Palmyra Atoll Reefs is an important undefined aggregation area for two shark species.

This area has a significantly higher biomass of reef sharks (i.e., Grey Reef Shark, Blacktip Reef Shark, and Whitetip Reef Shark) than neighbouring islands in the central and southwestern Pacific Ocean (Stevenson et al. 2007; Sandin et al. 2008; Nadon et al. 2012).

Between 2004–2010, reef sharks were recorded biennially around 46 US islands, atolls, and banks (e.g., Mariana, Hawaii, American Samoa, Wake, Phoenix, Jarvis, Johnston, and Line Islands) on surveys (divers towed behind a boat) that each covered >0.01 km² on forereefs at 15–20 m of depth (Nadon et al. 2012). Sites were grouped in 15 locations according to their geographic proximity. Line Islands, where this area is located, had the third highest abundance for all reef sharks (Line Islands = 4.5 individuals/0.01 km²; Palmyra atoll = 3.4 individuals/0.01 km²) and the third rank for Grey Reef Sharks and Whitetip Reef Sharks. Line Islands also had the second highest abundance of Blacktip Reef Shark among the 15 locations. The study suggested that the overall abundance of reef sharks is similar to the expected baseline shark density given no humans within 200 km. So, these shark populations are considered to be at their likely carrying capacity (Bradley et al. 2017b).

Between 2003–2005, three studies compared reef shark abundance, mainly Grey Reef Sharks, in the Line Islands (i.e., Christmas and Fanning islands, Kingman, Tabuaeran, and Kiritimati atolls) using belt-transect dive surveys at shallow depths (2–12 m) (Stevenson et al. 2007; DeMartini et al. 2008; Sandin et al. 2008). These studies demonstrated that this area had the highest abundance of reef sharks. One of these studies reported that top predators dominated the unpopulated Palmyra Atoll, where sharks represented 57% of top predator biomass, compared with the populated atolls of Tabuaeran and Kiritimati (Sandin et al. 2008). Grey Reef Sharks were the most abundant shark species on Palmyra, followed by Blacktip Reef Sharks and Whitetip Reef Sharks (DeMartini et al. 2008; McCauley et al. 2012; Nadon et al. 2o12).

Between 2004–2014, baited remote underwater video station (BRUVS) surveys (n = 47) were deployed at the forereef, backreef and lagoon environments and MaxN (the maximum number of individuals of a species observed in a single frame) was determined (Papastamatiou et al. 2018a). The highest MaxN was four for the Grey Reef Sharks, at the forereef, and five for the Blacktip Reef Sharks, at the forereef and backreef.

The Grey Reef Shark is a highly social species, reported in aggregations in this area (Papastamatiou et al. 2018b, 2020). From October 2006 to October 2014, 1,399 Grey Reef Sharks were captured using hand lines in the forereef, backreef, lagoon, and channel habitats on Palmyra Atoll. Most were adults, with females measuring 146 cm total length (TL) and males measuring 139 cm TL (Bradley et al. 2017a). The size class mode was 141–160 cm TL for both females and males. Females mature at 120–142 cm TL, and males mature at 130–145 cm TL (Ebert et al. 2021). Between 2004–2010, density estimates of Grey Reef Sharks in this area ranged from ~200 sharks/km² (Nadon et al. 2o12; McCauley et al. 2012) to >1,000 sharks/km² (Nadon et al. 2o12). Observations were recorded from stationary point counts, belt transects, video surveys, and towed-diver surveys.

From October 2006 to October 2014, 1,356 Grey Reef Sharks were captured over 88 days of fishing in this area (Bradley et al. 2017b). Of these, 389 individuals were recaptured during unique sampling periods. Grey Reef Sharks (n = 37) were acoustically tagged in 2010–2012 and tracked through August 2015 (Bradley et al. 2017b). Grey Reef Shark adult and sub-adult density was 21.3 sharks/km² (95% CI = 17.8–24.7), and total population abundance was 8,344 individuals (95% CI = 6,977–9,698). Density hotspots were located on the eastern and western forereefs, with shark densities up to an order of magnitude lower on the north and south forereefs, on the backreefs, and within the lagoons. Sharks used core areas that were highly stable over several years (Papastamatiou et al. 2018a, b). The unfished shark population was also stable over time; there were no significant differences in Grey Reef Shark abundance or density throughout the study period (Bradley et al. 2017b).

This area has one of the highest densities of Blacktip Reef Sharks in the central-western Pacific Ocean with animals regularly observed together (Stevenson et al. 2007; Sandin et al. 2008; Nadon et al. 2012). Line Islands had the second highest abundance for Blacktip Reef Shark among 15 locations surveyed (Nadon et al. 2012). Furthermore, in another survey, the average density of Blacktip Reef Sharks on the forereef of Palmyra was measured as 2.3 sharks per 0.01 km² (SE = ± 0.8) (McCauley et al. 2010). Regular observations of mating behavior and reproductive scars of Blacktip Reef Sharks have been recorded in this area. In October 2007, at the northern forereef of Palmyra Atoll at a depth of 5–20 m, a Blacktip Reef Shark mating event was recorded (McCauley et al. 2010). This was characterised by seven males following a single female, a male grabbing the female near the pectoral fin and positioning her head down on the bottom and copulating (McCauley et al. 2010). During sampling for a concurrent population study of Blacktip Reef Sharks, it was reported that many female sharks were caught with large amounts of scarring, generally concentrated on the anterior body flank and pectoral fins. This suggests that mating is frequent in this area; however, further information is needed to confirm this and determine whether these aggregations are for reproductive purposes.