Managing the development of sustainable shrimp farming in Australia: the role of sedimentation ponds in treatment of farm discharge water
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In Australia, the shrimp farming industry operates within strict environmental guidelines applied at national and state government levels. In this paper, we briefly review the current status of the development of an ecologically sustainable shrimp farming industry in Australia and the future research priorities to assist in maintaining sustainable development. The broad objectives of the current regulations governing shrimp aquaculture in Australia are to achieve appropriately sited and operated facilities. However, there are differences amongst the various jurisdictions, and these need to be reconciled. Currently, shrimp farm discharge regulations are based on permissible characteristics of discharge water quality. These parameters are highly variable over short time frames, and this presents a significant barrier to the development of cost-effective sampling strategies that accurately reflect trends in farm discharges. Furthermore, water quality parameters may provide little information about the effect of discharges on the ecology of the receiving waters. We describe more sophisticated indicators of the potential impacts of shrimp pond discharges, including the d15N-nitrogen isotopic ratios in marine plants, which integrate the high variability in water quality parameters. In many shrimp-farming countries, including Australia, there is an increasing trend towards the use of sedimentation ponds (=settling ponds, settling basins) for treating pond effluent prior to discharge or recirculation. Despite this trend, the performance of such systems has rarely been studied at farm scale, and therefore their effectiveness is poorly understood. We have examined the nutrient reduction performance of sedimentation ponds on a commercial farm over an entire production season. The results demonstrated that the sedimentation ponds were effective in reducing discharges of suspended particulates, but were less effective in reducing nutrient concentrations. Total suspended solids (TSS) were reduced by 60% with residence time of 0.7 day. The best reductions of total phosphorus (TP) and total nitrogen (TN) (35% and 23%, respectively) were in a sedimentation pond with 2 days residence time. The pond with the longest residence time, 4.6 days, had poorer removal rates for each of the three parameters. Although sedimentation ponds alone are a useful strategy, we advocate that sustainable development of shrimp farming requires an integrated approach involving appropriate planning, regulation and effective monitoring protocols by governments, and reduction of the volume of effluent and mass loadings of nutrients from shrimp farms. The key future challenges are to continue to improve feeds and feed utilization efficiency; reduce water usage through recycling and reduced exchange; and improve treatment of discharge water to remove nutrients.