Up to 900 reef sharks survive in a small patch of one of the most untouched reef environments in the world, thanks to an annual migration of groupers arriving at their door, researchers have found.

Key points: Shark numbers outnumber prey in a French Polynesian shark sanctuary

Shark numbers outnumber prey in a French Polynesian shark sanctuary Raises questions about how they find enough food

Raises questions about how they find enough food Study shows they feast on massive aggregation of spawning groupers in winter

Study shows they feast on massive aggregation of spawning groupers in winter Finding suggests other fish stocks need to be protected to conserve sharks

The discovery, which provides a new view of reef food pyramid dynamics, suggests fish aggregations for mating and spawning play a key role in maintaining shark numbers on reefs and has important implications for conservation, Dr Johann Mourier said.

Dr Mourier and his colleagues were initially studying the aggregation of groupers that occurs at the southern pass of Fakarava atoll in French Polynesia every June and July.

When they arrived at the pass — where up to 17,000 groupers arrive — they noticed an extraordinary number of reef sharks were present, said Dr Mourier, a shark researcher at Macquarie University.

"It is a very narrow pass in the atoll about 100 metres wide — when you are diving inside you can see both sides of the channel — and we were surprised to find a very huge population of sharks there," Dr Mourier said.

"We wanted to understand how the sharks can live in this small area. The question was how they can find the food they require for their energy and needs?"

Imbalance between predator and prey

The researchers used underwater surveys and acoustic telemetry to assess the number of sharks living in the pass and their feeding behaviour.

According to the study, published today in Current Biology, the reef shark population ranged from 250 in the summer months to up to 900 during the grouper aggregation in winter.

The average population of 600 was two to three times the number per hectare documented for any other reef shark aggregations.

Modelling by the team, led by the Insular Research Center and Environment Observatory in France (CRIOBE), showed the sharks required 90 tonnes of fish per year, whereas the total fish production in the pass was just 17 tonnes per year.

Researchers were surprised by the number of groupers they found ( Laurent Ballesta/Andromede Oceanologie et Blancpain Ocean Commitment )

Dr Mourier said with such a imbalance between predator and prey numbers, the sharks would need to undertake large journeys to forage for food.

However they found during the grouper aggregation the sharks remained at the site.

"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."

However he said once the aggregation was over they spent less time inside the pass in order to find enough food.

The team also found later aggregations at the site of other fish species such as surgeonfish and parrotfish also subsidise the sharks' diet.

Dr Mourier said while sharks had been known to target fish spawning aggregations, the research was the first to suggest this strategy played a key role in maintaining shark populations.

"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," he said.

Fish key to conserving sharks

Dr Mourier said French Polynesia was unusual because sharks had never been fished or exploited and were now protected in one of the biggest shark sanctuaries in the world and especially in Fakarava as part of a UNESCO Biosphere reserve.

The south pass of the Fakarava atoll was not fished except for subsistence fishing by less than 10 people, which meant the spawning aggregation there had remained healthy.

However, he said nearby atolls had also in the past similar healthy spawning aggregations, but these been exploited and were now very small with what appeared to be corresponding smaller shark populations.

"We think shark fishing bans might not be sufficient if not combined with protections of fish spawning aggregations too," Dr Mourier said.

If sharks don't have subsidies for their food they have to travel long distances to find food and that may be why the densities [on other reefs] are less."