Osprey Reef is important for undefined aggregations of one shark species.

Passive acoustic telemetry data show that Grey Reef Sharks regularly aggregate in this area (Barnett et al. 2012; N Lubitz et al. unpubl. data 2025). A total of 36 Grey Reef Sharks tagged with acoustic transmitters were detected in Osprey Reef between February 2021 and November 2024 (N Lubitz et al. unpubl. data 2025). Aggregations were defined as three or more tagged individuals detected on a receiver within five minutes. The approximate range in which receivers detect tagged sharks 100% of the time on Coral Sea reefs is ~400 m (Barnett et al. 2012). Aggregations in consecutive 5-min bins were grouped into an event and its duration was calculated.

There were ~1,230,000 detections recorded at this reef, but one station had few detections (n = 186; 0% of total) and was excluded. The remaining five receiver locations were used to define the boundary of the area. There were 79,454 five-minute bins with aggregations recorded within the area, grouped into 31,761 aggregation events that lasted between 0.2–537 min (mean = 10.8 min). Aggregations comprised 3–11 individuals at a time (mean = 3.5 individuals). As aggregations lasted up to several hours, these events comprised up to 22 unique tagged sharks (61% of total), but a maximum of 11 individuals (31% of total) were recorded at the same time. There was no evidence of clear seasonality in aggregation number, size, or duration. There were fewer aggregations during the night (28%) than during the day (72%), but there was no defined peak within the daytime. Aggregations were recorded on 72% of monitoring days, highlighting their regular occurrence. Although one station is located at a dive site (‘North Horn’) where tour operators sometimes attract and feed reef sharks, most days with aggregations were recorded at two stations further south at the entrance to the lagoon (833 and 551 days, respectively), highlighting that aggregations persist naturally. Only six of 81 Grey Reef Sharks tagged in the broader Coral Sea region (Osprey Reef, Holmes Reefs, Flinders Reefs, Bougainville Reef) were detected at multiple reefs, highlighting their high residency and site fidelity, and the individual importance of each reef (N Lubitz et al. unpubl. data 2025).

Additionally, historic passive acoustic telemetry data from 2008–2009 support aggregations and the space use of Grey Reef Sharks in this area, with 11 of the 18 tagged individuals recorded on >90% of monitoring days (Barnett et al. 2012). They were detected mostly in the north and west of the reef, within this area (Barnett et al. 2012). Aggregations of Grey Reef Sharks have also regularly been reported by divers and recorded on iNaturalist between 2000–2025 (iNaturalist 2025). Historic data from a cleaning station recorded 38 Grey Reef Shark cleaning events in 288 h of remote video footage captured in 2007 (O’Shea et al. 2010). This indicates that the area may also be important for cleaning. Juvenile Grey Reef Sharks are seen inside the lagoon, but not on the outside of the reef, and it is possible that this part of the area is important for the early life-stages of the species (Barnett et al. 2012). Additional information is required to determine the nature and function of these aggregations.