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Title: The Effects of Grazing by Parrotfishes (Family Scaridae) on Selected Shallow Hawaiian Marine Communities 
Author: Brock, Richard Eugene
Date: 1905-06-01
Publisher: University of Washington
Citation: Brock, Richard E. The Effects of Grazing by Parrotfishes (Family Scaridae) on Selected Shallow Hawaiian Marine Communities. University of Washington, 1979.
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
Description: Thesis (Ph. D.)--University of Washington, 1979. Bibliography: leaves [114]-126.
Pages/Duration: 202 pages
LC Subject Headings: Parrotfishes.
Coral reef ecology--Hawaii.

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