Abstract
This study evaluated the effects of replacing fishmeal with microbial single-cell proteins (SCPs) in diets for juvenile yellowtail kingfish (YTK, Seriola lalandi), assessing growth, feed efficiency, nutrient digestibility, gut histology and microbiome, blood biochemistry and gene expression. Eight experimental diets were formulated, using methanotrophic (SB) and hydrogenotrophic (LT) SCPs at inclusion levels of 4 %, 8 %, and 16 %, alongside a control diet (42.2 % fish meal) and a combination diet (4 % SB + 4 % LT). Fish were reared in 500 L tanks for 12 weeks, after which growth performance, feed intake, feed conversion ratio (FCR) and health indices were measured. Significant improvements in FCR were observed in the 8 % SB and 16 % SB groups (0.99 and 0.98, respectively), compared to the control diet (1.06). Protein and energy retention were also highest in the 8 % SB and 16 % SB diets, with respective retention efficiencies of 39.0 % and 37.8 %. Apparent digestibility was comparable across diets, with protein digestibility peaking at 87.9 % in the 4 % SB + 4 % LT diet. Histological analysis revealed no significant changes in intestinal or skin integrity. Blood biochemistry, hematology, gene expression profiles and microbiome were unaffected by SCP inclusion, except for a notable decrease in gut bacterial diversity in fish fed the 4 % SB + 4 % LT diet. Overall, these results demonstrate that SCPs, particularly SB at 8 % and 16 % inclusion levels, can effectively replace fishmeal in YTK diets without compromising health or growth, while enhancing feed efficiency and nutrient retention.
