Fish solubles (FS), a nutrient-dense functional feed ingredient derived from fish processing byproducts, can complement soybean meal (SBM)-based diets by improving growth performance, gut health, and nutrient absorption, ultimately supporting sustainable and cost-effective aquaculture practices. This current study aimed to study strain-specific (commercial strain 1, CT1 vs. CT 2) response of rainbow trout to high SBM diets supplemented with FS on growth, feed utilization, gut health, gene expression, and fillet quality.
In a 66 days feeding trial, 600 fish (22.1 g) were distributed into 10 treatments in triplicates (50 fish/tank) following a 2 ´ 5 factorial design, where two types of rainbow trout strains, i.e., CT1 vs. CT2 were fed with 5 different isonitrogenous (45% crude protein) experimental diets: Diet 1, FM-based diet; Diet 2, 20% SBM; Diet 3, 35% SBM; Diet 4, 20% SBM with 5% FS; Diet 5, 35% SBM with 5% FS. All the data were subjected to two-way ANOVA to determine the main and interaction effects of strains and diets (three-way ANOVA when analyzing the fillet quality for two different storage days) and Tukey’s HSD for pairwise comparison using R programming.
Results showed that fish fed Diet 5 exhibited significantly (p < 0.05) higher growth performances than other diets. The CT2 strain showed better growth performance than the CT1 strain, and a similar observation was followed for feed utilization. However, no difference between these strains was observed for daily feed intake. The body indices like viscerosomatic index and hepatosomatic index were significantly (p < 0.05) higher in the CT2 strain than the CT1 strain. Fillet yield was significantly (p < 0.05) higher in fish fed Diet 5 and in CT2 strain. Significant effects of Diet, Strain, and Storage days were observed on fillet quality parameters. Significantly (p < 0.05) lower liquid loss and highest pH were recorded for Diet 5, CT1, and Day 8 than their counterparts. A better color profile was observed for the CT1 and Diet 3. The texture profile analysis showed that the CT1 strain had higher fillet hardness than the CT2 strain, and the opposite trend was observed for cohesiveness. Gut histology and gene expression parameters are being analyzed.
Overall, Diet 5, with 35% SBM and 5% FS, showed better growth performance and feed utilization. The CT2 strain showed better growth and feed utilization, but fillet quality was found to be better in the CT1.