Seasonal diving behaviour and feeding rhythms of green turtles at Mayotte Island
Résumé
In long-distance migratory marine species foraging behaviour remains particularly difficult to study even though it has important consequences for individual life history. Indeed, studies assessing concurrent dive patterns and feeding behaviour remain rare. We investigated the daily and seasonal feeding rhythms of green turtles Chelonia mydas on a coastal seagrass meadow at Mayotte, in the South Western Indian Ocean. Between 2005 and 2008, the behaviour of 19 green turtles (body mass: 86.8 to 134.0 kg) was recorded using electronic time-temperature-depth recorders, concurrently validated by direct in-water observations. Additionally, fast- acquisition global positioning system units were deployed on 3 of these turtles. Green turtles showed a high fidelity to the foraging site and conducted predominately flat-bottom dives, in accordance with both local bathymetry and tidal regime. On a daily basis, 2 main dive categories were identified: shortshallow foraging dives (mean ± SE: 6.0 ± 0.0 min for 2.0 ± 0.0 m) occurred during the day on the seagrass meadow, and long-deep resting dives (50.9 ± 19.0 min for 10.3 ± 1.4 m) occurred at night in coral and rocky areas. On a seasonal basis, mean dive duration increased from summer to winter (from 9.5 ± 0.5 to 15.7 ± 1.1 min) and was negatively correlated with seasonal water temperature (range: 25.7°C in winter to 29.3°C in summer). However, the daily foraging duration (~11 h d-1) did not vary significantly with season. Investigating green turtle foraging patterns is crucial for an understanding of their contribution to coastal ecosystem functioning.