Rainbow trout (Oncorhynchus mykiss) is one of the most important species produced in aquaculture in the European Union. Its culture requires high-quality, well-oxygenated waters. Like any animal production, the fish culture generates waste. Hence, discharge of non-treated waters from fish ponds to a receiving water body may disturb its ecological homeostasis. This study was aimed at evaluating the effect of rainbow trout aquaculture in a two-stage flow system on water quality. The study was conducted at a trout farm composed of two sets of fish ponds made of concrete, which were fed with waters by gravitation in a continuous flow system. Waters inflowing to the fish farm were of good quality. Based on the calculated CCME-WQI values, they were classified to the category of water quality called Fair. Their quality met requirements set in EC Directive no. 2006/44/EC for inland waters constituting the habitat for the Salmonidae fish. Once water had flown through the fish ponds, its quality deteriorated. Water discharged from ponds A had a higher BOD5 value and higher concentrations of total phosphorus and ammonia nitrogen, which resulted in its quality class decline from Fair to Marginal. In turn, water outflowing from ponds B had higher concentrations of Ptot and N-NH3, which nevertheless did not result in a decrease of its CCME-WQI value. This was mainly due to an increased to 8 dm3·s-1 flow rate of waters through ponds B compared to ponds A, which facilitated faster water exchange in the ponds, caused higher dilution of contaminants present in the water and, simultaneously, their more rapid discharge outside the ponds. This study did not confirm the expected positive effect of the treatment ditch. Despite pre-treatment, the water discharged from the ponds can pose a threat of receiver waters contamination.        

24.2.2019