Cook Islands crusade against starfish, the latest scourge for the dying coral reef.

In the Cook Islands, divers armed with wooden spears are waging a rudimentary war to save coral reefs from starfish, a crucial battle for ecosystems already weakened by climate change. These improvised tools are the Korero O Te Orau association's best weapon ("Knowledge of Earth, Sky, and Sea" in the Maori language) in the war against the "crown of thorns," a species of starfish that feeds on coral and devours tropical reefs.

The Cook Islands, an island nation in the South Pacific covering just under 240 square kilometers and home to approximately 17,000 people, is gripped by a years-long invasion, according to marine biologist Teina Rongo. "They can completely destroy the entire coral reef surrounding the island," warns Rongo, who trains volunteer groups to protect the reefs of Rarotonga, the archipelago's largest island. "I believe the invasion is currently affecting the entire Pacific: we know other countries are facing similar problems."

A single adult crown-of-thorns coral reef shark can devour more than 10 square meters of coral each year, all the while compressing its stomach through its mouth to coat the coral with digestive juices. These invertebrates pose a serious threat to Australia's Great Barrier Reef, where scientists have developed robots to track them and inject them with venom.

"At the moment, we mainly kill them by injecting them," says Sven Uthicke, a researcher at the Australian Institute of Marine Science. "It could be vinegar, lime juice, or ox bile," while "others are developing chemical traps," he says. "This is all very promising, but these techniques are still being developed."

Rongo believes the fastest way to remove starfish is to use a stick made from Pacific ironwood, a particularly hard wood. "We've made some modifications over time because we were getting stung by these starfish. It's painful," he says.

Named for their hundreds of tiny venomous barbs, crown-of-thorns can grow to a size larger than a tire. And, according to the Australian Institute of Marine Science, they are reproducing at "epidemic proportions," making them a major cause of coral decline. Researchers suspect these infestations are triggered by a combination of factors, including agricultural waste dumped into the sea and fluctuations in the number of natural predators.

But their harmful effects are worsening with the weakening of coral reefs due to coral bleaching and ocean acidification, two phenomena linked to climate change. Starfish are sometimes difficult to spot, stuck in dimly lit crevices. Once detached from the coral by volunteer divers, they are skewered with a thick rope and hauled aboard a boat. The day's catch is dumped into a plastic container before the starfish are transported to shore.

The goal? Count them, measure them... but also grind them to make fertilizer for gardens. Every year, volunteers from the Korero O Te Orau association remove thousands of them. Rongo is motivated by the devastation caused by the country's last major infestation in the 1990s. He says he was already involved in the fight back then. But "we reacted too late. The process continued and ended up destroying the reef."