The Effects of Grazing by Parrotfishes (Family Scaridae) on Selected Shallow Hawaiian Marine Communities

Abstract

This study has been conducted to: (1) assess the quantitative effects that rasping parrotfishes in a coral reef ecosystem have on the structure of benthic communities; (2) describe the standing crop of parrotfishes and (3) examine scarid recolonization patterns on a fish depopulated patch reef. Field studies were conducted at both Johnston Atoll and Kaneohe Bay, Oahu, Hawaii from 1975 through 1977. These studies suggest that there are two principal groups of Hawaiian parrotfishes. i.e., those with heavy dentition (Scarus perspicillatus, S. sordidus and S. taeniurus) that consume large quantities of calcium carbonate and probably utilize endolithic resources and those possessing relatively lightweight dentition (Scarus dubius and Calotomus sandvicensis) that appear to feed primarily on epilithic organisms. The latter species are not particularly abundant possibly due to competitive interactions with other herbivorous fishes (e. g., acanthurids) on Hawaiian reefs. Using an acid dissolution technique of extraction, the cryptobiota are estimated to range from 10 to 1400 g/m2 (dry weight) and average about 50 g/m2 in most Hawaiian reef systems. This potentially large food resource is systematically harvested by few other large reef species besides some parrotfishes and sea urchins. Laboratory experiments conducted at the Hawaii Institute of Marine Biology in Kaneohe Bay using a flow-through seawater system suggests that parrotfish (Scarus taeniurus) at low density cause benthic community structure to proceed to macroalgal dominance. At intermediate density, (0.6 to 1. 5 parrotfish/m2 or 9 to 17 g wet weight/m2 ) a diverse, high biomass community (to 400 g/m2 dry weight) develops which may be enhanced by the presence of refuges. At Scarus densities greater than 1. 9 fish (20 g wet weight per m2 ) and in the absence of refuges, a benthic community of low diversity and biomass (3 to 8 g/m2, dry weight) develops. Under high grazing pressure coralline algae are competitively superior as manifested through greater coverage. Recruitment and growth of corals in the experimental situation correlates positively with increased grazing pressure and the presence of refuges (P < 0.01). These data suggest that parrotfishes may be important to the maintenance of the overall structure of coral reefs. thus acting as keystone species to other components of the benthic community. Parrotfish densities for optimum benthic community development in the laboratory are similar to those observed in some field situations (Kaneohe Bay, Oahu--1.1 fish or 10.8 g/m2 ), and maximal growth of juveniles occurs at such densities. Field experiments conducted at Johnston Atoll suggests that at normal field densities, parrotfishes may appreciably alter the benthic community structure in two dimensional (planar) systems. The addition of a third dimension (substratum depth) alleviates this negative impact. Coralline substratum samples exposed to average field grazing pressure tend to harbor a more diverse cryptofaunal community than is present in substrata protected from grazers. The presence of a third dimension appears to give the epilithic and cryptobiotic components the protection from grazers necessary for survival. Depopulation and recolonization studies conducted on an isolated Hawaiian patch reef suggests that the MacArthur-Wilson theory of island biogeography models the observed fish recolonization. The calculated wet biomass of fishes prior to depopulation was about 930 kg/ha which is in the range of other published studies. Fish community structure was dominated by planktivores (55% by weight) followed by carnivores (32%), herbivores (12%) and omnivores (2%). The large standing crop of planktivores was related to abundant plankton probably caused by local nutrient enrichment. Recolonization studies demonstrate that parrotfishes are one of the most successful groups to recolonize, suggesting that they are opportunistic in their habitat selection. A comparison of these data to those from the same reef 11 years earlier indicates that the structure of this community has been stable and has persisted in spite of local environmental change.