Overgrazing of Algae by Herbivorous Marine Fishes

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The growth of benthic algae in the intertidal zone in the Hawaiian Islands is usually luxuriant but in deeper regions, where suitable bottom affords attachment, it is often restricted to a low stubble. The paucity of algae is most evident in reef areas in which the available hard surface for plant growth is restricted by the presence of living coral. Since herbivorous animals of a size greater than a few inches do not graze extensively on algae in the intertidal zone (tides in Hawaii are small), the short length of the thalli on rock or reefs which are always submerged may result from heavy grazing by plant-teeding animals. The dominant herbivores in Hawaiian waters are fishes, especially the surgeonfishes (Acanthuridae) and certain parrottishes (Scaridae). These groups represent the most abundant fishes of moderate size regardless of food habits. Paradoxically, they are still the most abundant in coralliferous areas (an analysis of the populations of Hawaiian reef fishes by underwater transect has been made by Brock 1954). Other important plant-feeders are the omnivorous damselfishes (Pomacentridae) and various echinoids. 'lo test the overgrazing hypothesis, 4 enclosures of oneinch mesh wire one square toot in area were attached to coral-free sections of the rocky bottom of coral-rich Kealakekua Bay near Captain Cook's Monument on the island of Hawaii at depths of 4 to 6 feet in April, 1955. The enclosures prevented larger herbivores from feeding on the algae. Prior to being covered, the small areas were observed to be grazed repeatedly by herbivorous fishes, especially the surgeonfishes Acanthurus leucoparcins, A. antilles and Zebrasotna flavescens. The surgeonfish Ctenochaetus strigosus is at least as common as t^.e above species but it is a detritus feeder and the algae it consumes is mostly already detached from the bottom (Randall 1955). The most abundant acanthurid in inshore waters in Hawaii, Acanthurus triostegus, is not common in areas with high coral cover like the experimental site, probably because it is less inclined than other species of Acanthurus to crop close to the substratum (Randall, 1961). A study site on Hawaii was selected because a comparable area on Oahu is more apt to be disturbed by the many skin divers and fishermen at this more populous island. One month after the enclosures were affixed to the bottcm the author was informed by an acquaintance on