Methanotroph (Methylococcus capsulatus, Bath) bacteria meal as an alternative protein source for Japanese yellowtail, Seriola quinqueradiata

Abstract Two trials were carried out to determine the optimal replacement level of fish meal (FM) protein by the bacterial protein meal FeedKind® (FK), in the diet of juvenile Japanese yellowtail, Seriola quinqueradiata. FK protein was produced from the methanotrophic bacteria Methylococcus capsulatus Bath, which utilizes methane as its sole carbon and energy source. In Trial 1, six diets were formulated: control diet (C) with FM as protein source; four experimental diets in which the FM of diet C was replaced at 25 (FK25), 50 (FK50), 75 (FK75) and 100% (FK100) by FK; and a final experimental diet in which 3% of FK in diet FK100 was replaced by enzyme-treated FM (EFK). In Trial 2, seven diets were prepared: control diet C similar to Trial 1; three experimental diets in which the FM of diet C was replaced at 20 (FK20), 25 (FK25) and 30% (FK30) by FK; two experimental diets in which FK from diet FK25 was replaced either by further grinding FK (FK25J) or lower digestible FK (FK25L); and experimental diet EFK in which 3% of the FK in diet FK25 was replaced by enzyme-treated FM. Fifteen (ca. 126g) or ten fish (ca. 80 g) were stocked into each of eighteen 500 L tanks in triplicates in Trial 1 and 2, respectively, fed two times daily until apparent satiation, and cultured for 8 weeks. At the end of both trials, feces were collected after feeding with chromic oxide (Cr2O3) mixed diets. In Trial 1, all growth parameters and nutrients retention efficiencies showed similar patterns and no significant differences were observed between diets C and FK25 (P > .05), though feed efficiency (FE) was reduced by 10% in the latter diet. However, other diets showed significantly lower growth performance compared to diet C (P .05). FE was also either equal to or higher in all FK-based diets compared with control group. Overall, results indicated that the FK can comprise up to 20% of total diet, replacing 30% of the FM protein in diet for yellowtail, without compromising growth performance or FE.