Trophically-based assessment of the impacts of deepwater sewage disposal on a demersal fish community

Synopsis The impacts of sewage disposal on the demersal fish community were identified after approximately 2 years of operation of a deepwater outfall off Sydney, NSW Australia. Gut-content analyses were used as a basis for identifying groups of fish with dietary similarities. Similar results from multivariate analyses (Bray-Curtis dissimilarity measure and MDS) identified 8 trophically-similar groups of fish. The majority of the fish comprising these groups consumed organisms in the soft-bottom community. Only 5 of the 8 trophic groups identified could be analysed in detail as individuals comprising the remaining groups were not caught in sufficient numbers near the outfall. Asymmetrical analyses of variance showed that the numbers of individuals and species in one particular trophic group increased significantly following the commissioning of the Bondi deepwater outfall. However, the proportional representation of individuals in the 5 trophic groups examined did not change. Five hypotheses are advanced to account for the observed impacts. Of these, hypotheses centred around increased habitat complexity and a numerical response to increases in prey abundance warrant further investigation. Directions of future research and the limitations of various designs to assess environmental impacts are discussed in light of these results.