Off Puerto Rico's hurricane-battered east coast, divers are hand-mixing buckets of cement underwater to glue broken coral fragments back onto a reef — and the first expanded year of results already beat expectations.

  • 30,000 square meters of reef under restoration — about 5.5 football fields
  • 30,000 individual elkhorn corals planned by 2028
  • $5 million in NOAA Fisheries funding for the expanded project
  • 22 prior major coral restoration projects completed in Puerto Rico by Sea Ventures

A Reef That Survived When Others Didn't

Sea Ventures and NOAA have been restoring the Cayo Largo reef off Fajardo since 2018, but this year marked a major scale-up. With $5 million from NOAA Fisheries' Office of Habitat Conservation, the team is now rebuilding 30,000 square meters of reef habitat, aiming to outplant 30,000 elkhorn corals, or Acropora palmata, across 60 restoration plots by 2028.

The timing matters. Marine heat waves in 2023 and 2024 wiped out elkhorn populations at several restoration sites in southwestern Puerto Rico. At Cayo Largo, natural and restored elkhorn fared far better, for reasons scientists don't yet fully understand. That resilience is part of why the site was chosen for the expanded effort.

Elkhorn coral was once the dominant reef-building species across the Caribbean and is now considered threatened. Its sturdy, branching colonies build the three-dimensional structure that shelters fish, and reefs like it can reduce incoming wave energy by as much as 97 percent along a hurricane-exposed coastline.

How You Plant a Coral Underwater

The process is more construction job than science experiment. On a good day, a crew of six divers, a boat captain, deckhands, and scientists can place 300 to 500 coral fragments, ranging from tennis-ball to basketball-sized.

Deckhands mix buckets of a special cement mortar topside and pass them down to divers below, who mold the mixture into a small dome directly on the reef before setting the coral fragment on top. The cement hardens into a permanent, inert part of the reef structure.

"When you look at the areas where no restoration has happened, you see some young corals every now and then," said Sea Ventures coral biologist Manuel Nieves. "But when you see the areas where we've intervened, you see more corals growing, more fish. It reinforces your sense of purpose. I know I'm not wasting my time with what I'm doing."

Scientists then track each plot using photomosaics — divers swim a lawnmower pattern over the site shooting 2,500 to 4,000 photos, which are digitally stitched into a high-resolution 3D map used to measure coral growth and survival over time.

Why Elkhorn Coral Matters

Not every outplant survives, and future bleaching events remain a real threat to the young colonies. But early photomosaic data has encouraged the team studying it.

"It's very impressive — it goes from a small bit of palmata on cement to having branches and the whole cement mound overgrown in the first year and a half," said David Repollet, a University of Puerto Rico PhD student analyzing the data. "That's healthy, very motivated-to-grow palmata."

NOAA marine habitat specialist Michael Nemeth said the surviving natural corals at Cayo Largo point to a broader opportunity. "I think the fact that we have remaining healthy corals shows a potential for recovery," he said. "And the work that Sea Ventures is doing is helping that and speeding up recovery."

Much of the fragment stock comes from "rescue" corals — pieces broken loose by storms, coral settled on unstable rubble, or colonies salvaged from an abandoned Navy dry dock whose future is uncertain. Sea Ventures outplants at least 10 different elkhorn genotypes per plot, hedging against disease or bleaching wiping out any single genetic line. More detail on the project is available from NOAA Fisheries and Sea Ventures.