EFFECT OF SUBSTITUTING CHLORELLA SP. FOR REGULAR FISHMEAL ON GROWTH, BODY COMPOSITION, HEPATIC LIPID METABOLISM AND HISTOLOGY IN CRUCIAN CARP CARASSIUS AURATUS

The present study was designed to evaluate the effects of fishmeal (FM) substitution with Chlorella sp. on growth, body composition, hepatic lipid metabolism and histology in crucian carp Carassius auratus. Five isonitrogenous (CP, 38%) diets were formulated to substitue FM for Chlorella sp. with 0 (control), 21.8% (RM 21.8), 43.6% (RM 43.6), 65.5% (RM 65.5) and 87.3% (RM 87.3), respectively. Each experimental diet was randomly fed to triplicate groups with 20 crucian carp initial body weight of (5.540.08) g per tank for 8 weeks. Weight gain rate (WGR) increased with the substitution level from 0 to 43.6%, and then tended to decline with further increased replacement, but feed conversion ratio (FCR) showed the opposite trend with WGR. No significant differences were observed in IPR (intraperitoneal fat ratio) cross groups, while HSI (hepatosomatic index) and VSI (viscera somatic index) increased with the increased dietary Chlorella sp. The protein contents of muscle and liver decreased with the elevated dietary Chlorella sp. level, while the highest hepatic lipid contents were observed in RM 43.6 and RM 65.5 groups, which is associated with the increased expression of several lipogenic enzymes, such as G6PD (glucose-6-phosphate dehydrogenase), ME (malic enzyme), FAS (fatty acid synthase). Chlorella sp. had adverse effects on hepatic histology based on the cell size. Karyolysis, nucleus disappearance or necrosis occured in some hepatocytes of crucial carp fed Chlorella-added diets. Based on the quadratic regression analysis of WGR and FCR, the optimal dietary substitution levels were between 47% and 50% for best growth performance and feed utilization of crucian carp.