Optimizing Dietary Protein and Lipid Levels for Greenfin Filefish (Thamnaconus septentrionalis) in Practical Feeds:...

The study investigated the optimal dietary protein and lipid levels for greenfin filefish (Thamnaconus septentrionalis) using two sequential, growth trial-based experiments with practical formulations. Experiment I was designed to establish the quantitative protein requirement. Six isolipidic and isoenergetic diets with graded crude protein levels (38%, 41%, 44%, 47%, 50%, and 53% of dry matter) and a fixed lipid content of 10% were fed to triplicate groups of fish (5.56 g) for 56 days. Growth performance, feed utilization, and physiological responses were evaluated. Weight gain, specific growth rate, and feed conversion ratio all showed significant quadratic responses to dietary protein level, peaking at 47% protein. Quadratic regression analysis based on growth pinpointed the optimal dietary protein requirement at 45.93%-45.99%. This optimal level not only yielded the highest final biomass and best feed efficiency but was also associated with the highest hepatic total antioxidant capacity (T-AOC), indicating an enhanced systemic antioxidant defense without provoking oxidative damage, as malondialdehyde (MDA) levels remained stable across all protein groups. Subsequently, Experiment II was conducted to determine the lipid requirement at the established optimal protein level (46%) by testing six isonitrogenous diets with graded lipid levels (6%, 7.5%, 9%, 10.5%, 12%, and 13.5%). Quadratic regression based on growth revealed an optimal dietary lipid range of 9.69%-10.39%. While lipid levels up to 10.5% promoted growth and improved feed conversion through a protein-sparing effect, inclusion levels exceeding 12% induced metabolic dysfunction. This was evidenced by a sharp decline in T-AOC and superoxide dismutase activity, a significant increase in MDA, elevated hepatosomatic index (HSI), and reduced lipid retention efficiency. Notably, the concurrent accumulation of hepatic unsaturated fatty acids and rise in MDA at high lipid intakes suggested these endogenous pools became substrates for peroxidation, overwhelming the antioxidant system. Therefore, a practical formulation containing 46% protein, 9.7%-10.4% lipid, and 33% carbohydrate, with a crude protein-to-gross energy ratio (CP/GE) of ~23.93-24.59 mg/kJ, is recommended for juvenile greenfin filefish (<20 g).