Theoretical effects of feed composition, feed conversion and feed spillage on waste discharge in fish culture

Based on feed composition, fish composition and feed conversion, some theoretical effects of feeding on waste discharge are calculated to illustrate the appropriateness of farm effluent monitoring. The effects of feed conversion and feed spillage and feed composition on waste discharge (suspended and dissolved N, P & COD) are demonstrated. Slight variations in feed conversion at the moment of sampling, especially those evoked by feed spillage, have tremendous consequences for discharge values. It appears crucial to determine whether observed conversions are explained by inherent efficiencies of fish growth, or whether higher growth efficiencies are combined with feed spillage. Therefore, thorough knowledge of relationships between nutrient intake and growth should be applied to effluent assessment. A calculation is presented to serve as a preliminary assessment of the impacts of these variables on water quality monitoring. The concept is illustrated using measurements from a recirculation pilot eel culture system.