An explosion of sea urchins threatens to push coral reefs in Hawaii ‘past the point of recovery'

CNN

21-06-2025

The turquoise water of Hōnaunau Bay in Hawaii, an area popular with snorkelers and divers, is teeming with spiny creatures that threaten to push the coral reef 'past the point of recovery,' new research has found.
Sea urchin numbers here are exploding as the fish species that typically keep their populations in check decline due to overfishing, according to the study, published last month in the journal PLOS ONE. It's yet another blow to a reef already suffering damage from pollution as well as climate change-driven ocean heat waves and sea level rise.
Kelly J. van Woesik, a researcher at the North Carolina State University Center for Geospatial Analytics and a study author, first noticed unusually high numbers of sea urchins on snorkeling trips. 'I knew there was a story to be told,' she said.
She and her fellow researchers used data from scuba surveys and images taken from the air to track the health of the reef.
'We found on average 51 urchins per square meter, which is among the highest recorded densities on coral reefs anywhere in the world,' van Woesik said.
Sea urchins are small marine invertebrates, characterized by their spiny bodies and found in oceans around the world. They play a useful role in preventing algae overgrowth, which can choke off oxygen to coral. However, they also eat the reef and too many of them can cause damaging erosion.
In Hōnaunau Bay, the coral is already struggling to reproduce and grow due to ocean heat and water pollution, leaving it even more vulnerable to the erosion inflicted by sea urchins.
Its rate of growth has plummeted according to the study.
Reef growth is typically measured by the amount of calcium carbonate — the substance which forms coral skeletons — it produces per square meter each year.
The reef in Hōnaunau Bay is growing 30 times more slowly than it did four decades ago, according to the study. Production levels were around 15 kilograms (33 pounds) per square meter in parts of Hawaii, signaling a healthy reef, according to research in the 1980s.
Today, the reef in Hōnaunau Bay produces just 0.5 kg (1.1 pounds) per square meter.
To offset erosion from urchins, at least 26% of the reef surface must be covered by living corals – and even more coral cover is necessary for it to grow.
Gregory Asner, an ecologist at Arizona State University and study author, said what was happening in this part of Hawaii was emblematic of the mounting pressures facing reefs throughout the region.
'For 27 years I have worked in Hōnaunau Bay and other bays like it across Hawaii, but Hōnaunau stood out early on as an iconic example of a reef threatened by a combination of pressures,' he said, citing warming ocean temperatures, pollution from tourism and heavy fishing.
The implications of coral decline are far-reaching. Coral reefs are sometimes dubbed the 'rainforests of the sea' because they support so much ocean life. They also play a vital role protecting coastlines from storm surges and erosion.
'If the reef can't keep up with sea-level rise, it loses its ability to limit incoming wave energy,' said van Woesik. 'That increases erosion and flooding risk of coastal communities.'
Kiho Kim, an environmental science professor at American University, who was not involved in the study, said the findings highlight the fragility of reef ecosystems under stress.
'Dramatic increases in any species indicate an unusual condition that has allowed them to proliferate,' Kim said. That imbalance can undermine diversity and reduce the reef's ability to provide essential ecosystem services including food security and carbon storage, he told CNN.
Despite the challenges, researchers emphasize that the reef's future is not sealed. Local groups in Hōnaunau are working to reduce fishing pressure, improve water quality and support coral restoration.
'These reefs are essential to protecting the islands they surround,' van Woesik said. 'Without action taken now, we risk allowing these reefs to erode past the point of no return.'