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 varieties when land erodes and sinks beneath 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.
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Feminine sharks within the neighborhood of Rangiroa atoll principally gathered in a spot known as “hammerhead plateau” — an space 150 to 200 toes (45 to 60 meters) deep, in response to 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 world is an aggregation web site, in response to the examine. The sharks seemingly don’t have any relationship to one another however are drawn there by exterior elements 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 instructed. The sharks may additionally have responded to adjustments in Earth’s geomagnetic discipline because the moon waxes and wanes, they posited.
Massive gatherings of nice hammerheads within the space coincided with excessive numbers of ocellate eagle rays getting into the lagoons to breed. Sharks prey on these rays, whose mating season is “a predictable occasion that sharks might attempt to intercept,” the researchers wrote.
Elevated water temperature after the winter months may additionally 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 staff recognized a further 30 female and male sharks from these data and located that males had been primarily sighted from August to October, fairly than in the course of the summer time.
Segregation between women and men has beforehand been reported for scalloped hammerhead sharks (Sphyrna lewini), the researchers famous, however not for nice hammerheads. The researchers’ findings counsel 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 numerous shark species,” the researchers wrote. Whereas they may not affirm 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.