Quantifying regional feed utilization, production and nutrient waste emission of Norwegian salmon cage aquaculture

We analyzed growth performance and waste emissions of cage aquaculture responding to biophysical conditions that vary along the coastline of Norway. The analysis is based on a published mass balance model predicting waste emissions and open quality-assured data from the Norwegian Directorate of Fisheries between 2016 and 2020. Results showed significant regional variation in feed conversion rate, with a steady increase from the Northern to Southern region (p < 0.001) and a co-occurring reduction in waste emission. The retention rate of feed carbon (C), nitrogen (N) and phosphorus (P) in tissues of farmed Atlantic salmon Salmo salar Linnaeus, 1758 varied from 39 to 43% of consumed nutrients across regions. The estimated defecation rate of C, N and P was 18% of consumption, whereas excretion or respiration varied from 39 to 43% across regions. This variability, along with farming methods and location characteristics, could potentially impact regional differences in benthic ecosystems. The regional differences in feed and environmental cost could potentially attract salmon farming to shift towards the Northern region, and this trend might become strengthened as water temperatures rise due to climate change. Additionally, salmon growth and feed intake rate interacted significantly with temperature (p < 0.001) and showed regular seasonal variation. The regulation of maximum allowed biomass restricts salmon farmers from increasing biomass beyond their maximum allowed level at any time. Nevertheless, the same biomass may cause higher waste in the summer. Therefore, we suggest an awareness of the seasonal increase in waste emissions and consideration of implementing environmental assessment for surface waters, which might be beneficial for decreasing the risk of reduced water quality.