Blacktip reef sharks off Sanya show coral revival

Blacktip reef sharks have once again returned to the shallow waters off Wuzhizhou Island in Sanya, Hainan province, a seasonal appearance that marine scientists say reflects the steady recovery of one of China's restored coral reef ecosystems.

Over the past week, visitors have spotted several of the sleek predators cruising near the island's designated marine observation area, known as the sightseeing corridor.

Measuring about 1.2 to 1.4 meters in length, the sharks have become familiar summer visitors, with their annual return offering scientists encouraging evidence that the surrounding reef is regaining its ecological health.

"Blacktip reef sharks are highly selective about their habitat," said Zhou Zhuocheng, deputy director of the native aquatic biology and water ecology committee under the China Fisheries Association. "Their presence directly shows that the reef ecosystem here is recovering."

Healthy coral reefs provide food and shelter for the sharks while supporting the complex food web on which they depend. As predators near the top of the local marine food chain, blacktip reef sharks thrive only where fish populations are abundant and water quality remains high, Zhou said.

Known scientifically as Carcharhinus melanopterus, the blacktip reef shark is listed as "near threatened" by the International Union for Conservation of Nature because of global pressures from coral reef degradation and overfishing.

The annual return of the sharks is especially significant because the waters around Wuzhizhou Island were once heavily degraded. By the late 2000s, overfishing and land-based pollution had reduced live coral cover to below 15 percent, leading to a sharp decline in marine life.

The recovery began in 2010, when Wuzhizhou Island partnered with Hainan University to establish an experimental marine ranch. Since 2016, researchers and dive teams have transplanted cultivated coral colonies and deployed artificial reefs, including concrete structures, scuttled boats and volcanic rock platforms, to restore underwater habitats.

The restoration has produced measurable results. More than 68,000 coral colonies have been transplanted across 61,000 square meters of seabed, while live coral cover has rebounded to 37.08 percent, according to the Wuzhizhou Island Tourist Area. Fish diversity has increased to 265 species, and water quality has consistently met China's highest national marine standard.

Wang Aimin, a 65-year-old retired marine professor at Hainan University, said the recovery has also been supported by the expansion of marine ranching projects and China's annual summer fishing moratorium, which reduces disturbances from fishing vessels during the sharks' seasonal visits.

"Without nearby fishing boats disturbing them, the sharks are even more at ease," he said.

The healthier ecosystem is attracting more than just blacktip reef sharks. In recent years, wild dolphins, endangered hawksbill turtles, spotted seals, whale sharks and giant groupers have all been recorded in the island's waters.

Earlier this year, an underwater photographer also captured a leaf sheep sea slug, a tiny, 1-centimeter-long marine creature so sensitive to pollution that biologists regard it as another indicator of reef health. Together with the annual return of the blacktip reef sharks, its appearance provides further evidence that Wuzhizhou Island's underwater ecosystem continues to recover.