Unusual All-Female Gathering of Critically Endangered Hammerhead Sharks Discovered in French Polynesia
French Polynesia has become the stage for an extraordinary spectacle as an all-female assembly of critically endangered great hammerhead sharks has been congregating in the tropical waters every summer for over a decade. Scientists have recently made this remarkable discovery, noting that the number of sharks peaks around the full moon.
The gathering takes place during the austral summer, from December to March, in the Tuamotu archipelago, specifically around two neighboring atolls – Rangiroa and Tikehau. Atolls are ring-shaped islands or coral reefs that form when land erodes and sinks below the ocean surface, creating a lagoon.
In the summers of 2020 and 2021, researchers recorded a total of 54 female great hammerhead sharks (Sphyrna the mokarra) and one whose sex could not be determined, in the two atolls located 9 miles (15 kilometers) apart. More than half of the sharks were identified as seasonal residents, meaning they spent up to six days a month in the area for up to five months. These findings were published in the journal Frontiers in Marine Science on August 29.
The female sharks in the vicinity of Rangiroa atoll were primarily concentrated in a spot known as the “hammerhead plateau,” located 150 to 200 feet (45 to 60 meters) deep. The researchers observed that the sharks were mostly sighted independently from one another, roving around the bottom of the plateau.
Great hammerhead sharks are typically solitary creatures, so the high number of females congregating around Rangiroa and Tikehau atolls indicates that these locations serve as aggregation sites. However, the sharks likely have no relationship to each other and are drawn to these spots by external factors related to the lunar cycle and the presence of ocellated eagle rays (Aetobatus ocellatus), according to the study.
Interestingly, the number of sharks peaked in the days before and after a full moon during both summers. The researchers speculate that the increased moonlight may enhance the sharks’ ability to hunt around the atolls at night. Additionally, the sharks may be responding to changes in Earth’s geomagnetic field as the moon waxes and wanes.
The large gatherings of great hammerheads correlated with the influx of ocellated eagle rays entering the lagoons to reproduce. Since sharks prey on these rays, the study suggests that the sharks are intercepting them during their mating season, which is a predictable event.
The scientists also theorize that the rising water temperatures after winter could attract the sharks to the Tuamotu archipelago. By comparing their observations with long-term data, the researchers discovered that some sharks have been returning to the area every summer for 12 years. They also found that male great hammerhead sharks were primarily sighted from August to October, preceding the summer period.
While segregation between males and females has been reported for other hammerhead shark species, this is the first time it has been observed in great hammerheads. The researchers propose that the males remain at a distance from the sites occupied by females during the austral summer, possibly due to their breeding period.
The scientists also suggest that Rangiroa and Tikehau atolls may serve as nursery grounds for great hammerhead sharks. Although this aspect could not be confirmed in the current study, further investigation is ongoing to determine the significance of these locations for the survival and reproduction of this critically endangered species.
The discovery of this all-female assembly of great hammerhead sharks in French Polynesia provides valuable insights into the behavior and conservation needs of this vulnerable species. It highlights the importance of protecting marine habitats and understanding the complex ecological relationships that exist beneath the ocean’s surface.