On Oahu’s Sand Island a small team of coral specialists and technicians has been working for years to restore the slow-growing coral reefs in Hawaii that are being subjected to increasingly frequent bleaching events.

The corals in the state, like many around the world, are threatened by climate change-driven bleaching, in which food-providing algae that live in their tissues abandon them once ocean temperatures become too warm, potentially killing the corals.

The work of the Division of Aquatic Resources’ Anuenue Coral Restoration Nursery, which began in 2016, highlights the particular needs of Hawaii’s corals, which are among the slowest-growing in the world.

“If you go to Florida, coral grows 15-20 centimeters per year. If you go to Australia, it’s 20- 25 centimeters per year. Hawaii, it’s 1 to 2 centimeters per year on average,” said Dave Gulko, a coral biologist for DAR who runs the nursery. “When you see a coral that’s 1 meter here in Hawaii, that’s hundreds of years old.”

Coral reefs provide important ecological services to thousands of ocean species but also have important economic value. The National Oceanic and Atmospheric Administration estimated the total economic value of coral reefs in the main Hawaiian Islands to be over $33 billion, which factors in how much people are willing to pay to protect the reefs as well as the services and goods the reefs can provide.

But with warming oceans, bleaching events occur more frequently, giving surviving corals less time to recover from previous bleaching events. Shorter recovery times and slower growth rates of Hawaii’s corals exacerbate the problem.

The nursery is a small operation, and its seven- member staff, along with a few students and volunteers, works largely out of a few rooms in a building not much larger than a shipping container. There the team cuts, grows, houses and experiments on dozens of coral species.

The nursery is looking for ways to make Hawaii corals grow faster. Through its Fast-Grow Protocol, it can do that by taking fragments of endemic, reef-building corals and cut them into dozens of tiny fragments, a process called microfragmentation.

The fragments are often taken from harbors, where they provide fewer ecological services than corals growing out at sea, and go through a quarantine program. Harbor corals are more susceptible to carrying algae, invasive species and chemicals from ships.

“As long as the live tissue is clean for 30 days, then they’re deemed passable, and we move them on to the next stage, where we frag them in our fast-grow room,” said Chelsea Wolke, one of the nursery’s coral specialists.

Because of the way corals grow, the smaller the fragments are, the more surface area from which they can grow.

Using a band saw in one of the nursery’s rooms, the staff produces the fragments, which are then glued onto cement modules shaped into pyramids, serving as substrate on which they can grow.

“If you think of a pyramid, it has a nice, wide base,” said Norton Chan, the nursery’s senior coral specialist. “It’s more stable, lower center of gravity. So that way it’s very unlikely that it’ll flip over.”

The cement modules are made on-site and placed in buckets of fresh water so they can be leached of any chemicals before being put back into the ocean.

The fragments, glued a few centimeters apart from each other, are for eight months to a year given “5-star hotel” treatment: optimal lighting, tank cleaning, food and other necessary nutrients for growth.

As the fragments grow outward and cover the cement module, they eventually touch and form a single colony that covers the module completely. Then the modules are placed outdoors to acclimate to outdoor conditions, or “budget hotels.”

“Essentially, we take them from being fed regularly, pure, beautiful water quality and a lot of maintenance that goes into cleaning them, out into here,” said Callie Stephenson, one of the nursery technicians, shin-deep in an outdoor pool of water while handling a pyramid of coral.

The corals, Stephenson said, are slowly subjected to outdoor conditions, including simulated wave action, before being planted in the ocean.

The corals that go through microfragmentation grow about 40 centimeters in a year, much faster than the 1 centimeter they would grow on their own. The nursery plants about 80 of these modules a year, but with new facilities coming within the next few months, they’ll be ready to produce over 200 modules annually.

Still, the nursery has limited space, and most of the rooms have giant tanks of coral-studded cement pyramids lined against the walls. The rooms are also cast in blue, the color of the lights used to grow coral.

Planting large coral modules in the ocean is important, Gulko says, because they are ready to reproduce and provide important ecological services as soon as they are placed into the ocean.

“Imagine the difference between grass and a big koa tree. … With a koa tree you get shade, you get a place for birds to live, you get all the plants that can live in that shaded environment, you get a windbreak — you get all these cool things,” he said. “You get none of that with grass.”

The nursery’s primary focus is on microfragmentation, but it is working on a variety of other projects to accommodate Hawaii’s corals.

One of the more important ones is the nursery’s dedicated “coral ark,” housed in a few indoor tanks containing rare — some that have yet to be described — and endemic corals.

“We’re not trying to fast-grow these; we’re just trying to keep them alive,” said coral specialist Christina Jayne. “In case something happens to their wild counterparts, we essentially have a backup.”

The rare corals are kept as an “insurance pool,” which Gulko said started in 2015 after being warned of the major bleaching event that affected nearly half of Hawaii’s corals.

Other projects include the use of larvae to fast-grow the coral, different modules to grow branching corals and even expanding on the microfragmentation technique itself to grow 1-meter coral modules in a year, which represents over 100 years of growth in the wild.