Project Report PR-99-04
A Survey of Selected Coral and Fish Assemblages Near the Waianae Ocean Outfall, Oahu, Hawaii, 1998
Anthony R. Russo
September 1998
ABSTRACT
In 1998, coral growth and fish abundance were monitored at stations located at and in the vicinity of the Waianae Ocean Outfall. Comparisons of results with fish surveys done in previous years showed no significant differences in the species composition or relative abundances of fish populations at Station W-2 (the sunken ship Mahi), which is located 1.2 km south of the diffuser. Fish abundance and species richness at Station W-3, which is located at the diffuser, increased from 1990 to 1995, decreased in 1996, and increased again in 1997 and 1998. At Station WW, an inshore station located 0.8 km from shore, fish were abundant and speciose on the armor rock covering the pipeline. The fish species seen inshore are comparable to fish species seen in similar (boulder) natural biotopes around Hawaii. There were no significant differences in total mean coral cover at selected quadrats from 1994 to 1998 at Station W-2. However, when comparing 1991 Data with 1998 Data, there was a significant increase (4.6%) in total mean coral cover at this station. At the diffuser, corals were seen growing on the diffuser pipe and on the riser discharge ports. In 1986, when the diffuser began operation at a discharge rate of 1.5 mgd (0.07 m3/s), no corals were seen at this location. At inshore station WW corals off the pipeline were sparsely distributed but were numerous and thriving on the armor rock over the pipeline. The inshore transect (Alpha), off the armor rock, was moderately covered (less than 30%) with the alga Dictyopteris plagiogramma. This seaweed was also abundant at this location in 1995, 1996, and 1997. The water was clear at all stations surveyed (15 to 20 m horizontal visibility), and the surrounding sediments were clean and white. No significant deleterious effect due to outfall operation and discharge were seen on the biological community at the stations surveyed. The increase in fish diversity and abundance at the diffuser in 1998 may be due to natural fluctuations in abundance or to environmental conditions suitable to fish populations living there. To distinguish between outfall effects and natural fish population fluctuation patterns, several years of consecutive monitoring must be performed.