Comparison of two methods for evaluating waste of a flow through trout farm

European water legislation enforces increasingly restrictive measures with regards to reduction of water consumption and waste emission in order to minimise the potential environmental impact of the agro industry sector. Fish farms are particularly concerned, but legislation covering effluent discharge varies significantly from country to country. However, recommendations and directives from institutional, national or regional bodies suggest the enforcement of increasingly strict waste reduction measures and the development of waste treatment. Before treatment, it is necessary to evaluate waste production in terms of composition and quantity. The waste quantification methods used today for fish culture systems are either based on direct measurements of nutrient and suspended solid fluxes or on indirect evaluation based on the digestibility coefficients of the feed constituents. The objective of the present study is to evaluate the waste of a freshwater flow through farm using both approaches and to discuss their applicability, drawbacks and advantages from the viewpoints of fish farmers and control authorities. Waste production on the farm was monitored during several 24 hour cycles in order to characterise the effluents of the system. The predictions and measurements for the total nitrogen (TN) parameter were well correlated, but measured and predicted suspended solids (SS) and total phosphorus (TP) values presented a weaker correlation coefficient. The hydrobiological method gives details on the N and P forms of waste but this method is heavy and it is difficult to obtain representative samples and flow rate measurements. The nutritional method is the simplest to use, provided that feed data are available.