Abundance and spread of the invasive red algae, Kappaphycus spp., in Kane'ohe Bay, Hawai'i and an experimental assessment of management options

Summary

Study 1

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 are still spreading in Kane'ohe Bay and can form over 50% cover on some reefs, an investigation was undertaken to look at control options including the effectiveness of manual removal.

Study sites: To assess the efficiency and effectiveness of manual removal of Kappaphycus spp. as a control option, three 28 m transects were established in each of three reefs in Kane'ohe Bay, Hawai'i. The three reefs (0.5–2 m deep) were: Mark's Reef, Reef 29 and Reef 44. In April 2002, eight 0.25 m² plots were established on each transect roughly every 4 m, with the corner of each plot permanently marked with a stake. Plots were classified as being dominated by live coral, coralline pavement or unconsolidated rubble. The percentage cover of all benthic organisms within each plot was estimated by the point intercept method, using a double-strung 0.5 m quadrat with six lines every 7 cm giving a total of 36 intersections.

Manual removal: One snorkeler manually removed all Kappaphycus spp. within each plot, using forceps to remove algal attachment points, while a second snorkeler used a hand-net to capture algal fragments in the water column that were left during the removal. For each plot, the time required to clear the Kappaphycus was recorded, and all removed was spun 10 times in a mesh bag (to remove excess water) and weighed to the nearest 0.25 kg.

Monitoring of regrowth & benthic organisms: One plot from each transect was used to monitor the rate at which Kappaphycus re-grew from any residual algal material that was left after manual removal. These plots were left undisturbed except to remove any drifting algal fragments that settled on them. The cover of benthic organisms within plots was estimated at approximately 6-week intervals over the following 12 months.

Attachment-point re-growth: Remove all algal material was attempted during manual removal but some tissue remained on the substratum at the attachment point. To assess the ability of Kappaphycus to re-grow from these tissue remnants, 13 pieces of rubble (dead Porites compressa coral) with Kappaphycus attached, were brought to the Hawai'I Institute of Marine Biology in Kane'ohe Bay. The number of Kappaphycus plants attached to each piece of rubble was counted, and each peice of rubble was then scraped by hand, to remove all visible tissue. The rubble was then placed in outdoor, flow-through seawater tanks with natural lighting. After two months, the pieces of rubble were examined and the number of emergent Kappaphycus plants and branches growing on each counted. Due to the lack of sexual reproduction in these species, all algal growth observed could be attributed to growth of existing tissue as opposed to settlement of new algal spores.

Manual removal: The biomass of Kappaphycus spp. removed from the seventy-two 0.25 m² plots varied significantly with habitat type. Comparisons found that the rubble habitat contained significantly less biomass than the other two habitats (coral or pavement). Even the rubble, however, had on an average almost 15 kg/m² of wet weight Kappaphycus. The time required to remove all Kappaphycus from the 0.25 m² plots also varied with habitat type. Pavement habitat required significantly less time to clear than coral or rubble. The pavement habitat, however, still required an average of almost two person hours to clear to clear 1 m². Percent cover of Kappaphycus was reduced from an overall average of 56% (±6.7 SE) in the plots to effectively 0% by manual removal in May 2002.

In the subsequent 12 months, all three reefs showed substantial re-growth of the algae from tissue remaining following manual removal. Abundance of Kappaphycus in May of 2003, one year after initial removal was 39% cover (±10 SE), 57% cover (±23 SE) and 89 % cover (±23 SE) at Mark's Reef, Reef 29 and Reef 44, respectively.

Re-growth experiment: Rubble used in the experiment had an average of 7.7 (±1.1 SD) plants growing out of attachment points prior to removal. Two months after removing all visible algal tissue, there was a mean of 4.7 (±1.4 SD) branches re-growing from attachment points, representing a 61% recovery in branch density.

Conclusions: Manual removal is too time-consuming for large-scale removals. While the biomass of Kappaphycus spp. was least in habitats composed of rubble, most likely due to the scarcity of stable attachments, all habitat types on the experimental reefs contained much algae. While pavement habitats required the least amount of time to clear due to the lack of topographic complexity, all habitats still required prohibitive amounts of time to clear. As a potential means of increasing removal efficiency, a modified dredge capable of removing large quantities of algae via suction is currently undergoing testing.

Even if efficient means of removal are developed, the rapid re-growth of Kappaphycus following removal is a problem. In the experimental plots, extensive re-growth was observed within two months of removing all visible algal, Kappaphycus re-growing from tiny amounts of the residual tissue. Another factor that may have contributed to the rapid Kappaphycus regrowth is the low preference that native herbivorous fishes have for the algae. Within their native range, the primary grazers upon Kappaphycus are siganids (a family of fish not found in Hawai'i) and sea urchins, which are not abundant within Kane'ohe Bay. However, the use of a native sea urchin, Tripneustes gratilla, as a biocontrol agent appears quite promising (see Case 254).

 

Study 2

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.

Effects 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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