Replacement of fish meal with a methanogen bacterial single cell protein: Effects on nutrient utilization and performance in juvenile rainbow trout (Oncorhynchus mykiss)

Nutrient utilization and performance of juvenile rainbow trout was examined by a gradual dietary replacement of fish meal (FM) with a novel methanotrophic bacterial single cell protein (BSCP) (Uniprotein®) as only dietary protein sources. Six isoproteic and isoenergetic diets included 0 %, 7.2 %, 14.5 %, 21.9 %, 29.3 %, 60.4 % of FM protein replacement with BSCP and without optimization and balancing with crystalline amino acids. Two experiments were conducted: 1) A mass balance study and 2) a six week growth performance - and amino acid (AA) digestibility study. The apparent digestibility coefficient (ADC) of crude protein (CP) significantly decreased (P ≤ 0.04) with ≥ 14.5 % of FM CP replacement. Partial protein digestibility was 86.7 % for BSCP and 93.0 % for FM with tryptophan and cysteine having the lowest AA digestibility. Crude lipid (CL) ADC was lowest at 60.4 % FM CP replacement (P ≤ 0.005). Nitrogen free extract (NFE) ADC was highest at 7.2 % CP replacement (P ≤ 0.047) and lowest at 60.4 % CP replacement (P ≤ 0.001). Total phosphorus (TP) ADC followed dietary TP content and increased by inclusion of BPM (P < 0.02). Solid faeces N excretion (TAN, total ammonia nitrogen) increased with higher FM replacement, conclusive of the lower CP digestibility and utilization of BSCP. Solid P faeces excretion followed dietary content in contrast to dissolved TP, that increased by higher BSCP inclusion. Specific growth rate (SGR) was not affected (P ≥ 0.078), but a negative impact on feed conversion ratio (FCR; P < 0.001) at highest replacement. No adverse gut or histopathological changes were observed. Hepatosomatic index (HSI) increased by supplementation of BSCP. Uniprotein® BSCP can replace at least 7.2 % of FM protein (32 g kg diet⁻¹).