WRRCTMR No.65 Benthic Ecosystem and Fish Population Off the Mokapu Outfall: A Third Post-Installation Study

Abstract

An ecological study of the benthic and fish communities at Mokapu, O’ahu, was completed in the summer of 1980 approximately three years subsequent to the installation of an ocean outfall by the City and County of Honolulu. Data were obtained from five transects between Mokapu Point on the northeastern tip of O’ahu to Alala Point, approximately 6 034 m (3.75 miles) south, at depths of 6 to 24 m (20-80 ft). This study is subsequent to an initial survey completed in 1975 prior to outfall construction. Result, show little or no effect from the operation of the outfall on the benthic and fish communities. There are no significant differences in the abundance, diversity, or composition of fishes from 1975 to 1980 except at the outfall site where new substrate was formed by construction. Between the 1975 and 1980 studies there are some differences in coral species cover indices, which are attributed to patchy substrate distribution rather than stress from the sewage outfall. Differences in species composition and distribution of micromolluscan assemblages may also be explained, at least in part, by patchy distribution of the substrate. Analysis of micromollusk assemblages shows that the proportions of dominant species in the assemblages of shallow and deep waters sites have remained constant over the years of study. In general the biotic community has not changed in the area studied over three years and equilibrium seems to have been reached in the benthic and fish communities at least for the present. The construction of the outfall, surrounding rock substrate, and the swift removal of effluent by vigorous water movement have contributed, in the case of fish populations at least, to increases in abundance.