Individual study: The potential of a native sea urchin, Tripneustes gratilla, to control invasive red algae Kappaphycus spp. in Kane’ohe Bay, Hawaii, USA
Conklin E.J. & Smith J.E. (2005) Abundance and spread of the invasive red algae, Kappaphycus spp., in Kane'ohe Bay, Hawai'i and an experimental assessment of management options. Biological Invasions, 7, 1029-1039
Several species of red algae Kappaphycus spp. were intentionally introduced into Kane'ohe Bay, Hawai'i in the 1970s. Despite predictions that Kappaphycus would be incapable of effectively dispersing from the initial site of introduction, they have spread rapidly throughout the bay and are now found in a variety of reef habitats where they overgrow and kill corals. Because Kappaphycus spp. are still spreading in Kane'ohe Bay an investigation was undertaken to look at control options including the use of a native sea urchin, Tripneustes gratilla, as a potential biological control agent. (For attempts at manual remaoval, see Case 252).
Study site: An experiment was undertaken to assess the ability of a native sea urchin, Tripneustes gratilla, to control invasive red algae Kappaphycus spp. in Kane'ohe Bay, Hawai'i. Trials were undertaken on three shallow (0.5–2 m deep) reefs: Mark's Reef, Reef 29 and Reef 44.
Experimental design: In July 2002, on each of the three study reefs, three 0.25 m² plastic-coated chicken wire mesh fence enclosures were constructed in areas where Kappaphycus was abundant. Digital photographs were taken of each enclosure using a photoquadrat and the percentage cover of benthic organisms measured by tracing the area within each photograph occupied by each benthic species (Image-J software). When Kappaphycus abundance had been quantified, a single sea urchin was placed inside each enclosure. Photographs were subsequently taken approximately every six weeks until November 2002, to record changes in Kappaphycus abundance over time.
Effescts of sea urchin grazing: A single T.gratilla was able to substantially decrease the abundance of Kappaphycus spp. within an enclosure in five months. Across all three reefs, urchins reduced the cover of Kappaphycus from an initial average of 62.5% (±6.0 SE) to 15.9 % (±4.6 SE). Live coral cover remained similar throughout the study, with abundance ranging from 11.8% cover (±5.1 SE) in July 2002, to 14.3% cover (±6.2 SE) in Dec 2002. Sessile invertebrates, fleshy, and calcified algae all increased as Kappaphycus decreased. Urchin grazing scars were evident on Kappaphycus thalli within enclosures indicating consumption by sea urchins had occurred.
Conclusions: The native sea urchin, Tripneustes gratilla, reduced the biomass of Kappaphycus spp. in the small experimental enclosures and their use as a biocontrol agent appears quite promising. These urchins appear to prefer Kappaphycus over many other species of algae and the study demonstrated that they were highly successful in rapidly removing large amounts of algal biomass. There was no corresponding increase in coral cover, probably due to the experimental period being too short relative to the settlement and growth of coral species to observe recovery.
T.gratilla is common on many Hawaiian reefs but occurs in very low abundance on reefs in Kane'ohe Bay. It is unclear why this is, the urchin may have been over-fished and/or other physical or biological factors may be involved. To further examine their potential as biocontrol agents, more research would be needed to determine if these grazers can remove Kappaphycus spp. from large reef areas, and to establish any potentially negative effects of enhancing their populations in Kane'ohe Bay.
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