Spatial and temporal variation in fish assemblages exposed to sewage and implications for management

Underwater visual surveys, involving a Before/After/Control/Impact (BACI) experimental design, were used to investigate the abundance and species richness of temperate rocky reef fishes and the abundance of a sea urchin following the discharge of sewage effluent from a deepwater outfall. Multivariate analyses indicated that fish assemblages were significantly different at the outfall and control locations, and these differences were largely attributed to the declines in relative abundance of several common, resident species of reef fish, such as the eastern hulafish, Trachinops taeniatus and yellowtail Trachurus novaezelandiae and the sea urchin Centrostephanus rodgersii. Univariate analysis indicated that the abundance and richness of groups of fish and individual species varied over times, periods and locations. The abundance and species richness of fish from the outfall location were generally less than for fish assemblages at the control locations. We detected decreases of 33% in the species richness of fish and 50% in the abundance of Centrostephanus rodgersii at the outfall location and, conversely, the abundances of some cryptic (small, cave dwelling or camoflagued) fishes increased over time at the outfall. This study indicates that the discharge of sewage effluent resulted in an environmental impact affecting fish assemblages, some fish species and a sea urchin, and it was concluded that the outfall was not complying with current pollution management guidelines.