Estimation of waste outputs by a rainbow trout cage farm using a nutritional approach and monitoring of lake water quality

Abstract Nutrients released by cage fish farms to the aquatic environment are an issue of concern since these can result in deleterious environmental changes. In the present study, a mass balance model was used to calculate nutrient loadings from an experimental rainbow trout cage farm located in a freshwater, oligotrophic lake. Detection of these nutrient loadings using water quality monitoring was then investigated. The loading of total solids of faecal and feed origin (TS), solid phosphorus (SP) and nitrogen (SN), and dissolved P (DP) and N (DN) wastes from the farmed fish were estimated over two production cycles. Waste outputs were estimated using the Fish-PrFEQ feed requirement and waste output model using measured inputs including diet composition, nutrient digestibility, intake and retention by the fish, and water temperature. Nutrient loading predictions were compared with measured nutrient concentrations of lake water. In 2003, TS, SN and SP and DN and DP waste outputs were 236.0, 12.8, 5.3, 41.3, and 3.4 kg tonne − 1 of fish produced, respectively. In 2004, the TS, SN and SP and DN and DP waste outputs were 220.0, 12.2, 5.3, 38.0, and 3.4 kg tonne − 1 of fish produced, respectively. Over 60% of the P waste output from the cage was predicted to be solid for both years while over 65% of the total N waste from the cage farm was predicted to be excreted as ammonia. Concentrations of ammonia and of dissolved and particulate phosphorus were not reflective of waste loading of cage origin, suggesting efficient removal through uptake by biota and/or in the case of ammonia by nitrification. Fish-PrFEQ model is a valuable cage management tool that allows realistic estimation of waste outputs for cage farms and can be used to examine effect of management and feeding practices on waste outputs. However, the model is of limited use as a lake management tool, as it does not consider effects and fate of wastes released by fish. Similarly, reliance on periodic water quality monitoring at stations near cage farms may not be protective of the environment, as our results demonstrate that rapid diffusion, uptake, transformation and removal of nutrients resulted in water quality measures that were relatively insensitive to cage loading. Combining the Fish-PrFEQ model with a consideration of assimilative capacity of the system in addition to the monitoring of chemical and biological variables of the lake is recommended for environmental impact assessment of cage culture operations.