Dozens of hammerhead sharks mysteriously gather during full moon

Large numbers of female great hammerhead sharks are concentrated near the two atolls of Rangiroa and Tikehau, possibly related to hunting and breeding.

In the summers of 2020 and 2021, 54 female hammerhead sharks gathered around two atolls in French Polynesia. Photo: Alastair Pollock Photography/Getty

Scientists have discovered an unusual group of all-female great hammerhead sharks ( Sphyrna mokarran ) congregating in the tropical waters of French Polynesia every summer for more than a decade, with numbers peaking around the full moon. The new findings were published in the journal Frontiers in Marine Science on August 29.

Hammerhead sharks congregate during the Southern Hemisphere summer (December–March) around the open waters of Rangiroa and Tikehau atolls in the Tuamotu archipelago. An atoll is a circular island or reef surrounding a lagoon, formed when land erodes and sinks below the ocean surface.

In the summers of 2020 and 2021, the team found 54 female great hammerhead sharks and one of unknown sex around two atolls (which are 15 kilometers apart). More than half of the sharks were “seasonal residents,” meaning they were there for up to six days a month for up to five months, they said.

According to the study, female sharks near Rangiroa Atoll mainly congregate at a location called the “hammerhead plateau” – an area 45-60m deep. Experts mostly saw them moving independently around the bottom of the plateau.

The great hammerhead shark is listed as critically endangered on the International Union for Conservation of Nature (IUCN) Red List. It is a solitary species, so the presence of females around Rangiroa and Tikehau atolls suggests that the area is a gathering place. It is likely that they are not related to each other, but are attracted there by external factors. These factors appear to be related to the lunar cycle and the presence of the white-spotted eagle ray ( Aetobatus ocellatus ).

Sharks were most concentrated in the days before and after the full moon in the summers of 2020 and 2021. Photo: Gerard Soury/Getty

Great hammerhead sharks were most numerous on days around the full moon in both summers. The researchers think this may be because the brighter moonlight allows them to hunt better around the atoll at night. It’s also possible that they’re responding to changes in the Earth’s magnetic field as the moon changes.

The gathering of great hammerhead sharks around the two atolls coincides with the large influx of white-spotted eagle rays into the lagoon to breed. Great hammerhead sharks prey on these rays, and their mating season is a predictable event that the predators try to disrupt.

Additionally, rising water temperatures after the winter months may also attract great hammerhead sharks to the Tuamotu Islands. The scientists compared their observations with long-term data collected on the atolls and found that some returned every summer over 12 years. They identified 30 additional males and females from the old data, and found that males were mostly present from August to October rather than in the summer. This suggests that male great hammerhead sharks remain farther from the southern hemisphere locations where females live during the summer, which may be related to the breeding season.

The lagoon, with its warm, shallow, protected waters, is a breeding ground for many shark species. The team was unable to confirm this in the new study, but they are conducting further research to determine whether the atolls of Rangiroa and Tikehau are breeding grounds for great hammerhead sharks.

Thu Thao (According to Live Science )

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