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.

Stingray Habitat Use Is Dynamically Influenced by Temperature and Tides

“…Conversely, while sharks have also been shown to increase their use of the flats at high tide in the atoll (Lea, 2017;Lea et al, 2020), this habitat likely still provides refuge opportunities for dorsoventrally-flattened stingrays that can make use of shallow island fringes or sand banks that are more inaccessible to sharks. Further support for this predator avoidance hypothesis comes from the evidence that juveniles preferentially occurred on the flats, whilst adults (which are at lesser risk to predation) preferentially occurred in the lagoon (Elston et al, 2021). This behavior has been observed in an inter-tidal bay, where U. granulatus juveniles moved in a directed way during flooding and ebbing tides, remaining in shallow water, which was hypothesized to be a predator avoidance strategy (Davy et al, 2015).…”

“…Detections on the flats occurred throughout the tidal cycle (although were somewhat more frequent at high tides), while detections in the lagoon occurred at a significantly higher proportion at low tides. Given that stingrays are dorsoventrally flattened, they can occur in very shallow waters and indeed, the majority of detections for these stingrays occurred on the shallow flats (Elston et al, 2021). The high use of the flats by stingrays was likely a function of predator avoidance.…”

“…The lagoon is mostly comprised of a sandy benthos with numerous large poriitid and flaviid coral outcrops that rise to the surface. The atoll supports an abundance of marine megafauna including sharks, rays and turtles (Filmalter et al, 2013;Lea et al, 2016), and has been identified as a nursery area for three stingray species (Pastinachus sephen, Urogymnus granulatus, and Urogymnus asperrimus) (Elston et al, 2021).…”

“…Between March 2014 and May 2015, a total of 60 stingrays, comprising 20 individuals from three different species (P. sephen, U. granulatus, and U. asperrimus) were surgically implanted with acoustic transmitters. Details and of capture and surgery procedures can be found in Elston et al 2021). Individuals consisted of both males and females and spanned the range of sizes present in the atoll (Table 1).…”

“…These receivers were spread across the reef flats and lagoon habitats, as well as on the external fringing reef of the atoll. As detections on receivers along this fringe reef were negligible (Elston et al, 2021), analyses looking at habitat use only considered detections in the reef flat and lagoon habitat. Receiver installation was completed in November 2014 and only detection data recorded subsequent to this date were included in the analyses.…”