Saint-Gilles to Saint-Paul is important for undefined aggregations of two shark and four ray species.

From October 2012 to May 2014, 20 adult Bull Sharks (size range = 183–329 cm total length [TL]; sex ratio, 0.35:1 male:female; estimated size-at-birth for this species in Reunion Island is 60–80 cm TL and size-at-maturity is 234 cm TL for males and 257 cm TL for females; Pirog et al. 2019) were monitored with acoustic receivers along the west coast of Reunion island (Institute de Recherche pour le Développement [IRD] 2015). Receivers were deployed ~2 km apart from each other and at depths of 10–60 m. This area had the highest detections of Bull Sharks in Reunion Island during the tracking period (J Mourier unpubl. data 2021). Further, the spatial dynamics of Bull Sharks were found to be strongly influenced by size. Larger sharks, especially larger females, were mostly found within the area during the winter (Mourier et al. 2021). This geographical structuring of the population indicates an ontogenetic segregation in habitat, where older and younger sharks use different habitats and their space-use rarely overlaps, a common pattern observed in many shark species (Speed et al. 2010). More importantly, in this area the second highest number of female/female co-occurrences and occasional male/female co-occurrences were recorded among the receiver network (J Mourier unpubl. data 2021). These co-occurrences mostly occurred from April to July and altogether indicate that undefined aggregations of Bull Sharks occur in this area. The function of these aggregations is currently unknown. It is unclear why, in Reunion Island, larger sharks display a preference for Saint-Gilles to Saint-Paul, but it may be due to the presence of abundant food resources or a preference for the sandy bottom on a 50 m deep shelf between Saint-Gilles and Saint-Paul (IRD 2015). Alternatively, this area could represent a resting area due to optimal current conditions (Soria et al. 2019). The detection of these larger individuals increased during autumn and winter (Blaison et al. 2015; IRD 2015; Soria et al. 2019), which could also possibly be a result of increased mating activity. It has been suggested that a pre-mating aggregation could occur in the area from April onwards (Soria et al. 2019). The observed dynamics in movement patterns could also be driven by parturition with female Bull Sharks using some areas as pupping and nursery grounds, and parturition occurring in October–November (Pirog et al. 2019). Despite their ability to travel long distances, several studies have highlighted site-fidelity and reproductive philopatry in Bull Shark populations across the world (Brunnschweiler & Baensch 2011; Karl et al. 2011; Tillett et al. 2012).

Tiger Sharks were captured in the local shark control program along the west coast of Reunion Island using Shark Management Alert in Real Time (SMART) drumlines and benthic-set longlines. SMART drumlines were deployed at depths of 12–35 m and mostly set at night while longlines were deployed at depths of 10–70 m. From 2012 to 2017, a total of 150 Tiger Sharks (87 females, 63 males) were captured. These were almost exclusively adults (size range = 130–429 cm TL; L₅₀ for males = 270–280 cm TL, L₅₀ for females = 330–340 cm TL; Centre Sécurité Requin unpubl. data 2023). Most individuals (n = 112; 77.8%) were caught within Saint-Gilles to Saint-Paul (Centre Securité Requin unpubl. data 2023). From December 2011 to September 2014, 55 Tiger Sharks (average size, 277 cm TL; range, 111–397 cm TL; sex ratio, 0.9:1.0 male:female) were monitored with acoustic telemetry along the west coast of Reunion Island (IRD 2015; J Mourier unpubl. data 2021). The highest number of detections (> 10,000 detections) and the highest cumulative hours spent (> 400 h) within the proximity of a receiver were made in the area, particularly at the receiver located near the shelf edge of the area. Seventeen individuals were detected at this receiver in total, and in many cases, individuals were detected at the same time at this same receiver (up to seven individuals detected simultaneously; J Mourier unpubl. data 2023). This indicates that undefined aggregations of Tiger Sharks occur in this area. The function of these aggregations is currently unknown.

The Mascarene Archipelago Elasmobranch Observatory has maintained a citizen science and scientific diving database since early 2021 from the area. This has provided data on aggregations of ray species. Further information is required to determine the nature and function of these aggregations.

Spotted Eagle Rays are recurrent year-round in the area and have been recorded in a total of 34 observations, forming aggregations of 2–10 individuals.

Common Eagle Rays were sighted 14 times in the area, every year from September to April, forming aggregations of 2–20 individuals.

Pink Whiprays were also recurrently sighted (n = 25 observations), from November to April, forming aggregations of 2–40 individuals.

Blotched Fantail Ray were sighted 16 times, every-year, from January to August, forming aggregations of 2–14 individuals.