Plenty of fish in the sea: But what happens when the predators outnumber the prey?

1 August 2016

It stands to reason in a healthy environment that prey should always vastly outnumber the predators that eat them. But a study led by a researcher from Macquarie University has found that sometimes, even in the most untouched environments on the planet, the food pyramid can be turned upside down.

Dr Johann Mourier, from Macquarie University, alongside a group of researchers based at the Centre de Recherches Insulaires et Observatoire de l’Environment (CRIOBE) in Perpignan, France have confirmed that in pristine remote coral reefs, sharks can be numerous, sometimes even outnumbering their prey at local scales.

The discovery was made in the southern pass of Fakarava atoll, a reserve in French Polynesia that hosts as many as 900 reef sharks. French Polynesia is unusual in this regard as targeted shark fishing has never taken place there and sharks have also been protected since 2006 in what is now the largest shark sanctuary in the world.

“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,” Mourier recalls.

The researchers decided to expand the scope of their study, to jointly study the shark and spawning grouper populations and their interactions. Underwater surveys and acoustic telemetry were used to assess the number of sharks living in the pass and their feeding behavior.

While sharks had been known to target fish spawning aggregations, the findings are the first to suggest that this strategy plays such an essential role, the researchers say. The discovery also has important conservation implications.

“Implementing strong shark protection laws is unlikely to be sufficient to maintain high numbers of sharks if not jointly implemented with conservation of fish spawning aggregation,” Mourier says. If overfishing leads to a loss of those aggregations, sharks “will have no other choice than undertaking energetically costly, wider-range foraging to meet energy requirements.”

The findings suggest that the natural abundance of sharks living on coral reefs might have been lost in many places due to human influence. Conservation of fish spawning aggregations together with shark fishing bans might help to bring the predators back.

The researchers say they’d now like to follow long-term trends on this extraordinarily remote coral reef. They are also using novel data-loggers and animal-borne video cameras to better characterize predator-prey interactions between sharks and their prey over the course of a year.

Mourier, Johann; Maynard, Jeffrey; Parravicini, Valeriano; Ballesta, Laurent; Clua, Eric; Domeier, Michael L; Planes, Serge. Extreme Inverted Trophic Pyramid of Reef Sharks Supported by Spawning Groupers. Current Biology. July 2016.

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