Broadscale effects of marine salmonid aquaculture and introduced pests on macrobenthos and the sediment environment in Tasmania between 1998 and 2003

Around Tasmania, the benthic macrofauna of marine-influenced embayments and large estuaries exhibited a strong regional separation between Macquarie Harbour and the eastern and northern coasts, with the Macquarie Harbour fauna depauperate in species and animal numbers. • Introduced species comprised a dominant presence in terms of biomass across much of Tasmania, but were present in negligible densities in Macquarie Harbour. • The scale of fish farm impacts was substantially less than the scale of natural regional variation. • The inshore Tasmanian marine environment was undergoing a period of change, with increasing organic loadings to sediments, declining sediment redox potential, and increasing modal particle size at sites investigated over the period of study. • Associated with these environmental changes was an increasing density of macrofauna, which included an increasing proportional abundance of both introduced taxa and capitellid polychaetes. • Fish farm effects that extended to regional scales and affected reference sites could not be adequately assessed within the project because reference regions without fish farms were not monitored; however, significant increases through time at reference and compliance sites in sediment organic matter, surface redox potential, total macrofaunal abundance, proportional abundance of capitellid polychaetes, and proportional abundance of introduced species, were are all suggestive that organic enrichment associated with fish farms may have extended at low levels across regional scales. • Given the implications to biodiversity conservation of region-wide impacts, monitoring of reference sites in regions lacking fish farms is urgently warranted. • Fish farm effects were found to significantly affect the sediment environment and macrobenthic communities near farm leases in all major regions of the state other than Macquarie Harbour, where no effect of fish farm activity was detected. • Fish farm effects were most apparent in comparisons between reference sites and farm sites. • Fish farm effects were also detectable at compliance sites located 35 m out from lease boundaries, where sediment redox potential and faunal assemblage composition were intermediate between patterns found at farm and reference sites; however, farm effects were relatively minor at compliance sites with no indication of negative impacts on the biota. • The major physical effect of fish farm activity was a decline in redox potential of sediments to at least 4 cm depth. • The faunal community within fish farms generally exhibited increased faunal dominance, increased proportional abundance of capitellids, and decreased total bivalve/total mollusc ratio. • Species found to be consistently positively-associated with fish farm impacts included the capitellid polychaete Capitella sp., the leptostracan crustacean Nebalia sp., the nereid polychaete Neanthes cricognatha, the ostracod crustacean Euphilomedes sp., the introduced bivalve Theora lubrica, and the nassarid gastropod Nassarius nigellus. • Some species were also found to be negatively-associated with fish farm impacts, but such species tended to be localised in distribution and not generate significant results in statewide analyses. • Introduced species were increasing in proportional abundance within the macrofauna by 2-3% per annum. • The most abundant and widespread introduced species were the bivalves Theora lubrica and Corbula gibba, and the screwshell Maoricolpus roseus, while the bivalve Raeta pulchella and fanworm Euchone limnicola were also locally abundant. • Future taxonomic study of taxa identified during the project to the genus level will likely reveal additional introduced species that are abundant within the state but not currently recognised as introduced. • Populations of M. roseus were stable through time and unaffected or slightly negatively-affected by fish farm activity. • Populations of C. gibba appear newly established within Tasmania, with exponential growth between 1998 and 2001, and a subsequent population decline. This bivalve was positively associated with sites with high organic loading and fine sediments, but was not strongly associated with fish farms. • Populations of T. lubrica were positively affected by fish farm activity, increasing through time at farm and compliance sites but not at reference sites. • Increasing population numbers of introduced species were generally associated with increasing population numbers of native species, rather than the introduced taxa causing a detectable decline in native populations through competition. • The Tasmanian finfish monitoring program comprises a successful partnership between industry, management and researchers that has provided an invaluable state-wide baseline for assessing environmental impacts within estuarine and inshore marine habitats. Given the magnitude of ecological changes evident over the six-year period of study, we strongly recommend that this monitoring scheme be continued through the long-term and expanded to include regions that currently lack fish farms