NOAA chief scientist reviews coral reef monitoring cruise in Marianas

By Ron Rocky Coloma

Every morning for 75 days, just after sunrise, small boats launched from the NOAA ship Oscar Elton Sette. Divers zipped across open ocean, dropped into turquoise water and sank into silence — notebooks ready, cameras clicking, eyes scanning for coral and fish.

This was the daily rhythm of the 2025 Mariana expedition, part of NOAA’s National Coral Reef Monitoring Program. It’s not glamorous, but to research ecologist Thomas Oliver, it’s essential work. "We literally counted fish,” he said. “That’s one of the most important things we do.”

Oliver, based at the Pacific Islands Fisheries Science Center in Honolulu, led the expedition with a team of scientists tracking coral health, ocean chemistry and reef biodiversity across Guam and the Northern Mariana Islands. In a recent presentation, he walked through what they found, what surprised them and what it all means for the future of Pacific reefs.

The Oscar Elton Sette served as their home base, but most work happened from small boats — three launched daily to cover different sites. Divers collected 431 fish surveys and 313 photogrammetry images at random sites. Some dived into reefs brimming with life. Others found bare rock and fading coral.

“The ocean doesn’t always give you what you want,” Oliver said. “But it gives you the truth.”

The National Coral Reef Monitoring Program is a long-running project backed by NOAA and the Coral Reef Conservation Program. It tracks conditions across U.S. coral reef regions — in the Pacific, Atlantic and Caribbean — using a standardized approach. In the Pacific, it’s run from Pearl Harbor, Hawaii, where Oliver and his colleagues work out of a lab just steps from the water.

Their methods have evolved. Divers still use clipboards, but they now carry cameras, too. They build 3D maps of the seafloor using photogrammetry and feed images into machine-learning tools that help identify coral species and detect subtle changes in reef structure. They also track water chemistry, ocean temperature, reef erosion and fish biomass — combining biological and physical science with social and economic studies.

“You can’t protect reefs without understanding the people connected to them,” Oliver said. “That’s a big part of this.”

This year’s cruise covered a wide gradient, from the heavily populated reefs off Guam and Saipan to the uninhabited volcanic peaks of Agrigan and Maug.

“In places like Maug, the fish just feel different,” Oliver said. “You’re surrounded by sharks and parrotfish and clouds of damselfish. It’s like being inside a time capsule.”

The team also recorded data on coral cover. Unlike fish, coral health didn’t differ as sharply between populated and remote islands. Overall, the data showed a slow decline in calcifying organisms — corals and crustose coralline algae — and a slow increase in turf algae. Reefs, in other words, are losing ground.

The culprit is mostly heat. NOAA’s Coral Reef Watch program has documented five mass bleaching events in the region since 2010. The 2025 team recovered dozens of subsurface temperature recorders showing sharp summer spikes, even in the more temperate northern islands.

“You can see it in the data. You can feel it on the dives,” Oliver said. “The heat is everywhere.”

Not every reef is suffering. Maug showed high coral growth rates. Guam had strong pockets of coral cover in Tumon Bay and other marine preserves. Agrigan and Pagan, despite their remoteness, showed signs of erosion but also recovery.

In 2022, Pagan had a crown-of-thorns starfish outbreak. These venomous predators can devastate reefs when their population booms. This year, the starfish were still there — but fewer, less active. Coral cover remained low, but Oliver said it’s possible the worst has passed.

“There’s resilience out there,” he said. “You just have to look closely.”

Teams deployed small cement plates and blocks of dead coral across the archipelago, then came back years later to see what had grown or eroded. They used CT scans and microscopes to measure the tiniest changes. In many sites, the reef was still building, but others had tipped into net erosion — losing structure faster than it formed.

These findings help scientists model how reefs may grow, shrink, or collapse in the future. Oliver’s team uses demographic models to predict coral population trends and test what interventions — like protecting adult colonies or improving water quality — might work best.

“We’re asking, what would it take for this reef to recover?” he said. “It’s not just a research question. It’s a management question.”

All of the 2025 data, including fish surveys, coral imagery and chemistry readings, will be made publicly available through NOAA’s data visualization tool. Local governments, resource managers and community groups can use the site to explore conditions at specific reefs or download raw data.

In the Marianas, where fishing, tourism and culture are deeply tied to the ocean, that information matters.

“It’s their reef,” Oliver said. “We just help read the signals.”

Subscribe to

our digital

monthly edition