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Shark Witchcraft? Female Sharks Gather Each Year Beneath the Full Moon

Why are large numbers of female Great Hammerheads congregating beneath a full Moon?

By Cassidy Ward

In the original Sharknado, a hurricane hits Los Angeles (an infrequent but not unheard of event), filling the city with shark-filled floodwaters. Navigating L.A. traffic is bad enough without the threat of shark attack, and that’s before the tornadoes show up. Soon, a trio of twisters start sucking up water and sharks, transforming themselves into the titular sharknados.

There’s plenty to pick apart, scientifically speaking, but we’re going to focus on the congregation of sharks. We think of sharks as solitary creatures and often that’s true. The more we learn, however, the more we discover that sharks are sometimes more social than we previously supposed. Over the last few years, researchers have observed a rare all-female collection of hammerhead sharks congregating in the waters off French Polynesia, and whatever they’re doing is tied to the Moon.

Dozens of Female Hammerheads Gather Beneath the Full Moon

An international team of researchers passively observed a group of Great Hammerheads at the Rangiroa and Tikehau atolls — a ring of coral, island, or islands, which surrounds a lagoon — between December and March of 2020 and 2021. In the global north, that’s the peak of winter, but in French Polynesia, the sharks are enjoying the height of austral summer.

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Over the course of the study period, researchers identified 55 individuals and positively determined 54 of them to be female. The remaining individual was undetermined but likely female, given its company. Hammerhead sharks are typically solitary creatures, as we imagine, so finding this many hanging out together is unusual.

School of scalloped hammerhead sharks

In addition to two seasons of direct study, researchers relied on videos recorded by citizen scientists over a 15-year period. That footage allowed scientists to identify an additional 30 sharks, most of whom were positively identified as females, frequenting the area between 2006 and 2019. Roughly half of the identified sharks are considered seasonal residents, spending up to six days per month at one of the atolls, for up to five months at a time during the summer period.

Moreover, 32 sharks returned to the same atoll for up to 12 years. Researchers identified a number of potential factors driving the sharks to the area year after year. The first is the presence of prey animals, particularly ocellated eagle rays, which gather in the area every summer to mate. The abundance of prey makes the atolls an easy place to get a quick meal, and the ray mating season is consistent and predictable.

However, the ebb and flow of the local shark population more closely corresponds to the activity of the Moon. The total number of sharks peaked, both in 2020 and 2021, a few days before and after the full Moon. It’s not precisely clear why the Moon might bring all the sharks to the yard, but it does. It could have something to do with electrical fields influencing their navigation or it could be as simple as better visibility. More moonlight makes for easier hunting. Whatever the reason, researchers noted that the Moon’s cycle was the only significant factor impacting the number of sharks in the area.

For what it’s worth, we’re putting our money down on shark witchcraft. A few dozen powerful women gathering inside a naturally occurring circle beneath the full Moon… we know a coven when we see one.

See a group of gathered sharks for yourself in Sharknado: Heart of Sharkness, streaming now on Peacock!