Practical low-fishmeal diets for rainbow trout (Oncorhynchus mykiss) reared in RAS: effects of protein meals on fish growth, nutrient digestibility, feed physical quality, and faecal particle size

To evaluate strategies for optimizing waste in recirculating aquaculture systems, 1020 rainbow trout (initial live weight 17.2 ± 7.50 g/fish) were distributed into 12 tanks after 21 d of acclimation and fed during 84 days with four practical diets (crude protein: 49% DM; crude fat 26% DM; gross energy: 23 MJ kg-1) containing different rates of fishmeal and alternative protein meals, i.e. Diet FM (307 g kg-1 fishmeal, 61.2 g kg-1 poultry by-product meal); diet PBM (183 g kg-1 fishmeal, 168 g kg-1 poultry by-product meal); diet FeM (198 g kg-1 fishmeal, 61.2 g kg-1 poultry by-product meal, 76.5 g kg-1 hydrolysed feather meal); diet FeM+RM (171 g kg-1 fishmeal, 61.2 g kg-1 poultry by-product meal, 76.5 g kg-1 hydrolysed feather meal, 60.4 g kg-1 rapeseed meal). High structural integrity of extruded pellets and low oil leakage were measured in all diets, while the lowest water turbidity at 15 min after feed administration was recorded for FeM diet (2.7 vs. 12.7 mg L-1; p < 0.05). Diets did not affect fish specific growth rate (2.16% d-1). The lowest apparent digestibility of protein was measured with diet FeM (79.6%) and the highest with diet PBM (86.0%) (p < 0.001); apparent digestibility of lipids was higher in fish fed diets FM and PBM than the other diets (84.4% and 85.5% vs. 66.4% vs. 71.1%; p < 0.001). Fish fed diet PBM showed a higher percentage of retained faeces at mesh sizes 0.5–0.8 mm (33% vs. 30%; p < 0.005) and 0.3–0.5 mm (64% vs. 59%; p < 0.001) compared with the other diets. The replacement of fishmeal with poultry byproduct- meal had positive effects on nutrient digestibility and faecal particle size. The replacement with hydrolysed feathers and rapeseed meals impaired nutrient digestibility but had positive implications for water turbidity.