Ten years ago the patch reefs in Kaneohe Bay were in pretty sad shape — smothered and getting choked out by an invasive seaweed species introduced decades before.

Today, it’s a different story in the bay. Most of those reefs are pretty healthy and largely clear of the marauding algae, thanks in large part to a unique bio-control program started by the University of Hawaii at Manoa Pacific Cooperative Studies Unit and the state Division of Aquatic Resources.

It was 10 years ago that the first hatchery-raised sea urchins were released into Hawaii’s largest sheltered body of water, left to munch and mow down the seaweed like goats in a grass field.

To date, the sea urchin bio-control project has treated more than 227 acres of reef in Kaneohe Bay and has recently expanded to the Waikiki Marine Life Conservation District to control invasive algae.

Funded by the National Oceanic and Atmospheric Administration and the state, the hatchery has produced more than 600,000 sea urchins that have been released into the wild.

“This is one of the most effective marine invasive species control projects implemented in Hawaii,” said Shaya Honarvar, director of the university’s Pacific Cooperative Studies Unit.

The collector urchins (Tripneustes gratilla) are spawned, settled and raised at the state’s Anuenue Fisheries Research Center at Sand Island, a building that was once home to a prawn hatchery dating back to the 1970s.

The man in charge is David Cohen, a career aquaculture professional who accepted the challenge more than a decade ago to establish the world’s first hatchery for sea urchin used for bio-control.

It didn’t take long for Cohen to figure out how to raise the urchins, and he continues to work at improving the tricky process in a climate-controlled facility that handles the creatures from egg to maturity.

Parent urchins are collected from local reefs and taken to the hatchery to spawn. At any given time, the facility is occupied by as many as 15 million urchins in various stages of life, including some 20,000 to 60,000 young urchins earmarked for out-planting.

After about 26 days growing from larvae to urchin, they are moved into horizontal tanks lining the interior of the hatchery. After four to six months, the urchins are trucked to a boat for placement on reefs.

Kaneohe Bay is home to one of the only barrier reef systems in the United States, offering shelter for a wide variety of aquatic wildlife.

But the corals in the bay were ailing, under attack from non-native seaweeds brought to Hawaii in the 1970s for experimental aquaculture projects that were later abandoned. The seaweed escaped and spread throughout the bay.

Before the hatchery was created, two types of invasive macroalgae — Kappaphycus and Euchuema — grew in thick mats on top of reefs in Kaneohe Bay, blocking sunlight, killing coral and reducing the watery habitat’s ability to produce fish and marine life.

“Some places it was 2 or 3 feet deep,” Cohen said of the seaweed.

It was so thick that divers would first have to remove the algae, scooping it up in thick mounds before feeding it into a suction pump mounted on a barge nicknamed “The Super Sucker.”

The urchins would then be planted in a space where they could do their thing, paving the way for coral to regrow and allow for open spaces for fish, native seaweed and invertebrates to use as habitat.

While a 2018 UH study found that native sea urchins had successfully reduced the invasive, reef-smothering macroalgae by 85%, Cohen said he and others have seen patch reefs where most, if not all, of the invasive seaweed is gone.

Nowadays the Super Sucker machine isn’t needed. The bay’s reef areas are monitored for algae regrowth and urchins are out-planted as needed if invasive seaweed is detected.

In a few weeks, survey divers are scheduled to go out into the bay and determine where to out-plant sea urchins for the rest of the year.

“They will find plots that need spot-treating,” Cohen said.

Building on the success at Kaneohe Bay, the Division of Aquatic Resources last year began out-planting hatchery-raised urchins in the Waikiki Marine Life Conservation District. Some 100,000 urchins were placed in a 4-acre reef covered by invasive algae.

“We hope to see the same level of success in Waikiki to improve coral habitat and expand healthy reef coverage in the most visible MLCD in the state,” Wesley Dukes, DAR Habitat Monitoring Coordinator, said in a news release last year.

If other areas in Hawaii need invasive-algae cleaning, the $400,000-a-year project will consider sending urchins there as well, Cohen said.

Cohen said the project’s success has not gone unnoticed. Officials in Puerto Rico have contacted him and are hoping to replicate the effort following the pandemic using an urchin in the same genus. The Caribbean has been plagued by invasive seaweed over the past decade.

“I believe it will be a game changer in the Caribbean,” Cohen said.