Interactive Effects of Salinity and Dietary Energy on Growth Performance, Histomorphology and Nutrient Transporter Expression in Juvenile Nile Tilapia (Oreochromis niloticus)

Introduction: Nile tilapia (Oreochromis niloticus) exhibits good acclimatization to salinities up to 30 ppt and promising growth rates under commercial diets. However, fish in saline environments face increased maintenance demands due to osmoregulation. This study evaluated the interactive effects of dietary energy and salinity on growth performance, nutrient utilization, and the molecular expression of key nutrient transporters (PEPT1 and GLUT2) in juvenile Nile tilapia.

Methods: A 2 × 3 factorial trial was conducted using two identical Recirculating Aquaculture Systems (RAS) – one freshwater and one marine water (~25 ppt). Juvenile tilapia were fed three isoproteic diets with increasing gross energy levels (3,600, 3,800, and 4,000 kcal/kg). Growth performance metrics (final weight, weight gain, SGR, FCR) were analyzed. Histological and molecular analyses (RT-qPCR) were performed on liver and stomach tissues to assess relative expression of PEPT1a (peptide transporter) and GLUT2 (glucose transporter), comparing marine-reared fish to freshwater controls.

Results: Salinity and dietary energy independently and significantly enhanced growth, with no significant interaction between factors. Marine water rearing (~25 ppt) resulted in superior growth compared to freshwater (final weight: 9.1 vs. 7.39 g, p=0.023; SGR: 3.61 vs. 3.25, p=0.011; FCR: 1.46 vs. 1.85, p=0.039). Increasing dietary energy to 4,000 kcal/kg significantly improved all growth metrics regardless of salinity (p<0.01). Regarding gene expression, marine conditions were generally associated with reduced PEPT1 expression relative to freshwater controls, with this tendency becoming more pronounced at the highest energy level (4,000 kcal/kg). In contrast, GLUT2 expression showed a mixed pattern, varying across energy levels, suggesting greater sensitivity to combined environmental and dietary contexts.

Conclusion: Producers can optimize tilapia growth in marine water by increasing dietary energy to 4,000 kcal/kg, as both factors act as independent management levers. However, differential expressions of PEPT1 and GLUT2 indicate that osmoregulatory demands in saline environments modulate nutrient transporter regulation, with potential implications for diet formulation in saline aquaculture.