Ocean ecosystems have been severely damaged by human pressures (Roberts, Reference Roberts2003) and marine turtles are one of the most affected groups (IUCN, 2007). Major rookeries of loggerhead Caretta caretta (Turtle Expert Working Group, 2000), green Chelonia mydas (Troëng & Rankin, Reference Troëng and Rankin2005) and leatherback marine turtles Dermochelys coriacea (Fossette et al., Reference Fossette, Kelle, Girondot, Goverse, Hilterman and Verhage2008) are reported on the western shores of the Atlantic Ocean. In comparison, the olive ridley marine turtle Lepidochelys olivacea is less abundant, and its phylogeography indicates a recent colonization of the Atlantic (Bowen et al., Reference Bowen, Clark, Abreu-Grobois, Chaves, Reichart and Ferl1998).

The beaches of Suriname and French Guiana host major marine turtle aggregations (Reichart & Fretey, Reference Reichart and Fretey1993; Girondot et al., Reference Girondot, Godfrey, Ponge and Rivalan2007). Suriname was formerly identified as an important olive ridley turtle nesting site (Pritchard, Reference Pritchard1973) but the number of nests decreased from 2,875 in 1967 to 1,070 in 1975 (Schulz, Reference Schulz1975), and to 585 in 1989 (Reichart, Reference Reichart1993). Monitoring was interrupted during the early 1990s when the main nesting sites were occupied by rebels (Reichart, Reference Reichart1993). In 1995 only 335 olive ridley turtle nests were recorded (Hoeckert et al., Reference Hoeckert, Schouten, Van Tienen and Weijerman1996). Because of the difficulty of accessing some nesting sites only fragmentary information on olive ridley turtle nesting has been collected recently in Suriname (Fig. 1). Estimates for the early 2000s, based on limited monitoring efforts, were 150–200 olive ridley turtle nests per year (Hilterman et al., Reference Hilterman, Tordoir, Goverse, Reichart, Kalb, Rohde, Gayheart and Shanker2008).

Fig. 1 Olive ridley turtle nest numbers in Suriname and French Guiana, with median error bars (Gratiot et al., Reference Gratiot, Gratiot, Kelle and de Thoisy2006), during 1967–2007. Suriname data: 1967–1975 from Schulz (Reference Schulz1975), 1976–1989 from Reichart (Reference Reichart1993), 1994–1999 from Mohadin (Reference Mohadin, Kelle, Lochon, Thérèse and Desbois2000), and 2002 from de Djin (2003). Note that monitoring coverage is not detailed in Mohadin (Reference Mohadin, Kelle, Lochon, Thérèse and Desbois2000), and that de Djin (2003) considered the data to be incomplete. French Guiana data: 1987 from Fretey (Reference Fretey1989), when monitoring coverage did not include Cayenne Peninsula. Cayenne Peninsula was the only nesting area monitored in 2000 and 2001. Data for 2002–2007 are the minimum numbers of nests laid, as some minor nesting sites were not continuously monitored.

Historical data on olive ridley turtles in French Guiana are sporadic. Reports indicated the presence of the species on the coast (Fretey & Lescure, Reference Fretey and Lescure1979), and nesting appeared to be common on the Cayenne peninsula in the early 1970s (J. Fretey, pers. comm.). A survey in 1987 recorded 582 nests (Fretey, Reference Fretey1989; Fig. 1). Intensive monitoring was implemented in 2000 on the Cayenne peninsula (Talvy & Vié, Reference Talvy, Vié, Kelle, Lochon, Thérèse and Desbois2000); this facilitated the development of a model that defines the temporal distribution and numbers of olive ridley nesting, with a correlation of r ≥ 0.97 (Gratiot et al., Reference Gratiot, Gratiot, Kelle and de Thoisy2006). Fragmentary data from other beaches (Amana Nature Reserve and Kourou beaches, monitored since 2002) extrapolated using this model gave estimations for each nest site for the entire nesting season, with errors of 10–15% when the monitoring lasted ≥ 50 days (Gratiot et al., Reference Gratiot, Gratiot, Kelle and de Thoisy2006). Based on this extended monitoring coverage we estimated 1,716–3,257 olive ridley turtle nests per year for 2002–2007 in French Guiana. These numbers are the highest reproduction rates recorded for the species in the north-west Atlantic since 1967–1970, when Schulz (Reference Schulz1975) estimated 1,665–3,290 nests in Suriname (Fig. 1).

As suggested by Marcovaldi (Reference Marcovaldi, Eckert and Abreu-Grobois2001) we consider that three hypotheses could potentially explain the high number of olive ridley turtle nests recorded in French Guiana since 2002. Firstly, the population recovery could be due to long-term conservation efforts (Hays, Reference Hays2004). Because of over-harvesting of eggs (Reichart, Reference Reichart1993) and incidental catch related to shrimp trawling (Reichart & Fretey, Reference Reichart and Fretey1993) the olive ridley turtle was formerly considered the most threatened marine turtle in the Guianas (Guyana, Suriname and French Guiana; Tambiah, Reference Tambiah, Bjorndal, Bolten, Johnson and Eliazar1994). On beaches, egg poaching is limited to 5% of clutches (ONCFS, pers. comm.) in French Guiana. However, the olive ridley is the marine turtle most vulnerable to offshore shrimp trawling in French Guiana, and informal interviews suggested that c. 1,000 olive ridley turtles are caught annually (Gueguen, Reference Gueguen2000). Turtle Excluder Devices are not mandatory in French Guiana, as they have been in Suriname since 1992 (Mohadin, Reference Mohadin, Kelle, Lochon, Thérèse and Desbois2000). Thus, it seems unlikely that the resident population of olive ridley turtles in French Guiana has recovered because of long-term conservation.

Secondly, nesting females could have moved south from Suriname to French Guiana beaches. In the Guianas natural modifications of the coastline can cause marine turtles to change their nesting location (Hilterman et al., Reference Hilterman, Tordoir, Goverse, Reichart, Kalb, Rohde, Gayheart and Shanker2008), as happened with the leatherback turtles now nesting at Awala Yalimapo in French Guiana (Girondot et al., Reference Girondot, Godfrey, Ponge and Rivalan2007; Kelle et al., Reference Kelle, Gratiot, Nolibos, Thérèse, Wongsopawiro and de Thoisy2007).

Thirdly, the species’ occurrence in French Guiana could have been underestimated until 2000. Since the late 1970s marine turtle monitoring was mostly focused on leatherback turtle nesting beaches, and current major olive ridley turtle nesting sites (Kourou, Cayenne Peninsula) were rarely visited. Also, the olive ridley turtle sometimes adopts a particular nesting behaviour, coming ashore in large waves known as arribadas (Pritchard, Reference Pritchard1967). Between such events few adults come ashore, resulting in underestimation of nesting activity. As monitoring was limited in French Guiana until 2000, the species’ occurrence has probably remained underestimated there.

Only long-term monitoring at a large geographical scale is suitable for the assessment of marine turtle nesting trends (Schroeder & Murphy, Reference Schroeder, Murphy, Eckert, Bjorndal, Abreu-Grobois and Donnelly1999). In Suriname and French Guiana monitoring of olive ridley turtles has been both temporally and spatially heterogeneous and, therefore, no regional nesting trend can be identified. Based on nest numbers it appears that the population in French Guiana is only c. 2,000 adults, and is nearly extirpated as a breeding species in Guyana and Suriname. Despite a reported increase of the species in Brazil (de Castilhos & Tiwari, Reference de Castilhos and Tiwari2006) olive ridley turtles in the West Atlantic still face interactions with non-selective trawling gears, both in French Guiana and Brazil (Gueguen, Reference Gueguen2000; da Silva et al., Reference da Silva, de Castilhos, Lopez and Barata2007). As a consequence the olive ridley turtle continues to be threatened in the West Atlantic, despite a revision of its global IUCN Red List status from Endangered to Vulnerable (IUCN, 2008). Such an apparent discrepancy between local and global Red List assessments reinforces the need for regional assessments for such widely distributed species (Seminoff, Reference Seminoff2004; Mast et al., Reference Mast, Seminoff, Hutchinson and Pilcher2006).

The apparent increase in nesting by the olive ridley marine turtle in French Guiana therefore probably reflects the combination of a potential shift in nesting from Suriname to French Guiana and increased monitoring. Further investigation of the olive ridley turtle population structure in the West Atlantic is required. A transnational programme focusing on olive ridley turtles has recently been developed between Suriname and French Guiana, with joint genetic and satellite tracking research. This will produce a better understanding of the genetic signatures of rookeries and elucidate the species’ behaviour at sea. Similarly, such a collaborative initiative could be developed with Brazil to clarify gene flow between Brazilian and French Guiana nesting cohorts, the two main olive ridley turtle nesting aggregations in the West Atlantic. This would facilitate the identification of potential conservation units and the development of a comprehensive conservation strategy for the olive ridley marine turtle in the West Atlantic.