The effects of a selective corallivore on growth rates and competition for space between two species of Hawaiian corals

Chaetodon unimaculatus Bloch, in Kaneohe Bay, Hawaii, shows a strong preference for one of the two common corals, Montipora verrucosa (Lamarck). The impact of this selective corallivory was estimated by measuring coral consumption rate, effects on growth rates, and competitive interactions between the two common coral species. Coral consumption was measured by feeding laboratory-held fishes Alizarin stained corals. Bite size averaged 2.54 mg ash free dry wt · bite−1. Feeding rates were measured by field observations; 7.2 bites · min−1 during the “winter season” and 4.9 bites · min−1 during the “summer season”. An exclosure experiment demonstrated significant effects on vertical growth rates of M. verrucosa, with a decrease in uncaged replicates of 53% during the summer of 1983 and 86% during the summer of 1984. Reversals of the expected competitive outcome between branches of the two species in contact were also noted on uncaged experimental colonies of M. verrucosa. The corallivore restricts most of its activity to the edge of the reef, with 80% of the time spent < 15 m from the edge. The abundance of M. verrucosa increased with increasing distance from the edge of the reef, consistent with the hypothesis that selective corallivory by Chaetodon unimaculatus has a significant influence on the abundance and distribution of coral species on patch reefs in Kaneohe Bay.