Attempt to Control the Invasive Red Alga Acanthophora spicifera (Rhodophyta: Ceramiales) in a Hawaiian Fishpond: An Assessment of Removal Techniques and Management Options1

Acanthophora spicifera (Vahl) Børgesen was unintentionally introduced to Hawai‘i in 1950 and has since become the most common nonindigenous algal species in the main Hawaiian Islands. On the west coast of Hawai‘i Island it has been documented at three sites, including Kaloko Fishpond in Kaloko-Honokōhau National Historical Park. The fishpond has an open connection to the sea, increasing the risk that A. spicifera will establish itself on neighboring shallow coral reefs and rocky intertidal habitats. To diminish that risk and to develop an efficient management strategy, a range of approaches was assessed to control this invasive alga in Kaloko Fishpond. Removal techniques were labor intensive and had limited effect. All experiments showed a substantial initial decrease in algal density, but the long-term effect was minimal because of rapid regrowth. The most promising removal method was the use of submerged shelters to raise local densities of herbivorous fishes. Fishes grazed the alga and quickly reduced the biomass. However, the large number of predators and absence of topographical structure will make it challenging to provide sufficient shelters to increase the herbivorous fish population in the entire fishpond. A management strategy to substantially reduce the algal biomass in the fishpond includes a combination of biological control and periodic manual removal of the alga.