Effects of sewage discharge, fishing pressure and habitat complexity on coral ecosystems and reef fishes in Hawaii

Abstract

The major factor controlling the abundance of reef fishes in Hawaii (USA) appears to be habltat complexity (bottom topographic relief and interstitial space) Fishes aggregate In areas of h ~ g h hab~tat complexity at b~o~nass levels whlch, in areas protected from fishing, may significantly (by 2 to 5 times) exceed levels supported by pnmary production within the ecosystem. High relief natural areas 'act' like artificial reefs in terms of aggregating fishes. Reef fish abundance is also affected by fishing pressure in the main Hawaiian Islands. Areas protected from fishing (Marine Life Conservation Districts, MLCDs) support standing crops of reef fishes that average about 45 g m-2 higher than areas open to fishing. In areas unprotected from fishing, reef fish abundance has significantly declined in recent years. Conservation and management of reef fishes would be ~mproved if more hlgh relief areas were protected from fishing. Thus far. 11 MLCDs have been established in such areas In Hawaii. The discharge of primary or secondary treated sewage effluent into the ocean in Hawaii through deep ocean outfalls causes no apparent negative environmental impact to coral reef ecosystems. Deep ocean outfalls in fact attract reef fishes. Increases in abundance around the outfalls appears to result from increased habitat complexity brought about by the construction of the pipelines and surrounding caprock and concrete. A small amount of the variability (Increase) in reef flsh abundance around the outfalls may also be due to food subsidies (particulate organlc matter) in the effluent. No species of fish, coral, invertebrate or algae at the outfalls exhibited any pathological symptoms. Hence, statewide declines in reef fish abundance in areas unprotected from fishing are not related to the discharge of sewage effluents via deep ocean outfalls. Rather, these declines appear to have been caused by overfishing.