Fish growth is a consequence of hyperplasia and hypertrophy. In our previous study, based on the muscle fiber recruitment, we identified four growth phases (PH) in rainbow trout: hyperplasia (PH1: 2.2-15 g), hypertrophy (PH2: 15-50 g), hyperplasia (PH3: 50-150 g), and hypertrophy (PH4: 150-350 g). Based on these growth phases, this study aimed to determine the least-cost feeding strategy for rainbow trout to maximize growth by investigating its influence on muscle fiber recruitment and related myogenic gene expression.
In a 28-week feeding trial, 1400 fish (5.8 g) were distributed into seven treatments in triplicates (70 fish/tank), fed three diets (42% crude protein and 20% lipid) namely FM: fishmeal; PP: plant-based protein; and AF: additive mixture of krill meal, taurine, and organic selenium, via switching the diets at different phases (Figure 1). Treatments were T1 (FM: all phases), T2 (FM: PH1, PH3; PP: PH2, PH4), T3 (PP: PH1, PH3; FM: PH2, PH4), T4 (PP: all phases), T5 (FM: PH1, PH3; AF: PH2, PH4), T6 (AF: PH1, PH3; FM: PH2, PH4), and T7 (AF: all phases).
Results showed that both FM (T1) and AF (T7) individually or in any combination (T5 and T6) showed higher (p<0.05) weight gain than other groups in the last phase (Phase 4). A significant (p < 0.05) reduction in muscle fiber density across various treatments with the progress of the feeding trial. Dietary switching notably influenced muscle fiber density and recruitment patterns, with T7 followed by T5 showing the highest density and recruitment of smaller fibers. Myogenic gene expression varied significantly (p < 0.05) across treatments, with MyoD2, Myf5, MyoG, and MRF4 showing phase-dependent regulation. MEF2 family genes also showed notable changes, particularly in T2, T4, and T6. Growth inhibitors MSTN1 and MSTN2 exhibited differential expression patterns influenced by dietary switching, with T1, T2, and T4 showing higher expression.
Conclusively, during hyperplasia, the high-quality protein-demanding phase, AF performs better than FM. Additionally, dietary switching, particularly using FM during hyperplasia followed by AF during hypertrophy, can enhance muscle fiber recruitment.