Invasive, reef-smothering algae are being kept in check at Kaneohe Bay thanks to tiny, native sea urchins that graze on them the same way goats feed on grass.

The collector sea urchins, or Tripneustes gratilla, are found naturally in Hawaii waters but were bred in a Sand Island hatchery for a recently published study that shows they are part of an effective management strategy for controlling the algae.

For years divers have been removing the invasive algae manually and feeding them into an underwater vacuum to get them off the reef. It’s a labor-intensive task, according to Brian Neilson, one of the study’s authors, and the algae grows back within about six months.

The study found that a two-tiered approach — manual removal followed by “outplanting” of the sea urchins was a more effective way to keep the algae off the reef.

“They’re kind of like little goats of the sea,” said Neilson, who is also acting administrator of the state Division of Aquatic Resources under the Department of Land and Natural Resources. “They’ll mow down the algae.”

Over a two-year period, divers participating in the study manually removed invasive algae from nearly six acres of coral reef at Kaneohe Bay and then placed 99,000 hatchery- bred urchins.

Researchers found the algae declined by 85 percent after the project, with no apparent negative effects to the reef.

Scientists had previously tested the strategy in a laboratory, according to co-author Chris Wall, as well as in small enclosures and on small patches of the reef at Kaneohe Bay, but never on such a large scale.

“It’s safe to say our study is the first of its kind using native urchins at the scale that we used,” said Wall, a doctoral candidate at the University of Hawaii.

The other authors of the study are Frank Mancini and Catherine Gewecke.

Kaneohe Bay was ideal for the project because of its topography and long history of problems with invasive species.

The bay is home to five species of invasive algae the urchins eat, according to Neilson. Three, from the genus Kappaphycus and Eucheuma, are closely related and are generally called smothering seaweed. The other two are Gorilla ogo (Gracilaria salicornia) and prickly seaweed (Acanthophora spicifera).

The invasive algae Kappaphycus and Eucheuma were first introduced to the bay in the mid-1970s as part of an effort to cultivate carrageenan, an additive used to thicken food and drinks, according to the state.

While the collector sea urchins, so named because they collect items on their short spines, are native to Kaneohe Bay, their populations are low and they have not been found to spawn there naturally, according to Wall. However, they have been spawned successfully by the state at a Sand Island hatchery for the past eight years. In 2014, the hatchery raised and released 112,000 of the urchins.

After four to five months, the sea urchins have grown to the size of a quarter and are ready to be placed on the reef by divers.

Neilson said the state is looking at applying the same strategy to other areas, such as the coral reef in front of Waikiki Aquarium and possibly Maunalua Bay.

“There are invasive algae all along the south shore of Oahu,” he said. “There’s lot more work to do.”