Invasive seaweed is putting a deadly choke hold on Hawaii's coral reefs. In an effort to save them and the fish that rely on coral as a habitat, scientists have started breeding native sea urchins to eat the offending seaweed.

The culprits are two seaweed algae called Kappaphycus alvarezii and K. striatum. First brought to Hawaii in 1974 for commercial cultivation, the seaweed soon made its way into the wild and has now been plaguing the 50th state's coral reefs for years. The seaweed blankets the coral, smothering it to death, the Honolulu Star–Advertiser reports.

According to the University of Hawaii at Manoa Botany Department, Kappaphycus alvarezii grows up to two meters tall and can double its biomass every 15 to 30 days.

To combat the smothering seaweed, scientists from Anuenue Fisheries Research Center have now started breeding native collector urchins (Tripneustes gratilla), which can eat the invaders without damaging the local coral. The idea was first proposed in 2007, tested from August 2009 to August 2010, and is now being rolled out on a larger scale. The first 1,000 juvenile urchins, each about 15 milimeters wide, were placed on a 500-square-meter area of reef this weekend. As more urchins are bred, as many as 25,000 will be released each month.

They'll need more urchins than occur naturally—about four per square meter of reef—if they hope to wipe out the seaweed. (Full-grown collector urchins are less than 15 centimeters wide, so there's only so much seaweed each can eat.) They'll also need to be continually replaced, because as they feast on seaweed, they will also likely be eaten by fish and octopus.

David Cohen, Anuenue's urchin hatchery manager, told the Star–Advertiser it took a year to develop the techniques to breed the sea urchins in captivity.

Previous attempts at seaweed control included an expensive device called the Super Sucker, which can vacuum about 350 kilograms of seaweed from a reef in an hour. A pilot Super Sucker project removed more than 22,500 kilograms of the algae from the reefs a couple of years ago, but it was only intended as a temporary solution, and Hawaii's state budget cuts put an end to the project's funding. A smaller version of the sucker—called Super Sucker Junior—cost $50,000 to build and $150,000 a year to operate. By comparison, last year's test of the urchin solution was funded by a $60,000 grant from the National Oceanic and Atmospheric Administration and the U.S. Fish and Wildlife Service.

Photo: Kappaphycus alvarezii, via of the University of Hawaii