Residency and habitat use patterns by sympatric stingrays at a remote atoll in the Western Indian Ocean

Abstract

Information on the spatial ecology of batoids is lacking despite the need for this information for effective management and conservation of this vulnerable faunal group. We used acoustic telemetry to monitor the presence of dasyatids (Pastinachus ater, Urogymnus granulatus, and U. asperrimus) from March 2014 to November 2017 within and around the isolated St. Joseph Atoll, Seychelles. Results highlighted residency (median residency index = 0.75 and 0.57 for P. ater and U. granulatus) over periods of months to years for both juveniles and adults. Individuals displayed highly restricted movements; most detections occurred within 1 km of tagging locations, and movement networks were small and fragmented. However, juveniles increased their range of movements with size before dispersing to various locations on the Amirantes Bank. Within the atoll, resident adults and juveniles segregated ontogenetically by habitat; adults were more reliant on the deeper lagoon, while juveniles were largely restricted to the shallow sand flats, likely in an effort to avoid predators. Conversely, no inter-specific space partitioning was identified. This study provides preliminary evidence that this atoll is a dasyatid nursery; however, comparative research at other atolls is required to understand the role that atolls may play as batoid nurseries. This study was performed in the South West Indian Ocean, an understudied fisheries conservation hotspot where biodiversity is high but management effectiveness is low. The identification of a dasyatid refuge in this region highlights that the current marine protected area designation will likely provide conservation benefits for the dasyatid populations across the entire Amirantes Bank.