FRENCH POLYNESIA – Marine researchers have found that shark numbers outnumber prey in a French Polynesian shark sanctuary.

Up to 900 reef sharks can live 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, the Australian researchers have found.

The discovery, which suggests fish aggregations for mating and spawning play a key role in maintaining shark numbers on reefs, 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?”

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 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 sharks 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.

Dr Mourier said with such an 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 home 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.

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 similar healthy spawning aggregations in the past, but these had 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.” - ABC