HATCHERY PRODUCTION OF Tripneustes gratilla FOR SEAWEED BIOCONTROL AND CORAL REEF CONSERVATION
The sea urchin Tripneustes gratilla is raised as a biocontrol agent to mitigate the effects of invasive seaweeds in Kaneohe Bay, Hawaii.
In the 1970s the eucheumoid seaweeds Kappaphycus alvarezii, Kappaphycus striatum and Eucheuma denticulatum were introduced to Hawaiian waters during aquaculture research trials. These seaweeds have become invasive pest species smothering native corals and contributing to the decline of coral reef habitat within Kaneohe Bay. Depressed populations of grazing animals have resulted in low herbivory allowing the eucheumoids to grow relatively unchecked within the Bay. The native collector urchin Tripneustes gratilla was identified, tested and found to be an effective biocontrol species to reduce the spread of invasive seaweeds.
Tripneustes gratilla is raised at the Anuenue Fisheries Research Center in Honolulu. Wild broodstock are collected and spawned monthly. First feed is administered three days after fertilization. Larvae are grown in 200L cone-bottom tanks with mild aeration. Larvae are maintained in filtered, UV treated seawater within a temperature range of 25C to 26C. Water exchanges or tank changes are performed daily. Larvae are fed on a mixed diet of Rhodomonas sp. and Chaetoceros muelleri. Competency in a majority of larvae is usually observed at 23 days post-fertilization. Competent larvae are moved to settlement tanks for metamorphosis and grow-out. Settlement tanks are prepared with natural biofilms. Post-larval urchins graze on biofilms for eight to twelve weeks. At 5mm to 7mm juvenile urchins are switched to a diet of cultured macroalgae. Within three to five weeks of feeding on macroalgae, urchins more than double in size and are ready for outplanting in Kaneohe Bay.
When urchins are three to five months of age and at a size of 15mm to 20mm, they are released onto coral reefs. Previously invaded reefs remain >95% clear of nuisance seaweed once urchins are present.