Impact of Dietary Lipid Levels on Growth Performance, Metabolic Enzymes and Carcass Composition in Labeo rohita Fingerlings Reared at Low Temperature in a Recirculating Aquaculture System (RAS)

The present experimental study investigated the optimal dietary lipid levels to improve the growth rate and metabolic efficiency in Labeo rohita fingerlings raised at 18°C±1°C in a recirculating aquaculture system (RAS) simulating the winter conditions.

Six experimental diets, each with similar crude protein content (300 g kg^-1) but varying lipid levels (40-140 g kg^-1) and digestible energy (15-17.95 MJ kg^-1) were prepared. Over a two-month feeding trial, fingerlings (8±0.09 g) were fed until they were satiated twice daily.

Polynomial regression and broken-line analysis determined that 90 g kg^-1 lipid was optimal for growth and feed efficiency, with peak values for average final weight (AFW), protein efficiency ratio (PER) and feed conversion ratio (FCR). Important metabolic enzymes such as glucose-6-phosphatase (G6Pase), Glucose-6-phosphate dehydrogenase (G6PDH) and carnitine palmitoyltransferase1 (CPT1)) similarly peaked at this lipid level, whereas higher lipid intake significantly (p<0.05) impaired enzyme activity. The ideal lipid level for AFW, as estimated by the quadratic equation (y = -0.001x² + 0.1774x + 5.2264, r² = 0.98), was 90.33 g kg^-1 and 90.03 g kg^-1 by broken-line analysis (r² = 0.99), with an averaging 90.18 g kg^-1. For feed conversion ratio (FCR), the quadratic equation (y = 0.0005x² -0.09x + 6.35, r² = 0.96) predicted 91.82 g kg^-1, with the broken-line analysis indicating 89.34 g kg^-1, averaging 90.58 g kg^-1. For protein efficiency ratio (PER), the quadratic equation (y = -0.0003x² + 0.05x -0.734, r² = 0.95) estimated 90.14 g kg^-1, whereas the broken-line analysis suggested 89.34 g kg^-1, with an average of 89.74 g kg^-1. All things considered, the optimal dietary lipid requirement for optimizing growth rate and nutrient efficiency was approximately 90 g kg^-1. These findings underscore the critical role of dietary lipid optimization in improving growth, nutrient utilization and metabolic health of Labeo rohita under winter culture conditions. To confirm these findings under different production systems and temperatures, more investigation is required.