Auditing nutrient discharges from fish farms: theoretical and practical considerations

Nutrient discharges from fish farms can be determined retrospectively, simply and with a high degree of accuracy from records of fish production and feed conversion ratios (FCR), combined with chemical analyses of feed and fish. Prospective prediction of the inputs (via feed and outputs) via production and discharges) of given chemical elements, on a daily basis and over longer periods of time, would represent a valuable management tool for farmers, and for regulatory and planning authorities. A dynamic model is presented for Atlantic salmon, Salmo salar L., which integrates biomass, growth rate. FCR. and nutrient retention and discharges in relation to feeding rate, diet composition and environmental factors. In view of the importance of feeding rate on growth, FCR and nutrient discharges, three simulation alternatives were modelled; namely, feeding 50%. 75 or 100% of the estimated maximum daily ration of a specified diet to salmon smolts of 80 g initial weight over a period of one year in sea water. The 50 and 75% rations, in comparison with 100% ration, showed dramatic reductions in fish weight after one year (0.4 and 1.4 kg versus 3.3 kg). The discharges of nitrogen (N) and phosphorus (P) per unit biomass production were reduced to some extent in the first months of the simulation period, and thereafter, there were no clear differences in discharges of N and P per unit biomass at the three ration levels. However, the accumulated N and P discharges per unit weight gain were slightly increased on the 75% ration, and two-fold higher on the 50% ration compared to the 100% ration. The FCR was also higher in fish fed the lower rations. The simulation result indicates that reduced ration is not effective strategy for minimizing nutrient discharges from fish farms. In order to control discharges within any limits and to utilize the resources optimally, it is better to adjust the biomass on the farm and to feed the fish to appetite, at which growth rate is maximum and FCR is minimum.