| Summary: | Expansion of aquaculture production is largely the result of intensification of farming activities witha higher degree of feed input and waste output per rearing system. Concerns about the effects ofwaste output on the quality of receiving water systems have led to efforts to lower levels ofenriching nutrients in discharge water. Since the enriching nutrients originate in feed added to therearing systems, efforts to reduce levels in farm effluents must begin with feed. Phosphorus,nitrogen and fecal solids in farm effluent water are the “pollutants” of concern. Strategies to lowertheir concentrations in farm effluent water focus on decreasing the amount of uneaten feed, and onincreasing the retention of dietary phosphorus and nitrogen by the fish. Modifying feeding levelsand methods, plus increasing pellet water stability, are the approaches used to reduce waste fromuneaten feed. Reducing phosphorus levels in farm effluents requires effort in three areas, e.g.,information on the bioavailability of phosphorus in feeds and feed ingredients, accurate estimationsof requirements for phosphorus at various life-history stages, and formulation of feeds to match asclosely as is practical the available phosphorus level in the feed to the requirement of the fish orshrimp. Reducing nitrogen levels in farm effluents involves the development of feeds that supporthigh percentages of nitrogen (protein) retention by the farmed fish or shrimp. Another nutritionalstrategy useful in lowering nutrient levels in farm discharge waters is phase feeding, involving theuse of diets containing sub-optimal levels of phosphorus or protein during the final grow-out phase.Using appropriate nutritional strategies, levels of enriching nutrients in farm effluents can besignificantly reduced without lowering fish performance or farm profitability.
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