Scientists have discovered that a population of reef sharks in French Polynesia has tipped the local food pyramid on its head thanks to prey delivering themselves straight onto the sharks’ dinner plates.
An international team of researchers, including experts from Macquarie University, were investigating the ecology of the Fakarava atoll in French Polynesia, and found that the local grey reef sharks greatly outnumbered their prey. The study was published today in Current Biology.
“We noticed the massive number of sharks in this channel, especially grey reef sharks, and questioned how such a large number of sharks can be maintained and where they find their food,” says lead author Dr Johann Mourier, a marine biologist from Macquarie University.
Mourier, who was previously with the CRIOBE research centre in Perignan, France, tells SBS Science that his team had originally travelled to the southern pass of the Fakarava atoll to study grouper fish, but changed their plans when they saw the thriving reef sharks.
“We found that these huge populations of reef sharks needed lots of food to survive… otherwise they have to do longer excursions to find food elsewhere,” he says.
They used underwater surveys and acoustic telemetry to determine the size of both predator and prey populations – the reef sharks and grouper fish, respectively.
Under normal conditions, the sharks would actually have been starving, because food is scarce around their location. But for some reason spawning aggregations (breeding groups) of fish from other reefs were directly delivering themselves to the sharks.
Scientists are still unsure about why the spawning grouper fish would keep going to those reefs where there’s a high chance to be devoured, but it is thought that a combination of ‘site fidelity’ and rapid water movement in the atoll’s southern pass play a role here.
Hence, the sharks had no reason to venture out of the remote reef to search for food because the ongoing supply of breeding fish kept serving itself up for the sharks to gobble up.
"Shark bans alone are not sufficient if they are not jointly implemented with conservation of these spawning aggregations"
“We have to recognise that predator and prey interactions are very complicated mechanisms to understand,” says Mourier.
He explains that the sharks have largely been aided by French Polynesia’s ban on targeted shark fishing, and the Fakarava atoll reserve has since become the largest shark sanctuary in the world. But protecting the sharks alone is not enough.
“We see that shark bans alone are not sufficient if they are not jointly implemented with conservation of these spawning aggregations,” Mourier says.
He says without complementing shark fishing bans with efforts to conserve other fish species populations, these instances of local food pyramids being turned upside-down might start becoming more common.
Professor Colin Simpfendorfer, a shark expert from James Cook University, says the study provides a rare insight into how sharks exploit fish spawning aggregations.
“This study demonstrates that sharks take advantage of the spawning aggregations of some fish species and rely on this to maintain their abundance,” Simpfendorder tells SBS Science.
“That means that making sure those [spawning aggregation] populations are not overfished is important to maintaining shark populations.”
Dr Mourier says the next step for the researchers will be to replicate the study elsewhere.
“We will focus on doing the same type of study but in different reefs around Fakarava with different levels of exploitation of fish aggregations,” he says.