Johnston Atoll is an important area for undefined aggregations of one shark species.

Historical data from surveys conducted between 1992–1995 highlight the presence of Grey Reef Shark aggregations in this area (Randall et al. 1985, Economakis & Lobel 1998). This species was also reported as common in outer reefs and shallow lagoons of Johnston Atoll (Randall et al. 1985). Visual surveys from the beach were conducted at least once per week and were focused exclusively on counting Grey Reef Sharks. These surveys showed that aggregations of Grey Reef Sharks are common in Johnston Atoll between March–May with up to 160 individuals observed in a single day in multiple aggregations (Economakis & Lobel 1998). Aggregations were regularly observed during the day around Sand Island and in flat habitats without coral coverage. Larger aggregations coincided with the maximum water temperatures. Observations made by snorkelers revealed that all individuals observed were females (Economakis & Lobel 1998). Between 2001–2003, 25 Grey Reef Sharks (15 males and 10 females) measuring 56–135 cm total length (TL) were also captured in the area showing that males also occur there (Skomal et al. 2007). Of these, five measured 56–76 cm TL and were considered young-of-the-year as the reported size-at-birth for this species is 45–64 cm TL (Ebert et al. 2021).

Diving surveys and visual surveys conducted from the beach have revealed that Grey Reef Shark aggregations continue to regularly occur in the area (Nadon et al. 2012; CREP-PIFSC 2017; Brainard et al. 2019). Towed dive surveys (undertaken up to 30 m depth and covering ~2.5 linear km in ten five-minute segments; Brainard et al. 2019) conducted biennially in January between 2004–2012 and in 2015 recorded aggregations of up to 27 Grey Reef Sharks (Nadon et al. 2012; CREP-PIFSC 2017; Brainard et al. 2019). Individuals with estimated sizes of 88–225 cm TL (average = 110 cm TL) were mostly sighted in forereefs on the northern side of the atoll at depths 5–20 m. Occasionally, they were also sighted in backreefs and lagoon habitats (Brainard et al. 2019). Between 2004–2012, Grey Reef Sharks were observed in 34 of 98 towed dive surveys (2004: 7/25; 2006: 7/24; 2008: 7/11; 2010: 9/21 and 2012: 4/17) conducted in the area. Aggregations were recorded in all years (2004: 3; 2006: 4; 2008: 3; 2010: 3; 2012: 1) and ranged from four to 27 individuals (CREP-PIFSC 2017; Brainard et al. 2019). Between 2004–2010, Johnston Atoll held the fourth largest density (2 individuals/ha) of Grey Reef Sharks in all the US Pacific (Nadon et al. 2012). During 2015, no Grey Reef Sharks were recorded in towed dive surveys which agrees with the apparent decline of piscivore biomass recorded in that year in Johnston Atoll (Brainard et al 2019). However, in 2015, Grey Reef Sharks were recorded in five of the 31 stationary point count dive surveys (25 m transect lines during a timed five minute with four replicates). These are small-scale surveys that cover small areas and were aggregations are not well captured (Brainard et al. 2019). The presence of Grey Reef Sharks in those surveys confirms their contemporary presence and suggests that aggregations still occur in the area. Since 2015, no additional surveys have been conducted in the area. Additional information is required to determine the nature and function of these aggregations.