Iftekhar Ahmed Fagun

Reducing fishmeal (FM) use in aquafeeds is essential for sustainable aquaculture, but excessive replacement with alternative proteins may impair fish growth and physiological performance. This study evaluated the performance thresholds of graded FM replacement in probiotic‐supplemented alternative protein diets for climbing perch ( Anabas testudineus ). Four approximately iso‐nitrogenous and iso‐energetic diets were formulated using SBM and poultry by‐product meal. Diet D1 served as the control (0% FM replacement without probiotic), whereas D2, D3, and D4 contained 70%, 85%, and 100% FM replacement, respectively, and were supplemented with a multistrain probiotic containing Lactobacillus spp. and Corynebacterium glutamicum (0.2%). A total of 360 juveniles (2.52 ± 0.01 g) were fed the experimental diets for 56 days. Growth performance differed among treatments, with D1 producing the highest final body weight (9.65 g) and D4 the lowest (6.09 g). Fish fed D2 maintained final body weight (8.35 g), weight gain (231.81%), and specific growth rate (2.14% day ^{− 1} ) comparable to the control. Plasma glucose, total protein, GOT, GPT, triglycerides, and cholesterol did not differ significantly among treatments. Nitro blue tetrazolium activity did not differ significantly, whereas plasma peroxidase activity was significantly higher in D1 than in D3 and D4, with D2 showing an intermediate response. Salinity stress tolerance (LT ₅₀ ) was significantly higher in D1 (41.95 min) than in D4 (35.43 min), while D2 and D3 showed intermediate values. These findings indicate that approximately 70% FM replacement within the present probiotic‐supplemented dietary framework maintained growth performance and selected physiological responses broadly comparable to the control under the present experimental conditions, whereas complete FM replacement reduced growth performance and salinity stress tolerance. The study provides practical guidance for developing cost‐effective and environmentally sustainable feed formulations for climbing perch within a functional dietary framework rather than evaluating additive‐specific effects.