It appears an invasive seaweed that has plagued the reefs of Kaneohe Bay for more than 30 years has finally met its match all quarter-million of them.

The state Department of Land and Natural Resources recently put another 6,000 hatchery-raised sea urchins on selected reefs in the bay, bringing to roughly 250,000 the number of native collector sea urchins released so far in a program developed over the last decade.

Officials said they are ecstatic with the results so far. And, they said, as long as the spiny creatures are left alone to multiply and munch on the seaweed, the coral reefs of Kaneohe Bay can look forward to continued recovery and health.

Brian Neilson, state aquatic invasive species coordinator, said three major reefs have been cleaned with the help of the sea urchins and that now they’re working on a fourth one.

It’s all part of a two-tier approach that was developed in 2005 by the DLNR, The Nature Conservancy, and the University of Hawaii.

First, divers remove the algae. They scoop up thick mounds of kappaphycus seaweed from coral heads and feed it into a suction pump mounted on a barge nicknamed "The Super Sucker." This seaweed is then offered to farmers as a potassium-rich fertilizer for their fields.

Second, the sea urchins, or Tripneustes gratilla, are placed strategically on the cleared reef patches to gobble up the remaining seaweed. The urchins act as nature’s control measure.

The urchins are raised in hatcheries at DLNR’s Anuenue Fisheries Research Center on Sand Island, which was established in 2009.

"These native, herbivorous urchins maintain the areas like ocean gardeners or little goats of the sea. They keep the seaweed in check and give the corals a chance to recover," said David Cohen, DLNR sea urchin hatchery manager.

Called hawae maoli in Hawaiian, the urchins are black, softball-size creatures with stubby, blunt spines. They are safe to touch, but in other parts of the world, a similar-looking urchin can deliver a nasty sting.

Although this urchin is native to Hawaii, there aren’t enough of them on the Kaneohe reefs to control the algae. Their numbers used to be greater there, but unknown causes have made their populations decline.

Kaneohe Bay is home to one of the only barrier reef systems in the United States, offering shelter for an abundance of aquatic wildlife. But the corals have been ailing in recent years, under attack from non-native seaweed that was brought to Hawaii in the 1970s for experimental aquaculture projects that were later abandoned. The seaweed escaped and spread in the bay, forming thick mats choking out native seaweeds and smothering and killing corals and other organisms.

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, while scientists worked on coming up with a long-term solution.

Officials had a pretty good idea the sea urchin program would work after seeing the spectacular results of an experiment in 2010. Biologists placed 1,200 of the spiny creatures on a reef and left the other half of the test area urchin-free. Within five or six months the urchin side had little invasive seaweed left, while the algae on the other half grew back thicker than before.

The experiment was funded by a $60,000 federal grant from the National Oceanic and Atmospheric Administration’s Coral Reef Conservation funds and the U.S. Fish and Wildlife Service with matching funds by the Hawaii Invasive Species Council.

Since then production at the Anuenue Fisheries Research Center has increased to 60,000 urchins in 2012 and 93,000 in 2013 from 5,000 urchins in 2011, officials said.

This summer a mysterious disease was detected in the Kaneohe Bay sea urchin population, killing about 5 percent of the creatures. But the disease apparently ran its course after several weeks and is no longer a problem, said Neilson, the state aquatic invasive species coordinator.

With about 50 or so reefs in Kaneohe Bay and most of them needing help, Neilson said the state is planning to ramp up production of sea urchins in an effort that will probably take years to complete.

It costs about $400,000 a year for the Super Sucker and $335,000 a year for the urchin hatchery for a total of $735,000 annually.

"It’s not cheap, but it is worth it in restoring some of the best coral on Oahu," he said.

BIGGER BATCHES

The number of urchins being grown at the Anuenue Fisheries Research Center has increased dramatically:

2011

5,000

2012

60,000

2013

93,000