Measuring the footprint of land-based abalone aquaculture discharge points

A small but well-established land-based abalone aquaculture industry has existed in Tasmania since the 1980s. As this industry looks to expand and innovate, a better understanding of the environmental performance of these sites has become important. The Institute for Marine & Antarctic Studies (IMAS) was commissioned by the Department of Natural Resources and Environment Tasmania (NRE Tasmania) to undertake a thorough survey of environmental conditions surrounding each of the four land-based abalone aquaculture facilities in Tasmania. This project was developed to examine the potential impact of nutrients discharged into the environment through farm outfalls, along with the incidence of escaped farm abalone in the environment. Specifically, the project had the following aims: · Quantitatively assess subtidal and intertidal macroalgae communities to determine the impacts of nutrient discharge from farm outfalls; · Determine the presence or absence of farm origin abalone adjacent to farm outfalls; and · Examine the abundance, size structure and species composition of abalone populations in regions surrounding farms. Two environmental surveys were undertaken at each land-based abalone site within a twelve-month period; during spring when nutrient effects were likely to be the most prominent and again across autumn and winter when farms were approximately at peak carrying-capacity. Multiple survey methods were used to characterise nutrient impacts and locate escaped farmed abalone in the environment. At each location, the effects of mild nutrient enrichment on the environment, along with evidence of farmed abalone around outfall pipes was observed. Nutrient effects were highly variable depending on the location of interest, with wave exposure and/or water movement clearly a mitigating environmental factor. While evidence of nutrient enrichment was found at each discharge site, the variability in the data and lack of consistency between surveys would suggest that impacts are within the assimilatory capacity of the receiving environment at current levels of farm biomass. Overall, the environmental survey provides a valuable baseline for the effects of nutrient enrichment from land-based abalone aquaculture at current levels of on-site farm biomass. A better understanding of the magnitude and effects of nutrient inputs from abalone aquaculture within the context of all other nutrient inputs into receiving environments will aid management into the future, particularly if expansion of the industry is likely to occur. There was evidence of stock escaping from farms at the discharge point of each facility. Live escaped stock were found at the discharge point of farms at Dunalley, Bicheno and Stanley, suggesting that abalone can escape the farm via effluent treatment systems. A thorough search of cryptic habitat at the Clarence Point discharge point did not occur during the April/May survey, meaning the occurrence of stock escape cannot be conclusively ruled out at this location. Subsequent compliance surveys by NRE Tasmania found live abalone at this location. Empty farmed shells were also found on beaches near all farms. This suggests that stock may also be removed via birds or rodents directly from tanks or mortality bins, or through farm drain cleaning operations, where any accumulated abalone (empty shell or recently deceased) in lateral and main drains may be flushed into the environment. This was particularly evident at Clarence Point, where multiple shells, often very fresh, were found in vicinity of the discharge location. The complexity of abalone populations, particularly at Stanley, has highlighted the challenges of assessing abalone escapes from aquaculture facilities using visual surveys alone. It becomes difficult to distinguish farmed stock from natural hybrids beyond the discharge point, with limited existing knowledge around the effects of diet and environment on shell morphology. Therefore the movement of farmed stock into the environment beyond the discharge point cannot be conclusively ruled out based on the results from this study. As all four farms have demonstrated evidence of escaped stock, the risk of disease transfer from farmed abalone to wild stock remains. There is only limited methodology available for determining the origin of individual animals, particularly for separating wild from hybrids in the broader environment and no methodology is available for successfully tracing larval export from farms, or F2 or F3 backcrosses into wild populations. Better understanding of the relationship between escaped abalone and wild stock will help to better assess the risk of this interaction in a Tasmanian context.