WRRCTR No.132 Benthic Ecosystem and Fish Population off the Mokapu Outfall: A Second Postinstallation Ecological Study

Abstract

An ecological study of the benthic and fish populations at Mōkapu, O’ahu, Hawai’i was completed in the summer of 1979. This is the second post-installation study of the Mōkapu Outfall area since the commencement of effluent discharge in December 1977. An initial baseline survey was completed in 1975 (Russo et al. 1977, 1979). Data were obtained from five stations between Mōkapu Point on the northeastern tip of O’ahu to Alāla Point approximately 6 034 m (3.75 miles) south, at depths of 6 to 24 m (20-80 ft) in all surveys. Results show little or no effect from the operation of the outfall on the benthic and fish communities two years after commencement of discharge. There are no significant differences in abundance, diversity or composition of fishes from 1975 to 1979 except over the outfall site where fish abundance increased significantly especially over the diffuser where the snapper Lutjanus kasmira was counted in large numbers (>500). The relative abundance of herbivorous fishes (acanthurids) increased near the outfall. This may be commensurate with an increased coverage of a fine microalgal mat and an increase in the biomass of Lyngbya, a blue-green alga near the outfall site. The biomass of dominant macroalgae did not significantly change at any of the five areas surveyed. Between the 1975, 1978, and present (1979) studies, there are some differences in coral cover which are attributed to patchy substrata distribution rather than stress from the sewer outfall. In comparison with the samples of 1975 and 1978, the 1979 transect D samples and that from the diffuser show a lower, overall abundance in micromolluscan assemblages than was recorded in either 1975 or 1978, and that Tricolia, Vitricithna, Rissoina miltozona, and Bittium increased in relative proportions at the 18 to 16 m (60-80 ft) depth levels and at the diffuser. The differences reflect increasing numbers of micromollusks more usually associated with depths of less than 15 m (50 ft) at greater depths and of species associated with frondose algae and substrata. This trend was noted in 1978 and is apparent again in 1979. Large differences among some of the 1979 samples and between the 1979 and other year samples reflect the patchiness of the substratum and cannot be attributed to effects of the sewer outfall.