Individual variation in home-range across an ocean basin and links to habitat quality and management.
Abstract
Overgrazing may lead to management intervention (e.g. culling, animal relocation) to try and prevent habitat destruction. Overgrazing leading to seagrass meadow collapse has been recorded for green turtles (Chelonia mydas) at several sites around the world, although the generality of this phenomenon and the need for intervention to prevent widespread seagrass destruction is unknown. Where turtles have degraded seagrass meadows, home-ranges are expected to be large and turtles will relocate as meadows are destroyed. We used high resolution Fastloc GPS tracking (n = 32 individuals, mean = 171 days per individual, SD = 99) to record the home-range of adult green turtles at foraging sites spanning 4523 km of longitude across the Western Indian Ocean. Contrary to predictions if overgrazing was occurring, we recorded small home-ranges and turtles rarely relocated their daytime foraging areas. Based on all locations received, the mean 50% and 95% utilisation distributions (UD50 and UD95) were 2.4 km2 (SD = 2.7) and 15.4 km2 (SD = 17.7). Space use was often particularly small at night, when turtles rest, averaging 11% of the overall space use with the mean night-time UD50 and UD95 being 0.15 km2 (SD = 0.1) and 1.1 km2 (SD = 0.8), respectively. Variation in home-range across individuals was not influenced by the data volume (number of locations per day, duration of tracking) or animal size (carapace length) but increased significantly as the distance between the centre of day and night areas increased, that is individuals that had a larger daily commute had the larger home-ranges. Synthesis and applications. Comparisons with home-range estimates from 16 previous studies, showed that those we recorded are among the smallest for adult green turtles globally. These results suggest that despite population size increases at several major nesting sites in the Western Indian Ocean, green turtles are generally not destroying the seagrass meadows on which they forage and so management intervention to prevent overgrazing is not needed. In this way, our work illustrates how movement data may inform management decisions for green turtles. Further targeted work on the seagrass ecosystem health could help confirm this suggestion.