Nutritional Balancing Strategies in Low-Fishmeal Aquafeeds: A PRISMA 2020-Based Systematic Review Beyond Lysine and Methionine Supplementation.

Fishmeal reduction is central to sustainable aquafeed development, but low-fishmeal diets may not reproduce all the nutritional and functional properties of fishmeal after lysine and methionine correction. This PRISMA 2020-based systematic review synthesized peer-reviewed feeding trials evaluating nutritional-balancing strategies beyond these two amino acids in low-fishmeal, fishmeal-free, or high-alternative-protein diets. Alternative protein ingredients were treated as dietary matrices rather than interventions, and studies evaluating ingredient replacement alone were excluded. Twenty-three studies were included. Study-level effect sizes were estimable for 21 studies, providing 24 representative performance-related comparisons. Eight comparisons had 95% confidence intervals excluding zero in favor of the intervention, whereas 16 crossed the null; estimates were not pooled because of substantial heterogeneity in species, diet formulation, intervention, dose, and outcome definition. The evidence covered essential and functional amino acids, taurine and related nitrogenous compounds, palatability enhancers, enzymes, phosphorus and mineral balance, antioxidant support, lipid-associated nutrients, and gut-targeted strategies. Phytase provides the clearest mechanism through which phosphorus utilization can be improved in phytate-rich or phosphorus-limited diets. Taurine responses were generally favorable but imprecise in the four marine finfish species. Protein hydrolysates, fermented ingredients, yeast products, probiotics, hydroxyproline, and other additives produce context-dependent benefits for performance, nutrient utilization, intestinal conditions, immunity, stress resistance, or product quality. Across strategies, efficacy depended mainly on matching the intervention to the nutritional or functional limitation created by the complete diet. Nutritional equivalence should therefore be evaluated via integrated production, mechanism-specific, health, product-quality, environmental, and economic outcomes rather than the fishmeal-replacement percentage alone.