Millennium Atoll is an important area for undefined aggregations of two shark species.

Aggregations of Grey Reef Sharks and Blacktip Reef Sharks are regularly observed in this area and have been documented by underwater visual censuses and baited remote underwater video station (BRUVS) surveys (Kerr & Wragg 2008; National Geographic 2009; Friedlander et al. 2022).

In 2008, two underwater visual censuses were undertaken at three islands in the Southern Line Islands (Malden, Flint, and Millennium Atoll). A diver recorded all shark observations, during a period of 30 minutes between depths of 8–25 m (Kerr & Wragg 2008). Of these surveyed islands, Millennium Atoll had the highest abundance of sharks, which dominated the fish community. In this area, the mean abundance was 7.5 for Grey Reef Sharks and 3.0 for Blacktip Reef Sharks.

Two further expeditions were undertaken in the Southern Line Islands (National Geographic 2009; Friedlander et al. 2022). Between March–May 2009, underwater visual censuses were conducted by a team of paired divers at ~15 m depth at 29 sites (National Geographic 2009). During each survey, divers counted all sharks encountered along three fixed-length (25 m) corridor transects separated by ~10 m. Between 9–20 October 2021, these surveys were undertaken at 17 sites (Friedlander et al. 2022). Each site was surveyed using three transects. Sharks were also measured and the average body size was reported for each survey.

In 2009, total fish biomass ranged between 2.1–5.8 tonnes per 0.01 km² for the five islands assessed in the Southern Line Islands. Millennium Atoll had the second highest biomass (~ 5.5 t per 0.01 km²). This area is among the top seven highest fish biomass ever reported globally (National Geographic 2009). Top predators, including sharks and other fish (e.g., Twinspot Snapper Lutjanus bohar, trevally Caranx ssp.), accounted for most of the biomass (~70%). The most abundant top predators were Grey Reef Sharks and Blacktip Reef Sharks (National Geographic 2009). Fish biomass was highest on the windward side of the island or near the island tips, where dominant currents form eddies and enhance water motion.

In 2021, BRUVS were deployed and the maximum number of each species in a single video frame (MaxN) was recorded (Friedlander et al. 2022). Mid-water BRUVS were deployed at 10 survey sites in the area at a depth of 10 m and ranged in distance from shore from 500 m to 5 km (Friedlander et al. 2022). BRUVS were deployed in mesophotic reefs at nine survey sites in this area at depths between 50–65 m and in habitats from plate coral reefs to rubble/sand slopes (Friedlander et al. 2022). Despite a more than six-fold decrease in shark biomass between 2009 and 2021 across the Southern Line Islands, Grey Reef Sharks and Blacktip Reef Sharks were still present in relatively high numbers compared with most other regions around the world (Friedlander et al. 2022).

The underwater visual censuses showed that the Grey Reef Shark was the first-ranked species with an average of 14.7 (SD = ±38.3) individuals per 0.01 km² (Pristine Seas unpubl. data 2022). In 13 surveys, 100 sharks per 0.01 km² were recorded, and in one survey, 200 individuals per 0.01 km² were recorded (Pristine Seas unpubl. data 2022). In 15 surveys, Grey Reef Sharks had an average body size that ranged between 68–158 cm total length (TL) with a mean average of 120 cm TL (Pristine Seas unpubl. data 2022). Considering the size-at-maturity (120–142 cm TL for females, 130–145 cm TL for males), and the size-at-birth (45–64 cm TL) for this species (Ebert et al. 2021), individuals in this area were young-of-the-year, juveniles, and adults. From the mid-water BRUVS surveys, it was estimated that Grey Reef Sharks had a maximum MaxN of 12 and an average of five individuals, with an average size of 117 (SD = ±11.7) cm TL and a range of 98–143 cm TL indicating most individuals were juveniles. Sites close to the northern and southern tips of Millennium Atoll with more active sea states had higher numbers of Grey Reef Sharks compared to the other areas surveyed. From the BRUVS surveys in mesophotic reefs, it was estimated that Grey Reef Sharks had a maximum MaxN of 12 and an average of four individuals (Pristine Seas unpubl. data 2022).

The underwater visual censuses revealed that the Blacktip Reef Shark was the second-ranked species with an average of 2.9 (SD = ±16.9) individuals per 0.01 km² (Pristine Seas unpubl. data 2022). In three surveys, 100 sharks per 0.01 km² were recorded (Pristine Seas unpubl. data 2022). During these underwater visual censuses, an aggregation of at least eight Blacktip Reef Sharks was recorded (Friedlander et al. 2022). The Blacktip Reef Shark aggregations in this area could be for reproductive purposes. The lagoon could be used as a nursery for the Blacktip Reef Shark (Barott et al. 2010). Numerous Blacktip Reef Sharks, many of which were estimated to be juveniles (<100 cm TL), were observed inhabiting shallow back reef habitats throughout the lagoon’s perimeter (Barott et al. 2010). The body size of Blacktip Reef Sharks was visually estimated to range from 30–90 cm TL (G Wragg pers. obs. 1991). The size-at-birth for the Blacktip Reef Shark is 30–52 cm TL (Ebert et al. 2021). In addition, many of the larger (>100 cm TL) adult females on the forereef during concurrent surveys exhibited mating scars and appeared to be pregnant (Barott et al. 2010). Further information is needed to determine the nature and function of these aggregations and determine the importance of the area for reproductive purposes.