An Attempt to Increase Numbers of Herbivorous Fishes as a Means of Controlling Populations of Fleshy Macroalgae on Coral Reefs in Kane'ohe Bay, Hawai'i

Abstract

This study was conducted to determine the feasibility of enhancing fish populations as a means of controlling macroalgal populations in Kane'ohe Bay, O'ahu, Hawai'i. Fleshy macroalgae have overgrown corals on reef slopes of Kine'ohe Bay. Such shifts to fleshy macroalgal domination are often thought to be due to a decrease in abundance of herbivorous fishes. This experiment added 650 herbivorous fishes (acanthurids and scarids) to two reefs, constituting a potential addition of approximately 70% to the total populations of the two reefs. Fish censuses and grazing assays were used to assess the effectiveness of these additions in increasing grazing on these reefs and thereby diminishing the abundance of macroalgae. Fish censuses showed a smaller than expected increase in acanthurid abundance across all reefs, including the control reef, and no increase in scarid abundance. Grazing assays did not show any significant differences between pre- and postaddition. The fishes did not appear to remain on the small isolated reefs to which they were added. It is possible that habitat degradation and lack of shelter on the experimental reefs made them unsuitable for enhanced herbivore populations, because initial and postaddition biomass/ unit area was smaller than the published values for many sites. Increasing the abundance of shelter may be necessary to increase the number of fishes on these reefs.