Single-cell protein (SCP) meals from Methylovorus menthalis as feed ingredients for freshwater Atlantic salmon ( Salmo salar L.): Digestibility, growth performance, nutrient utilization, and fish health

This study provides nutritional evaluations of single-cell protein (SCP) meals produced from a naturallyoccurring methylotrophic microorganism (Methylovorus menthalis) in feeds for freshwater-phase Atlantic salmon (Salmo salar L., <30 g initial weight). Three trials were conducted (in triplicate) to: 1. determine 'singleingredient' apparent digestibility coefficients (ADCs) in an 80:20 ingredient substitution digestion assay (Trial 1), 2. examine effects of partial (50 %) or complete (100 %) substitution of major plant-protein ingredients on feed consumption (a proxy for palatability) and digestibility using nutritionally-balanced test diets (Trial 2), and 3. evaluate effects of graded inclusion levels (up to 30 %) at partial or complete substitution of marine and plantbased ingredients on growth performance, nutrient utilization, and fish health (Trial 3). In Trial 1, dry matter (DM), crude protein (CP), gross energy (GE), and essential amino acids (EAAs) in M. menthalis SCP meal were highly digestible at 85, 92, 88, and 86-96 %, respectively. In Trial 2, feed consumption of nutritionally-balanced diets were unaffected by up to 26 % M. menthalis SCP meal inclusion (100 % substitution of major plantproteins), but compared to the control diet showed higher ADCs for DM, GE, and methionine, the same ADC for tryptophan, and with only minor reductions in ADCs for CP and other EAAs. For Trial 3, inclusion of up to 30 % M. menthalis SCP meal did not impact fork length, Fulton's condition factor, survival, haematocrit, packed red blood cell counts, viscerosomatic index, feed conversion ratio, or protein efficiency ratio, however apparent feed intake, protein intake, final body weight, weight gain, and specific growth rate decreased, most notably at levels exceeding 10-20 % of the diet. Feed consumption (and nutrient/energy intakes) by fish fed increasing dietary SCP meal levels was linearly suppressed and had a cascading effect in reducing whole-body protein, lipid, and energy gains. However, this was not reflected in their corresponding apparent net retention efficiencies for protein, lipid, or energy, where no differences were observed at any SCP meal inclusion level, suggesting a possible 'correctable' diet palatability issue at high inclusion levels. Feeding diets with up to 30 % M. menthalis SCP meal had no impacts on plasma analyte biochemistry or intestinal histopathology. Collectively, results demonstrate that SCP meals derived from a naturally-occurring methylotrophic microorganism (M. menthalis) produced under scalable aerobic fermentation can be effectively included in Atlantic salmon pre-smolt feeds at 10-20 % at substitution of conventional marine and plant-protein ingredients with minimal or no effects on feed intake, nutrient digestibility, production performance, whole-body composition, nutrient utilization, intestinal histopathology, and fish health.