A model for predicting the quantities of dissolved inorganic nitrogen released in effluents from a sea bass (Dicentrarchus labrax) recirculating water system

Fish excretions and the transformation of nitrogen by bacteria in the nitri fying biofilter are two of the main sources of dissolved inorganic nitrogen (DIN) in fish farms that use recirculating water systems. In this study, t he DIN concentration in an experimental Dicentrarchus labrax aquaculture sy stem was calculated using empirical sub-models for fish growth, ingested fo od and water replacement. The specific growth rate (SGR) (% day(-1)) and th e daily feeding rate (DFR) (% day(-1)) both depend on the average weight, W (g), of the fish: Y = aW(b), where Y may be SGR or DFR, and a and b are em pirical constants. The DIN discharge rate, Gamma(N) (% of ingested nitrogen ), in the experimental aquaculture system was expressed as a function of in creasing replacement water flow rate, theta (day(-1)): DIN = c theta(d), wh ere c and d are empirical constants. Only three variables (the number of fi sh, the initial fish weight and the replacement water flow rate) are requir ed to run the general model, which was tested over a period of 12 months (J une 1997-June 1998). This model, calibrated and validated on independent se ts of data obtained from the same experimental system, accurately predicted the concentration of DIN in the effluent (r(2) = 0.92). (C) 2000 Elsevier Science B.V. All rights reserved.