0.749, p= 0.013). However, a larger sample size and/or analyses with satellite telemetry is necessary to confirm that larger turtles travel greater distances than smaller ones.
Based on number of recaptures and the additional information collected, we suggest that the area of Corisco Bay is a frequent destination of post-nesting green turtles from Bioko, and probably represents one of the main foraging grounds for this population. The area harbours extensive beds of seagrass and algae, forming a suitable feeding habitat for this species (Formia 1999). However, the recaptures in Ghana and southern Gabon suggest that the post nesting dispersal from Bioko is not restricted to Corisco Bay, and that there may be other important green turtle feeding grounds on the Atlantic continental shelf of Africa. Green turtles from the same nesting beach can disperse to different foraging areas, as shown by the present study and in other parts of the world (Cheng & Balazs 1998; Solé 1994). Moreover, more than one nesting population can share the same feeding grounds (Bass et al. 1998). Thus, the Bioko nesting population may be mixing with turtles from other nesting populations while feeding in Corisco Bay. Genetic analysis is currently being carried out by A.F. in order to identify the breeding stock origins of the Corisco Bay feeding population, as well as the distribution of the Bioko nesting population and the extent of its contribution to the feeding aggregate. In addition, we recommend tagging and satellite tracking programs in this feeding area to further elucidate its composition.
Some of the recapture information obtained from fishermen was imprecise,
probably because they lacked the means to effectively communicate tag data or
because of conflicting interests. In fact, fishermen may be reticent to admit
capturing a tagged turtle and may keep a tag for months, until they see the
opportunity of obtaining economic benefit from it. Offering rewards for tag
recovery information might be a useful strategy to maximise recapture data, but
should be used with caution. However, based on our experience, in some cases an
acknowledgement letter may be enough reward. Nonetheless, our data are useful as
a basis for future work in the area and to strengthen the case for protection
The green sea turtle is seriously threatened throughout the Gulf of Guinea due to capture for consumption both at nesting beaches and in foraging habitats. We recommend the implementation of efficient educational programmes, as well as the establishment of compensatory funds to provide economic alternatives to turtle hunting. Encouraging the collaboration of local people is essential to achieving conservation goals.
In addition to local fisheries, oil exploitation, with its corresponding seismic surveys, is becoming a major threat to turtles in the Gulf of Guinea. Such exploitation and exploration activities are developing offshore of Bioko and Rio Muni (continental Equatorial Guinea), and even in the Corisco area <http://www.eia.doe.gov/emeu/cabs/eqguinea.html>. Although the physical effects on sea turtles of seismic surveys and other activities related to the oil industry are still relatively unknown, significant impacts may include noise disturbance and increased collisions with vessels (Pendoley 1997), and also water contamination from spills and light pollution from platforms and gas flares. Long-term drilling for oil may result in the removal of the sea turtles from their natural nesting and feeding habitats.
Acknowledgements: Special thanks to Ramon Castelo, of the Asociación Amigos de Doñana, and Jacques Fretey for supplying information, comments and field assistance. We also thank Guy-Philippe Sounguet, of Aventures Sans Frontières and the local fishermen of the different recapture sites, for their collaboration. We wish to express our gratitude for the economic support of the project ECOFAC (funded by the European Union) and the collaboration of the Equatorial Guinea authorities (especially the Ministry of Fisheries and Forests) and the people of Ureca village (South Bioko).
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