Performance of diets based on alternative protein sources for rainbow trout (Oncorhynchus mykiss) in recirculating aquaculture systems

With the general aims of controlling waste production and increasing efficiency in recirculating aquaculture system (RAS), this study compared growth performance, diet digestibility, and faeces characteristics in rainbow trout (Oncorhynchus mykiss) fed four diets including fish meal (FM) as the main protein source or partially replacing FM with three alternative protein meals, i.e. poultry byproduct meal (PBM), hydrolysed feather meal (HFM) and rapeseed meal (RM). A total of 1020 fish (17.2 g ± 7.50 g) were randomly distributed into twelve experimental RAS units (3 tanks per treatment, 85 fish per tank) and fed during 94 days with diets including 30% FM (diet FM, control), or 18% FM and 17% PBM (diet PBM), or 20% FM and 8% HFM (diet HFM), or 18% FM and 8% HFM and 6% RM (diet HFM+RM). Good structural integrity (DORIS test: 0.91%) and low oil leakage (<1.5%) were found for all diets, while the lowest water turbidity (2.7 vs. 12.7 mg L-1; P<0.05) at 15 minutes after feed administration was recorded for HFM diet. Fish specific growth rate (2.16 % d-1, on average) and feed conversion ratio (1.05) were not affected by the dietary treatment, whereas carcass yield was higher in fish fed FM diet compared to the other diets (77.2% vs. 74.8%; P<0.05). Apparent digestibility of lipids (77.5% vs. 61.2%; P<0.001) and of main fatty acids (P<0.001) was higher in fish fed FM and PBM compared to the other diets, whereas no differences were found for protein digestibility (on average 80.4%). The rate of faeces retained by a 0.5 mm-diameter sieve was lower in fish fed PBM diet compared to the other diets (67.4% vs. 70.0%; P<0.05), i.e. PBM diet generated larger particles of faeces. As for waste production in RAS, the replacement of FM with PBM is worth of further investigation given the positive effects on nutrient digestibility and faeces particle size. Regarding production costs, FM diet was the most expensive both in terms of feed (921 €/ton) and fish live weight production (0.95 €/kg), whereas HFM+RM diet was the cheapest one (feed cost: 821 €/ton; fish production: 0.87 €/kg).