Fertilization fallout and fish fertility in low-silt ponds: studying the impacts on soil properties, water quality, immune-physiological response, and reproductive performance in red tilapia broodstock under saline conditions and plant-based diets

A 210-day reproduction trial tested the effect of organic fertilizer composed of fish sludge + compost of Beta vulgaris leaves (FS + BVL) on water quality, soil properties and fertility of red tilapia Oreochromis sp. fed a plant-based diet under varying salinities in low-silt ponds. A 1:3 sex ratio (500♂: 1500♀) was stocked into 32 hapas (24.3 m³ each) placed in 8 earthen ponds (150 m² each). Four ponds held 18‰ saltwater (2 fertilized, 2 unfertilized) and four ponds held 36‰ saltwater (2 fertilized, 2 unfertilized). Each fertilized and unfertilized group were fed two diets (fishmeal-enriched diet, FM₁, and plant-based fishmeal-free diet, FM₀) at 1% of body weight daily. Eight groups were administered in four replicates. The findings revealed that the use of FS + BVL fertilizer significantly (P < 0.05) improved water quality (higher oxygen, lower ammonia/nitrite, more chlorophyll and zooplankton) and enhanced nutritional composition of red tilapia (higher muscle protein, lower lipid). Fertilization also improved reproductive indices (testosomatic and gonadosomatic), reduced stress and metabolic markers (cholesterol, liver and kidney enzymes, cortisol), and boosted immunity, antioxidant defenses, and digestive enzyme activities. Consequently, broodstock in FS + BVL ponds achieved markedly (P < 0.05) higher sex hormones, fecundity, fry output, and reproductive success. Overall, FS + BVL proved highly effective in promoting red tilapia health and reproduction across 18‰ and 36‰ salinities and different diet types in low-silt ponds.