DIETARY PROTEIN AND LIPID ON GROWTH PERFORMANCE, BODY COMPOSITION AND DIGESTIVE ENZYME ACTIVITIES OF WINTERLING CULTER ALBURNUS ♀×MEGALOBRAMA TERMINALIS ♂

To optimize the feed formulation of a hybrid fish, Culter alburnus ♀ × Megalobrama terminalis ♂, a 3×3 factorial feeding trial was conducted. Nine diets were prepared with varying protein levels (36%, 39% and 42%) and lipid levels (6%, 8% and 10%) to evaluate production responses, including growth, feed utilization, body composition, and activity of digestive enzymes. The diets, labeled as P36L6, P36L8, P36L10, P39L6, P39L8, P39L10, P42L6, P42L8, and P42L10, were fed to 9 triplicate groups of fish with an initial average body weight of 8.59 g for 60d. Weight gain rates (WGR) and feed conversion ratios (FCR) of fish fed diets with protein levels of 39% and 42% (P39L6, P39L8, P39L10, P42L6, P42L8, and P42L10) did not show significantly differences among treatments (P>0.05). However, fish fed diets with a protein level of 36% (P36L6, P36L8, and P36L10) exhibited lower WGR and higher FCR (P<0.05). Protein retention (PR) of fish fed diets with one level of dietary protein were not significantly different, but fish fed diets with a dietary protein level of 39% (P39L6, P39L8, and P39L10) had higher PR compared to those of fish fed diets with 36% dietary protein (P36L6, P36L8, and P36L10). PR were not significantly different between treatments of fish fed diets with one level of dietary lipid and dietary protein level of either 39% or 42%; Viscera-somatic index (VSI) positively correlated with dietary lipid level (L) (VSI=0.223L+4.611, R=0.746, P=0.000). Whole fish body lipid tended to decrease with increasing dietary protein level and increase with increasing dietary lipid level. Fish fed diets with a dietary lipid level of 6% (P36L6, P39L6, and P42L6) had lower muscle lipid compared to those fed diets with 8% and 10% dietary lipid levels. Fish fed diets with a protein level of 36% (P36L6, P36L8, and P36L10) exhibited higher intestinal amylase activities compared to those fed diets with higher dietary protein levels. Intestinal protease and lipase activities were not significantly influenced by dietary protein level, lipid level or their interaction. Based on these results, it can be concluded that a dietary protein level of 39% and a dietary lipid level of 6% are suitable for maintaining good growth performance and feed utilization in the trial fish. Low dietary protein and high dietary lipid may lead to additional body lipid deposition.