Every July under the light of the full moon something magical happens in the French Polynesia island of Fakarava. Grouper come together in a colossal aggregation in the south pass of the island, an atoll in the Tuamotu Archipelago, to spawn. Biologists believe that numbers could surpass 20,000 fish and may be the largest known aggregation of large reef fish. In 2014, Laurent Ballesta and his team dove beneath the waves at night to record this spawning event… but they weren’t there just for the grouper.
Aptly named the “Wall of Sharks”, Ballesta and the other researchers were fascinated to find over 700 sharks feasting on the fish. The average population of feasting sharks was 600, which is two to three times the number per hectare documented for any other reef shark aggregations according to the research the team published in Current Biology. The reef shark population ranged from 250 in the warmer summer months to up to 900 individuals during the grouper aggregation that falls during the Southern Hemisphere’s winter months.
The researchers used underwater surveys and acoustic telemetry to identify how many sharks there were, as well as document their feeding behavior. And boy, do these sharks eat! Modelling was led by the Insular Research Center and Environment Observatory in France (CRIOBE) and indicated that the sharks needed 90 tons of fish per year. However, the total fish production in the pass was just 17 tons per year, which means that this massive aggregation played a key role in keeping these sharks thriving. What the scientists found surprising was that the sharks stayed at the pass even after the aggregation, and subsidized their diets with species like surgeonfish and parrotfish. “We did some night dives and found the sharks were still there and hunting. They use the new food source like a delivery, they don’t need to move anymore to look for food they just have to stay in the pass and there is always fish,” Dr Johann Mourier, a shark scientist who was working with Macquarie University at the time of this study, told ABC.
However he did note that once the aggregation was over the sharks spent less time inside the pass in order to find enough food. While this is not the first documentation of sharks using spawning events as feeding groups, it is the first to show how heavily some sharks rely on them. ”Lots of reefs have lost their spawning aggregation [through overfishing] so maybe that is one reason why we don’t see such density of sharks in these reefs,” Mourier said. But why does this phenomenon occur in Fakarava and nowhere else in the world? One big reason is that the French Polynesia area protects their sharks. Since 2006, the area has been protected as a designated shark sanctuary, one of the largest in the world, and the south pass of the atoll is fished by less than a dozen people, which means the spawning aggregation has remained healthy.
Fakarava is also part of a UNESCO Biosphere reserve! Biosphere reserves are terrestrial, marine and coastal ecosystems that act as ‘science for sustainability support sites.’ Basically they are special places for testing interdisciplinary approaches that come with the management of biodiversity, such as conflict prevention. These internationally recognized reserves are nominated by national governments and remain under the sovereign jurisdiction of where they are located. Currently there are 701 biosphere reserves in 124 countries, including 21 transboundary sites. Due to these protections, the south pass of the Fakarava spawning aggregation has remained healthy. The narrow pass also creates a unique marine environment with the transition between pelagic (open sea) and lagoonal life as outgoing tides deliver a mass of food – from plankton to fish of many sizes – making it an ideal breeding ground for groupers, which in turn attracts the sharks.
While scientists await for data that will help shed a light on the residency, social interactions, and hunting behavior of the grey reef sharks in this area… it’s almost feeding time again. Afterall, it is July. The sharks, many of which have acoustic tags inserted in their bodies for several projects, are still out there swimming in the protected waters and the full moon is on July 16.
Who is ready for a dip?