The Role of Geomagnetic Cues in Green Turtle Open Sea Navigation

Background: Laboratory and field experiments have provided evidence that sea turtles use geomagnetic cues to navigate in the open sea. For instance, green turtles (Chelonia mydas) displaced 100 km away from their nesting site were impaired in returning home when carrying a strong magnet glued on the head. However, the actual role of geomagnetic cues remains unclear, since magnetically treated green turtles can perform large scale (.2000 km) post-nesting migrations no differently from controls. Methodology/Principal Findings: In the present homing experiment, 24 green turtles were displaced 200 km away from their nesting site on an oceanic island, and tracked, for the first time in this type of experiment, with Global Positioning System (GPS), which is able to provide much more frequent and accurate locations than previously used tracking methods. Eight turtles were magnetically treated for 24–48 h on the nesting beach prior to displacement, and another eight turtles had a magnet glued on the head at the release site. The last eight turtles were used as controls. Detailed analyses of water masses-related (i.e., current-corrected) homing paths showed that magnetically treated turtles were able to navigate toward their nesting site as efficiently as controls, but those carrying magnets were significantly impaired once they arrived within 50 km of home. Conclusions/Significance: While green turtles do not seem to need geomagnetic cues to navigate far from the goal, these cues become necessary when turtles get closer to home. As the very last part of the homing trip (within a few kilometers of home) likely depends on non-magnetic cues, our results suggest that magnetic cues play a key role in sea turtle navigation at an intermediate scale by bridging the gap between large and small scale navigational processes, which both appear to depend on non-magnetic cues

Oceanic survival and movements of wild and captive-reared immature green turtles (

The ability of captive-reared turtles to survive in the wild is not precisely known, nor are movements of immature turtles in the open ocean. To provide information on these issues, a satellite tracking experiment was conducted in the western Indian Ocean to monitor oceanic movements of immature green turtles. Two wild turtles and four captive-reared individuals were tracked. The latter had been displaced after birth from nesting sites to a distant rearing site. Wild turtles survived after release, but did not move far away from release site. We hypothesize that this resident behaviour may be explained by stage-specific habitat requirements. Captive-reared turtles survived after release and migrated over thousands of kilometres. Among these, the oldest immature turtles retrieved the foraging sites of their native population, with movement patterns similar to those displayed by adults. Observed movements may be linked to hydrographic conditions such as general oceanic circulation, sea temperature and thermal fronts

Ingestion and defecation of marine debris by loggerhead sea turtles, Caretta caretta, from by-catches in the South-West Indian Ocean

Marine debris, caused by anthropogenic pollution, is a major problem impacting marine wildlife worldwide. This study documents and quantifies the ingestion and defecation of debris by 74 loggerhead sea turtles, Caretta caretta, in the South-West Indian Ocean. Debris was found in 51.4% of gut or fecal samples of loggerheads by-catch from Reunion Island long liners. Anthropogenic debris was ubiquitous in our samples with plastics accounting for 96.2% of the total debris collected. No significant relationship was detected between the characteristics of ingested debris and the biometric characteristics of loggerheads. The number, weight, volume and mean length of debris were higher in gut content of deceased loggerheads than in fecal samples of live turtles, but not significantly, except for the mean length. This is the first record of debris ingestion by sea turtles in the Indian Ocean and our results highlight the magnitude of this pollution of the marine environment.7 page(s

Green sea turtle, Chelonia mydas, feeding on Synapta maculata (Holothuroidea: Synaptidae) on a seagrass bed (Syringodium isoetifolium) at Reunion Island, western Indian Ocean

Young green turtles, Chelonia mydas, frequent the Syringodium isoetifolium seagrass beds on Reunion’s inner reef flats of fringing reefs, where they come to feed. Photographs of a green turtle looking for and eating the snake sea cucumber. Synapta maculata, were taken for the first time at Reunion Island. S. maculata is a sea cucumber species that was frequently observed on the inner reef flats and on the back-reef depression, but these abundances seem to be decreasing in recent years. Studies of S. maculata populations are needed to monitor changes in densities of this species, which has an important ecosystem role, as well as other observations to determine whether this case of green sea turtle predation is an isolated one or whether S. maculata may occasionally be part of the diet of young C. mydas in Reunion

Aerial and snorkelling census techniques for estimating green turtle abundance on foraging areas: A pilot study in Mayotte Island (Indian Ocean)

Monitoring the abundance of green turtles (Chelonia mydas) is necessary to assess population trends and risks of collapse. This note presents a study aimed at comparing three techniques for the direct estimation of green turtle numbers in their foraging habitats (seagrass beds and reef flats). The experiment was carried out at Mayotte Island, Western Indian Ocean. The techniques involved were surveys by snorkel, and aerial surveys using a microlight aircraft and a paramotor. Each technique had shortcomings and advantages. While each technique provided estimations of turtle numbers only surveys by snorkel permitted identification of species and sex, whenever visibility and turtle behaviour permitted. Along the shorelines, and over foraging areas, the paramotor was found to be most suitable for direct estimations of turtle numbers. The major advantage of this technique lied in its capability to obtain a synoptic snapshot of turtle distribution over foraging areas. Linear surveys from a microlight aircraft are better suited to monitor foraging areas located further away from the shore

Habitat use of a multispecific seagrass meadow by green turtles Chelonia mydas at Mayotte island

International audienceWe investigated the habitat use in green turtles exploiting a 13-ha multispecific seagrass meadow at Mayotte Island, south-western Indian Ocean. A phytoecological survey shows the occurrence of eight seagrass species, dominated by Halodule uninervis and Syringodium isoetifolium, distributed according to four distinct seagrass communities along the depth gradient. Direct underwater censuses show that green turtles occurred all over the meadow. Yet when community relative surface area was taken into account green turtles preferentially frequented the most seaward, biomass-richer S. isoetifolium-dominated community, suggesting that green turtles compensate for their intrinsically nutrient-poor herbivorous diet. Additionally, smaller (\80 cm standard curved carapace length, SCCL) individuals also preferentially occurred in the most shoreward H. univervis-dominated community where no larger ([80 cm SCCL) individuals were sighted, suggesting habitat use is indicative of diet selection and may reflect size-specific food requirements and physiolog

Individual Based Modeling of Green Turtle Spatial Dynamics in the Southwest Indian Ocean

International audienceSouthwest Indian Ocean (SWIO) represents a noteworthy region to study green turtle Chelonia mydas spatial ecology. Indeed, spatial complexity of the region and presence of numerous nesting sites (mostly islands) as well as vast nesting areas (mostly East-Africa and Madagascar) is a unique opportunity to understand the role of space in turtle ecology. For more than twenty years now, a number of effective studies have been conducted locally in major areas. Due to recent progress in wildlife tracking, movements of marine turtles between those sites are starting to be well-documented. Here, we propose an original approach to integrate this knowledge using a spatially explicit individual-based model

Modeling Spatial Population Dynamics of Green Turtle (Chelonia Mydas) in the Southwest of Indian Ocean

International audienceMore than twenty years of active research on green turtle (Chelonia mydas) in the South West Indian Ocean (SWIO) have contributed to improve the knowledge of the species biology and ecology. However, lots of gaps still remain regarding the links between the main behavioral processes: alimentation, reproduction and migration, while shifts in these processes at individual scale have major impacts at population scale. On this basis, we have started implementing a spatially explicit individual-based model (IBM) to assess population dynamics of the green turtle in the SWIO. Our simulation experiment aims to unify, in a single model, alimentation, navigation and reproduction constraints as well as potential changes in foraging or breeding sites