Holmes Reefs is an important area for undefined aggregations of one shark species.

Passive acoustic telemetry data show that Grey Reef Sharks regularly aggregate in this area (N Lubitz et al. unpubl. data 2025). A total of 19 Grey Reef Sharks tagged with acoustic transmitters were detected on the wider Holmes Reefs array, with 16 individuals recorded on the receivers within this area between February 2021 and October 2024. 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 five-minute bins were grouped into an event and its duration was calculated.

There were ~650,000 detections recorded at Holmes Reefs, but two receivers had few detections (1% and 1.2%, respectively) and these were excluded. The remaining two receiver locations were used to define the boundary of the area. There were 27,588 five-minute bins with aggregations recorded within the area, grouped into 9,121 aggregation events that lasted between 0.2–339 min (mean = 13.7 min). Aggregations comprised 3–5 individuals (mean = 3.2 individuals) or 19–31% of the 16 tagged Grey Reef Sharks that were detected in this area. There was a seasonal signal, with more aggregations and aggregation events persisting longer during March–July. Most aggregations were recorded between 7:00–13:00 (51%), with fewer records at night (28.5%). While aggregations were recorded in both sections of this area, they were more numerous in the narrow pass between two reefs in the western section, although receiver coverage also lasted longer there (N Lubitz et al. unpubl. data 2025). Combined, the data show that specific locations (i.e., this area) at this large offshore reef regularly host aggregations of Grey Reef Sharks that last up to ~5.5 h. Only 6 of 81 Grey Reef Sharks tagged in the broader Coral Sea region (Holmes Reefs, Osprey Reef, 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, a community-led underwater visual count survey of sharks on Coral Sea reefs conducted in December 2007 recorded most Grey Reef Sharks at Holmes Reefs, with 127 individuals counted in 8 h of surveying (Smith et al. 2008). The survey was a stationary point count that used flashers and berley to attract sharks to the observer. Most Grey Reef Sharks were seen at Holmes Reef (i.e., eastern reef; 102 sharks in 5 h) compared to the western reef (25 sharks in 3 h). Observers also noted the smallest individuals of ~50 cm total length on the eastern reef, indicating that it may be important for the reproduction of the species. More information is required to understand the nature and function of these aggregations.