Effects of dietary carbohydrate to lipid ratios on growth performance, digestive enzyme and hepatic carbohydrate metabolic enzyme activities of large yellow croaker (Larmichthys crocea)

An 8-week feeding trial was conducted to evaluate the effect of dietary carbohydrate to lipid (CHO:L) ratios on growth performance, feed utilization, digestive enzymes and hepatic carbohydrate metabolic enzyme activities of large yellow croaker, Larmichthys crocea. Six isonitrogenous and isoenergetic diets (39.0% crude protein and 16.0 MJ kg(-1) gross energy) were formulated to contain various CHO:L ratios which ranged from 0.39 to 5.97. Triplicate groups of 50 fish (initial weight 7.06 +/- 0.48 g) were stocked in 18 floating net cages (1.5 m x 1.5 m x 2.0 m), and were fed twice daily to apparent satiation for 8 weeks. The results indicated that growth performance and feed utilization were significantly influenced by the dietary CHO:L ratios (P < 0.05). Maximum weight gain (WG), specific growth rate (SGR), protein efficiency ratio (PER) and feed efficiency (FE) occurred at 1.34 dietary CHO:L ratios. There were no significant differences in VSI and HSI among all treatments, however, the lowest condition factor (CF) was observed at 0.39 dietary CHO:L ratios. Moisture, protein, and ash contents of whole body were not significantly affected by the dietary CHO:L ratios (P > 0.05), and lipid content in whole body significantly decreased with dietary CHO:L ratios increasing from 0.39 to 1.69 (P > 0.05). Hepatic glycogen and muscle glycogen significantly increased with dietary CHO:L ratios from 0.39 to 1.34 then plateaued from 1.34 to 5.97. Fish fed the 0.39 CHO:L ratio diet had higher cholesterol concentration in serum than those fed the diets containing 1.69, 3.00 and 5.97 dietary CHO:L ratios; the lowest glucose concentration in serum was observed at dietary 0.39 CHO:L ratio, however, there were no significant differences in triglyceride and total protein concentrations of serum among all diets. Amylase, pepsin and lipase activities were significantly influenced by the dietary CHO:L ratios, and maximum amylase activity occurred at the 1.34 CHO:L ratios. Glucokinase (GK), pyruvate kinase (PK), and phosphoenolpyruvate carboxykinase (PEPCK) activities significantly increased with dietary CHO:L ratios increasing from 0.39 to 1.34, and then significantly decreased with further increase of dietary CHO:L ratios; no significant difference of phosphofructokinase (PFK), fructose-1,6-biphosphatase (FBPase) and glucose-6-phosphatase (G6Pase) activities was observed among all treatments. Based on weight gain, the optimal CHO:L ratio was determined to be 1.34 (approximately dietary 12.56% starch and 12.15% lipid) of dry diet for juvenile large yellow croaker. Statement of relevance The manuscript addresses the effect of dietary CHO:L ratios on growth performance, feed utilization and hepatic carbohydrate metabolic enzymes. Although this study investigated a common nutritional research topic, it still had somehow research originality because such nutritional topic has not been studied in large yellow croaker yet. The results give some valuable data for this marine fish. What's more, the information obtained from the present study would be helpful in developing low-cost diets for this species. (C) 2015 Elsevier B.V. All rights reserved.