Scientists have noted that an all-female gatherings of hammerhead sharks have been taking place in French Polynesia each summer for more than a decade. Interestingly, the numbers of these hammerhead participants have been observed to be at their highest during full moon.
Female Hammerhead Sharks Gather Annually in French Polynesia During Summer and Full Moon
This female hammerhead convention has been to take place between December and March. They take place in the openings of two atolls, which are ring-shaped coral reefs or islands that surround a lagoon and that forms after the erosion of land and its seeking beneath the ocean, within the Tuamotu archipelago. These two atolls, Tikehau and Rangiroa, are close to each other.
As such, during 2020 and 2021 summer, scientists were able to observe 54 great female hammerhead sharks within the atolls, while they also noted one whose sex could not be definitely determined.
Over half of the gathering participants were observed to be seasonal residents of the area. This meant that they spent six days up to a month in the area for as long as five months.
In the Rangiroa atoll, these females gather in a spot known as the "hammerhead plateau," which has a depth of roughly 150 to 200 feet or 45 to 60 meters. The scientists note that the creatures were independently roving in the plateau's bottom.
Why Do These Female Hammerheads Gather in the Area?
The study further notes that the high number of female hammerhead sharks observed simultaneously among the Tikehau and Rangiroa atolls shows that the areas serve as aggregation sites. While all of them do not have any relation to each other, they are drawn to the area due to external conditions that seem to be connected with ocellated earle ray presence as well as the lunar cycle.
The exact number went up before and after the full moon in the two summers. The researchers suggest that the boosted moonlight could have boosted the creature's capacity to hunt in the atolls when it was night. These creatures may have also responded to alterations in the geomagnetic field of the earth as the moon experienced waxing and waning.
The heightened female hammerhead shark count also matched the high presence of ocellate eagle rays going to lagoons for reproduction. These rays are preys of sharks. The eagle rays also have quite a predictable mating season that the sharks could be capable of intercepting.
Comparisons were made with long-term data retrieved from the atolls. This showed that there were sharks that went back each summer for 12 years. The scientists also observed an addition of 30 female and male sharks in the records. They observed that male hammerhead sharks could be spotted during August to October.
While female and male segregation was observed in scalloped hammerhead sharks, this was not seen in great hammerheads. The findings suggest that males stay at a particular distance from the area that their female counterparts occupy over summer, which might be connected to the breeding period of these females.
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