Tiny sea urchins are now devouring blankets of algae smothering some of the best coral reef in the state.

State scientists, environmentalists and a local canoe club released about 1,000 collector sea urchins yesterday at Kaneohe Bay with the goal of restoring the bay.

It was first time anyone successfully bred sea urchins to counter an alien seaweed in Hawaii.

David Cohen, urchin hatchery manager at the Anuenue Fisheries Research Center, said he used about a million larvae to produce 25,000 sea urchins that reached at least 15 millimeters in diameter in about five months.

Of that group, 1,000 juveniles were gently placed on a 500-square-meter patch of reef yesterday by divers with the state Division of Aquatic Resources. The Kaneohe Canoe Club helped by carrying the trays of urchins.

Jonathan Blodgett of the state Anuenue Fisheries Research Center said the goal is to expand to 10,000 to 25,000 urchins being released per month.

Christy Martin, spokeswoman for the Coordinating Group on Alien Pest Species, said the urchins are needed to control the algae genus Kappaphycus, also known as "smothering seaweed," which is invading Kaneohe Bay, the only barrier reef system in the United States and a coral system that provides shelter for breeding fish and replenishing surrounding areas with aquatic life.

"This whole system is a nursery for the fish," she said.

The culprit is a seaweed introduced to Hawaii for its commercial applications, such as keeping ice chunks out of ice cream, that escaped from the lab when the industry failed here, she said.

The seaweed acts like a blanket, covering coral reefs and killing them, said Kate Cullison, state aquatic invasive species coordinator. Without intervention, the reefs would die and lead to a dead zone.

"It’s really gnarly stuff," she said.

Scientists chose the collector urchin, or Tripneustes gratilla — a species native to Hawaii — because it stays on the reef and is an effective grazer on the algae.

For years, the state used its Super Sucker machine to remove the algae as a temporary measure, at one time taking away 10,000 pounds of it, until scientists could come up with a long-term solution.

In 2009, the scientists gathered urchins from other parts of the state and released them at Kaneohe Bay. They found the urchins had successfully kept the seaweed down a year later, exposing the three-dimen- sional aspect of the reef, such as the puka that provide shelter for fish and lead to diverse ecosystems. Cullison said the method could eventually be used in other parts of the state.

But lots of sea urchins are needed. About four urchins per square meter is desirable, and at the same time, the urchins face the danger of becoming dinner for puffer fish, octopus and humans.

Cohen, the urchin hatchery manager, said he spent about a year innovating the method to breed sea urchins in captivity. The delicate young have to be kept suspended in the water column for weeks. Cohen is working on another batch and hopes to have about 20,000 urchins ready to release by April.