Insights into the structure of grey reef shark aggregation, Carcharhinus amblyrhynchos (Bleeker, 1856), in two distinct channels of the Maldivian archipelago, Indian Ocean

The grey reef shark Carcharhinus amblyrhynchos is one of the most frequently encountered reef-associated shark species in the Maldives, yet very little is known about its local size structure or population dynamics. This study provides new insights into two aggregations of C. amblyrhynchos in the Maldives, based on biometric measurements obtained with laser photogrammetry, standardized visual surveys, opportunistic behavioural observations, and video analyses of citizen science data spanning 2013–2024. In particular, between 2022 and 2023, biometric data and visual observations from two distinct sites, Villingili Kandu in North Huvadhu Atoll and Miyaru Kandu in Vaavu Atoll, were collected to preliminarily characterize the composition of these aggregations and to provide information on the ecology of grey reef sharks. The results revealed a pronounced female-biased sex composition and occurrences of mating in both channels, as well as a potential nursery area in Miyaru Kandu. Combining these data with predation and cleaning events recorded, we suggest that these channels might constitute important key habitats for grey reef shark aggregations. Laser photogrammetry data from 353 individuals were analysed, and a robust relationship between pre-caudal length and total length (R2 = 0.97) was established, enabling the derivation of a correction index through linear regression to predict and enhance the accuracy of size estimates. Finally, long-term video records demonstrate that Villingili Kandu currently hosts the largest documented aggregation of C. amblyrhynchos in the Maldives. These initial findings provide a baseline for further studies on the drivers influencing the aggregations of this species and its life history within the archipelago.