Abstract
Fish serves as one of the important sources of protein for people across the world. Fish production comes from two sources: capture fisheries and aquaculture. Aquaculture has become the main source of fish production due to the stagnant production of global capture fisheries in the last few decades. To sustain the growth of aquaculture, a consistent supply of nutritionally balanced feed is a must. But relatively stagnant fishmeal production compared to total aquaculture production annually, fish feed producers are looking for alternatives to fishmeal, including animal by-products, microbial proteins, insect-based proteins, and plant-based protein sources. Among these, plant-based protein is considered more sustainable than the other alternatives. However, the presence of anti-nutritional factors, imbalanced essential amino acid profile, reduced digestible nutrients, and lower palatability, limits their optimum utilization in aquafeed. Moreover, plant-based protein may negatively impact the fillet quality of fish. Profit in the salmonid industry does not only depend on the quantity of production; better quality of the final product helps to fetch high market value and consumer acceptance. To address how the high plant-based protein diets impact the growth and fillet quality of rainbow trout, two long-term experiments were conducted in this Ph.D. dissertation. In the first experiment, a 30-week feeding trial, we investigated two additive mixtures supplemented with plant-based protein diets to select the best-performing additive mixture. Additionally, we identified four distinct growth phases based on the muscle fiber recruitment pattern in rainbow trout. In the second experiment, a 28-week feeding trial, we developed a least-cost dietary switching feeding strategy to provide a growth phase-specific diet to maximize growth performance and fillet quality concomitant with reduced use of fishmeal. Briefly, in the first experiment, dietary supplementation of additive mixtures, i.e., a mixture of krill meal, taurine, and organic selenium (A1) and a mixture of proline, hydroxyproline, and vitamin C (A2), to plant-based protein (PP), influenced the growth performance, fillet quality, and intestinal microbial community of rainbow trout. Results showed that PP negatively affected growth, fillet quality, and intestinal microbial community compared to fishmeal-based diets (FM), possibly due to an imbalanced nutrient profile. However, supplementing PP diets with additive mixtures (PP+A1 and PP+A2) mitigated these negative effects and improved growth performance, fillet quality, and intestinal microbial composition, which was comparable to FM. Based on the muscle fiber recruitment pattern, the study identified four distinct growth phases, and the muscle histology results were supported by the myogenic gene expression patterns. The identified four growth phases were- Phase 1 (2.2-15 g): Hyperplasia; Phase 2 (15-50 g): Hypertrophy; Phase 3 (50-150 g): Hyperplasia; and Phase 4 (150-350 g): Hypertrophy. In the second experiment, fish were fed three diets (FM: fishmeal; PP: plant-based protein; and AF: an additive mixture of krill meal, taurine, and organic selenium) via switching the diets at different phases (PH) comprising seven treatments. 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 dietary switching between FM and AF showed a superior growth performance, nutrient utilization, and fillet quality, along with supporting better muscle fiber recruitment in rainbow trout. This current Ph.D. dissertation is the first in aquaculture nutrition to provide insight into how fish muscle physiology could be used for precision nutrition to enhance the growth performance as well as fillet quality of rainbow trout.