Characteristics of effluent and sludge from two commercial rainbow trout farms in Minnesota

Waste loads resulting from groundwater-fed raceway production of rainbow trout Oncorhynchus mykiss at two commercial trout farms in Minnesota were monitored. Loads of solids, organic carbon (C), and dissolved and total nitrogen (N) and phosphorus (P) were measured directly in effluent and in accumulated sludge and were normalized to fish biomass and production. Most of the observed nutrient waste (solid and dissolved) and about half of the observed solids load were present in the effluent fraction. Total annual loading rates (effluent plus sludge) observed per metric ton of production were 289-839 kg for solids; 47-87 kg for N; 4.8-18.7 kg for P; and 101-565 kg for C. These rates are similar to those reported for other salmonid raceway facilities. Differences in waste load and fractionation between sites and between study periods may be a result of facility and management factors.