Approximately 900 reef sharks in a French Polynesia marine reserve live in a place where there isn't nearly enough food to feed 900 reef sharks. And yet, there they are – predators far outnumbering prey in an upside-down food pyramid.
But, a new study finds that sometimes an environment's food pyramid can be flipped and that the sharks get around the inversion problem by letting meals come to them.
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Researchers from France's Centre de Recherches Insulaires et Observatoire de l'Environnement were studying grouper spawning populations in the southern pass of the Fakarava atoll – a site known for such gatherings of the fish in June and July each year – when they noticed an extremely high concentration of a certain type of predator resident in the area.
"We noticed the massive number of sharks in this channel, especially gray reef sharks, and questioned how such a large number of sharks can be maintained and where they find their food," said the study lead author Johann Mourier, currently at Macquarie University in Sydney, Australia, in a statement.
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Fish that typically live in the pass don't represent enough food for so many sharks, the researchers said. But, through study they found that it was the grouper populations passing through that helped balance out the resident predators' needs. The food came to them, during the spawning events.
When spawning season subsides, the scientists found that the resident sharks tended to turn to foraging expeditions outside of the pass.
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The researchers stressed that it's not news that sharks would stake out a fish spawning ground.
"Fish spawning aggregations are known to be targeted by sharks," they wrote, "but were believed to only represent occasional opportunistic supplements."
What's striking, they say, is how important the tactic is – how important the spawning grouper are – for these sharks, which live in an unusual circumstance.
The environment where all of this takes place is considered "pristine" by the scientists. Targeted shark fishing does not occur in French Polynesia, and since 2006 sharks in that area have been protected. They live in the world's largest shark sanctuary.
WATCH VIDEO: Sharks Attack Groupers Spawning on the Fakarava Atoll: