WRRCTR No.104 Survey of Benthic Coral Reef Ecosystems, Fish Populations, and Micromollusks in the Vicinity of the Wai’anae Sewage Ocean Outfall, O’ahu, Hawai’i - Summer 1975

Abstract

A marine survey was conducted in the vicinity of the present and proposed Wai'anae sewage ocean outfall site on the southwest coast of 0'ahu, Hawai'i. The survey at depths of 6 to 30 m (20-100 ft) characterized quantitatively the coral reef community, the reef fish population, and micromolluscan assemblages. Diversity indices were calculated for substrate, fish, and micromollusks. Similarity indices were computed and a dendrograph was generated for all stations. The stations could be clustered into three distinct regions: (1) a deep-water region (18-30 m depth) with smooth hard algae-covered limestone substrate and sand, low fish abundance and diversity, low micromollusk diversity with high abundance of dialids, and low abundance of archaeogastropods and rissoids; (2) a shallow-water region (6-18 m depth) with comparatively high coral cover, high fish abundance and diversity, high micromollusk diversity with low abundance of dialids and high abundance of rissoids and archeogastropods; and (3) the region along the harbor channel at Pōka'ī Bay where coral cover, fish abundance and diversity, and micromolluscan species abundance were distinctly different from those of other stations, indicating a probable influence by the boat harbor and channel on the benthic community. The sewage effluent has not adversely influenced coral growth in the vicinity of the outfall diffuser, although increased nutrients may account for high abundance of frondose algae at this station.