Techniques for farm-based assessment of sediment health associated with the commercial culture of Atlantic salmon ( Salmo salar L.) in Tasmania

Many studies have been carried out to evaluate the effects of organic enrichment on the marine environment, several of which specifically investigated the environmental impacts of cage fish farming. These studies have generally been conducted from a government or regulatory standpoint and to date, none have been undertaken from a farm-based perspective. Thus, there have been no studies aimed at improving the self-assessment capability of farms or developing farm management protocols to ensure environmental sustainability. The current study was undertaken with both these objectives in mind. Initially, the project reviewed techniques routinely employed for monitoring of aquaculture operations as well as methods that have been used to evaluate other sources of organic enrichment in the marine environment. These techniques were then assessed according to three basic criteria; simplicity, reliability and robustness, to identify those that could be considered applicable for farm-based use. The methods thus selected included macrofaunal assessment, evaluation of sedimentation rates, determination of organic matter content, and measurement of sediment redox potential. These techniques were then evaluated at two fish farm leases to determine how they would respond to a) the spatial variability at each site, and b) the temporal effects of operational variability at the two sites over the production cycle. The performance of each technique was judged against species level evaluation of the macrofaunal community structure as an indicator of the sediment condition. The results suggested that both sedimentation rate and organic matter were unsuitable as farm-based measurements. Measurement of redox potential was found to be a simple and reliable indicator of sediment condition, accurately reflecting the benthic condition. However, the redox results should be interpreted with caution, particularly when taken in isolation. Time series redox measurement showing a clear pattern of effect is preferred. However, isolated redox measurements can be used when viewed in conjunction some other substantiating evidence. Further examination of the macrofaunal results suggested that Capitella capitata complex abundance could also be a good indicator of sediment condition. However, once again, evaluation of the significance of this species complex is most useful when the interpretation incorporates a time series of observations. The macrofaunal results also indicated that assessment of annelids to family level alone may be sufficient to determine site condition, an outcome that could markedly reduce the costs of benthic assessment to farmers. Finally, the results from other major faunal groups, showed some very interesting patterns which could prove useful in evaluating sediment condition. The abundance of echinoderms appeared to be directly related to environmental conditions; total absence indicating highly enriched conditions, dominance by Echinocardium cordatum suggesting moderately impacted conditions whilst a more diverse echinoderm fauna seemed to be indicative of unimpacted conditions. In addition, the molluscan community structure at each of the study sites exhibited a shift from bivalve to gastropod domination. This change was reflected at all sample stations and consequently suggests either that the reference locations for each of the sites were influenced by the farm or that the final gastropod species, an introduced species, may itself have induced the change.