Evaluation of a novel methanotroph bacteria meal grown on natural gas as fish meal substitute in rainbow trout, Oncorhynchus mykiss

In the present study, we evaluated the dietary fishmeal (FM) replacement potential of String Pro(R), a novel methanotrophic bacterial protein meal (BM), in rainbow trout, based on growth, feed utilization, digestibility, nutrient balance and histological observations. Five isonitrogenous (49.4-51.1%) and isoenergetic (21-22.1 MJ/kg) practical diets containing 0, 12.5, 25, 37.5 and 48% BM, corresponding to 0, 25, 50, 75 and 100% FM replacement, respectively, were fed to triplicate groups of rainbow trout juveniles for 9 weeks. At the end of the feeding trial, growth rate and feed utilization were high in all the dietary groups (i.e. thermal growth coefficient of 2.55-2.96 and feed efficiency of 1.2-1.3). Feed efficiency and protein efficiency ratio did not differ between dietary groups; however, the decreasing growth trend due to reduction in feed intake was reflected in body condition and carcass estimates of protein and lipid retention, gain and whole-body content, which decreased with increasing BM inclusion levels. There was no significant difference in apparent digestibility of protein, although, dry matter digestibility was reduced at the 48% dietary BM inclusion. Intestinal leucine aminopeptidase activity decreased; whereas, intestinal alkaline phosphatase activity and phosphorus retention increased with BM inclusion, with favourable reduction in phosphorus loss. Histological observations of the gastrointestinal tract, liver and kidney did not show any differences between dietary treatments. Based on these findings, we conclude that 12.5% of this novel BM ingredient can be safely included in the diets of rainbow trout, as a sustainable substitute for fish meal.