Cook Islands | The coral reefs threatened by the starfish

(Rarotonga) In the Cook Islands, divers armed with wooden spears lead a rudimentary war to save coral reefs from starfish, a crucial struggle for ecosystems already weakened by climate change.

These fortune tools are the best weapon of the Koro O Te Orau association (“Knowledge of the land, the sky and the sea” in the Maori language) in the war against the “thorns crowns”, a kind of starfish which feeds on coral and devours tropical reefs.

The Cook Islands, island countries of the southern Pacific of some 17,000 inhabitants, are in the grip of an invasion that has lasted for years, according to the marine biologist Teina Rongo.

“They can completely destroy the entire reef that surrounds the island,” alerts Mr. Rongo, which forms groups of volunteers for the protection of reefs on the island of Rarotonga.

“I think it is currently an invasion on a Pacific scale, because we know that other countries are faced with similar problems. »»

A single adult “crown of thorns” can eat more than 10 square meters of reef each year, pressing its stomach through its mouth to cover the coral with digestive juices.

These invertebrates constitute a major threat to the great barrier of Australian coral, where scientists have developed robots to spot them and inject them with poison.

“For the moment, they are mainly killed by injection,” explains Sven Uthicke, researcher at the Australian Institute of Marine Sciences.

“It can be vinegar, lime juice or beef bile” while “others are developing chemical traps,” he said.

“All of this is very promising, but these techniques are still at the stage of development. »»

“Epidemic proportions”

Teina Rongo finds that the quickest way to detach star of sea is to use a stick cut in Pacific iron wood – a particularly hard wood.

“We have made some modifications over time, because we were pricked by these starfish. It is painful,” he says.

Named thus because of their hundreds of small poisonous peaks, “thorns crowns” can reach a size greater than that of a car tire.

And, according to the Australian Institute of Marine Sciences, they reproduce in “epidemic proportions”, thus constituting one of the main causes of the disappearance of corals.

Researchers suspect that these infestations are triggered by a combination of factors, including discharges into the sea of agricultural substances and fluctuations in the number of natural predators.

But the damage they can cause worsen as the reefs are weakened by the laundering of corals and the acidification of the oceans, two phenomena linked to climate change.

Stars are sometimes difficult to spot, stuck in unlit cracks.

Once detached from the coral by volunteer divers, they are pierced with a thick rope in order to be brought up on a boat awaiting them.

The grip of the day is poured into a plastic chest before the starfish are transported to the ground. The objective? Count them, measure them … but also grind them to make it fertilizer for gardens.

Each year, the volunteers of the Kortero O Te Orau association withdraw thousands.

Teina Rongo is motivated by the ravages caused by the country’s last major infestation in the 1990s. He said he had already participated in the fight at the time.

But “we reacted too late. The process continued and ended up destroying the reef ”.