An uncommon, all-female meeting of nice hammerhead sharks has been gathering within the tropical waters of French Polynesia each summer time for over a decade, with numbers peaking across the full moon, scientists have found.
These critically endangered sharks convene in the course of the austral summer time, between December and March, round openings in two neighboring atolls — Rangiroa and Tikehau — within the Tuamotu archipelago. An atoll is a ring-shaped island or coral reef enclosing a lagoon that kinds when land erodes and sinks under the ocean floor.
Within the summers of 2020 and 2021, researchers recorded 54 feminine nice hammerhead sharks (Sphyrna mokarran) and one whose intercourse couldn’t be decided on the two atolls, that are 9 miles (15 kilometers) aside.
They famous that greater than half of the sharks had been seasonal residents, which means they spent as much as six days a month there, for as much as 5 months. The findings had been printed Tuesday (Aug. 29) within the journal Frontiers in Marine Science.
Feminine sharks within the neighborhood of Rangiroa atoll principally gathered in a spot known as “hammerhead plateau” — an space 150 to 200 ft (45 to 60 meters) deep, in accordance with the examine. “They had been primarily sighted roving across the backside of the plateau, independently from each other,” the researchers wrote.
Nice hammerhead sharks are a solitary species, so the excessive variety of females sighted on the similar time round Rangiroa and Tikehau atolls signifies the realm is an aggregation web site, in accordance with the examine. The sharks seemingly don’t have any relationship to one another however are drawn there by exterior components that seem like linked to the lunar cycle and the presence of ocellated eagle rays (Aetobatus ocellatus), the examine discovered.
The variety of sharks peaked within the days shortly earlier than and after a full moon throughout each summers — maybe as a result of the elevated moonlight enhanced their potential to hunt across the atolls at evening, the researchers steered. The sharks might also have responded to modifications in Earth’s geomagnetic area because the moon waxes and wanes, they posited.
Giant gatherings of nice hammerheads within the space coincided with excessive numbers of ocellate eagle rays coming into the lagoons to breed. Sharks prey on these rays, whose mating season is “a predictable occasion that sharks may attempt to intercept,” the researchers wrote.
Elevated water temperature after the winter months might also lure sharks to the Tuamotu archipelago, the scientists added.
The researchers in contrast their observations with long-term information collected on the atolls, which revealed that some sharks have been returning each summer time for 12 years. The crew recognized a further 30 female and male sharks from these information and located that males had been primarily sighted from August to October, quite than in the course of the summer time.
Segregation between men and women has beforehand been reported for scalloped hammerhead sharks (Sphyrna lewini), the researchers famous, however not for nice hammerheads. The researchers’ findings recommend that males stay at a distance from the websites occupied by females in the course of the austral summer time, which may very well be linked to their breeding interval.
“Lagoons and their protected warm-shallow-coastal waters are identified to function nursery areas for varied shark species,” the researchers wrote. Whereas they may not verify this within the new examine, additional investigation is underway to find out whether or not Rangiroa and Tikehau atolls present nursery grounds for nice hammerhead sharks, they added.