Resumo. Abstract Aim: The mass balance has recently been applied in Brazilian aquaculture. In relation to trout farming, few studies have used this methodology as environmental indicator, especially when considering each development stage of fish. The daily mass balance allows an estimate of nutrients discharge, because the sampling effort over 24 hours enables to monitor the variations throughout the day. This study aimed to identify the breeding sector (fingerlings, juveniles and adults) in trout farming contributes with the great discharge of phosphorus and nitrogen in the effluent. Methods The concentrations of total phosphorus (TP), orthophosphate (PO4-P), total nitrogen (TN) and ammonium (NH4-N) were determined in affluent and effluents of the production system. The environmental indicators were calculated based on the values of nutrients loads and by quantifying the daily discharge of nutrients from the mass balance method. Results Over the 24 hours, the trout farming exported an average of 0.26 kg TP, 0.11 kg PO4-P, 3.11 kg TN and 0.18 kg NH4-N. The discharge of phosphorus and nitrogen in the effluent was higher at night associated to feeding management (hours and feeding frequency) and the nocturnal habits of trout. The juvenile and adult sectors accounted for the greatest contribution of these nutrients, related the largest fish biomass and metabolism, supported by the feed conversion ratio of 1.95:1 and 2.01:1, respectively. Conclusion In order to reduce exports of phosphorus and nitrogen there is a need to adequate the feeding management (feeding frequency from two to four times per day according to the trout development stage; carry out feeding in times close at twilight due to the nocturnal habits of this specie; reduce excess of ration offered and use good quality ration considering the distinct nutritional requirements of each trout stage of development). Besides the mass balance, the feed conversion ratio is also an efficient tool to detect the efficiency of feed utilization; therefore, it is proposed its use as environmental indicator, aiming the aquaculture environmental sustainability.