Application of biotic and abiotic indicators for detecting benthic impacts of marine salmonid farming among coastal regions of Tasmania

Analysis of sediment and macrofaunal samples collected during the Tasmanian marine farming finfish monitoring program – a six-year partnership between industry, management and researchers – revealed several univariate indicators to be useful for detecting effects of aquaculture on the benthic environment. Comparisons with reference sites revealed a significant decline in sediment redox potential to at least 4 cm depth at farm sites, and increased proportional abundance of capitellids and decreased bivalve/total mollusc ratio. At compliance sites located 35 m out from lease boundaries, sediment redox potential and faunal assemblage composition were intermediate between patterns found at farm and reference sites. Redox potential at the sediment surface declined on average by 178 eV at reference sites converted to farm sites, with this indicator proving the most sensitive for detecting regional impacts of farming activity.Fish farm effects that extended to regional scales could not be adequately assessed within the project because reference regions without fish farms were not monitored; however, a significant decrease through time at reference and compliance sites in surface redox potential, and increases in sediment organic matter and total macrofaunal abundance, were suggestive that organic enrichment may have extended at low levels across regional scales. Given the implications to biodiversity conservation of region-wide impacts and a need to distinguish fish farm effects from unrelated long-term environmental change, monitoring of reference sites in regions lacking fish farms is urgently needed.