Effect of Grazing and Browsing Fishes on the Zonation of Corals in Guam

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Abstract
Controlled coral transplant experiments were used to determine the effect of piscine corallivores on growth, zonation, and distribution of scleractinian corals in Guam. Pocillopora damicornis L. generally occurs only on the reef flat, reef margin (crest), and in lagoon habitats of the Indo—West Pacific. Although P. damicornis grows in deeper reef habitats in the eastern Pacific, it is seldom found in fore reef zones of the Indo—West Pacific. Colonies were transplated from a depth of 1.3 m in a barrier reef lagoon to depths of 15 and 30 m in fore reef zones of three fringing reefs, where P. damicornis was absent. All transplanted colonies were heavily fed upon and branch tips were removed primarily by chaetodontid and balistid fishes, but none of the colonies was killed. This feeding removed about one—fourth of the colony wet weight in 1 wk (°x 23.7% ± 2.4, .99 confidence limits, N = 118). There was a very significant differnt in the amount of fish feeding on transplants between three different study reefs (Fs = 459.30, P < .001). There was no significant difference in the amount of feeding between the two depths at each site on any one run. Polyp and mucus browsing by species which do not physically damage the corol skelton was also substantial. When colonies of P. damicornis from Apra Harbor (1.3 m) were transplanted with colonies of Pocillopora sp. from Fafai Reef (15 m), to Double Reef and attached to naturally occurring Pocillopora verrucosa colonies (15 m), fish discriminated between the three similar species and fed only on P. damicornis. While some authors have indicated that these species may comprise a species complex, the fish clearly discriminted between the species. Coral feeding by many reef fishes is a regular event and exerts great influence upon reef community structure. Although P. damicornis can survive and grow well in deeper habitats in fish exclusion cages, it generally does not occur in these zones in the Indo—West Pacific. It is therefore concluded that piscine corallivores are important in restricting the growth, local zonation, and general distribution of some coral species.

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