| Feeding trials were conducted in floating sea cages to investigate the effect of dietary carbohydrate level on growth performance, feed utilization, plasma parameters, digestive enzyme and hepatic carbohydrate metabolic enzyme activities of large yellow croaker(Larimichthys croceus) at two growth stages, and compared the difference in the optimal level of dietary carbohydrate for large yellow croaker between two different experimental diet designs. There were three feeding trials in the present study:1. An 8 week feeding trial was conducted to evaluate the effects of dietary carbohydrate level on growth performance, feed utilization, plasma parameters, digestive enzyme and hepatic carbohydrate metabolic enzyme activities of large yellow croaker (137.5±0.35g). Triplicate groups of fish were randomly fed six isonitrogenous (45% CP) and isoenergetic (18kJ/g GE) diets containing graded levels of carbohydrate (1.75%,6.67%,13.64%,21.15%,26.69% and 32.25%; Table 1-1). Fish protein concentrate, casein and gelatin were used as the protein sources. Fish oil and wheat starch were used as the lip id and carbohydrate sources, respectively. The results showed that the specific growth rate (SGR) increased with the increasing dietary carbohydrate levels up to 26.69%, and thereafter declined significantly (P<0.05). Both feed efficiency ratio (FER) and protein efficiency ratio (PER) of large yellow croaker fed diets with 13.64% to 26.69% carbohydrate were significantly higher than those of the other groups (P<0.05). The whole-body lipid content decreased significantly (P<0.05) with the decreasing dietary lipid and dropped to the lowest in fish fed diets with 32.25% carbohydrate. Hepatosomatic index (HIS) and liver glycogen content of fish increased with the increasing dietary carbohydrate, and reached to the highest in fish fed the highest-carbohydrate diet, which were significantly higher than those with 1.75% and 6.67% carbohydrate (P<0.05). Plasma total cholesterol and triglyceride levels significantly decreased with the increasing dietary carbohydrate levels (P<0.05), while no significant difference was observed in plasma glucose (P>0.05). Activities of serum lysozyme, lipoprotein lipase and hepatic lipase were significantly (P<0.05) stimulated by the increasing dietary carbohydrate while activities of intestinal tract amylase and hepatic hexokinase (HK) increased remarkably (P<0.05) first and then decreased, which rose to the highest value at 26.69% and 21.15% dietary carbohydrate inclusion, respectively. Activities of pyruvate kinase (PK) were significantly up-regulated by the increasing dietary carbohydrate, and reached to the peak in fish fed the diets with 32.25% carbohydrate, which were significantly higher than those withl.75% and 6.67% carbohydrate (P<0.05). Based on second-order polynomial regression analysis of SGR against dietary carbohydrate content, the optimal level of dietary carbohydrate (isonitrogenous and isoenergetic diet) for large yellow croaker (137.5±0.35g) was estimated to be 22.7%.2. An 8 week feeding trial was conducted to evaluate the effects of dietary carbohydrate level on growth performance, feed utilization, plasma parameters, digestive enzyme and hepatic carbohydrate metabolic enzyme activities of juvenile large yellow croaker (7.60±0.10g). Triplicate groups of fish were randomly fed six iso nitrogenous (45% CP) and isoenergetic (18kJ/g GE) diets containing graded levels of carbohydrate (1.2%,7.2%,14.4%,21.3%,27.9% and 33.9%; Table 2-1). Fish protein concentrate, casein and gelatin were used as the protein sources. Fish oil and wheat starch were used as the lipid and carbohydrate sources, respectively. The results showed that the SGR of juvenile increased with the increasing dietary carbohydrate levels up to 21.3%, which was significantly higher than those with 1.2% and 7.2% carbohydrate (P<0.05). The whole-body lipid content decreased significantly (P<0.05) with the decreasing dietary lipid and dropped to the lowest (5.75%) in fish fed diets with 33.9% carbohydrate. HSI and liver glycogen content of fish increased with the increasing dietary carbohydrate, and they both reached to the highest in fish fed the highest-carbohydrate diet, which were significantly higher than those with 1.2% carbohydrate (P<0.05). Plasma total cholesterol, triglyceride and low-density lipoprotein cholesterol (LDL-C) levels significantly decreased with the increasing dietary carbohydrate levels (P<0.05), while plasma glucose level significantly increased (P<0.05). The activity of intestinal tract amylase was significantly stimulated by the increasing dietary carbohydrate, which reached to the highest in fish fed diet with 27.9% carbohydrate (P<0.05). The increasing dietary carbohydrate contents significantly stimulated the activity of hepatic PK, while depressed the activity of hepatic PEPCK of fish (P<0.05). HK,6-Phosphofructo 1-kinase (PFK) and Fructose 1,6-bisphosphatase (FBPase) activities of juvenile were independent of dietary carbohydrate contents (P>0.05). Based on second-order polynomial regression analysis of SGR against dietary carbohydrate content, the optimal level of dietary carbohydrate (i so nitrogenous and isoenergetic diet) for juvenile large yellow croaker (7.60±0.1 Og) was estimated to be 21.13%.3. An 8 week feeding trial was conducted to evaluate the effects of dietary carbohydrate level growth performance, feed utilization, plasma parameters, digestive enzyme and hepatic carbohydrate metabolic enzyme activities of juvenile large yellow croaker (26.0±0.03g). Triplicate groups of fish were randomly fed six isonitrogenous (45% CP) and isolipidic (11% crude lip id) diets containing graded levels of carbohydrate (1.4%,6.7%,14.1%,21.6%,28.8% and 34.3%; Table 3-1). Fish protein concentrate, casein and gelatin were used as the protein sources. Fish oil and wheat starch were used as the lipid and carbohydrate sources, respectively. Results showed that the highest specific growth rate (SGR) was found in treatment with 21.6% of dietary carbohydrate, which was significantly higher than those with 1.4%,6.7% and 34.3% carbohydrate (P<0.05). The PER of juvenile showed the similar variation trend as SGR, which reached to the highest value in fish fed diet with 21.6% carbohydrate (P<0.05). The increasing dietary carbohydrate significantly decreased the whole-body moisture content, while increased the whole-body lipid content, which showed the highest value in fish fed diet with 21.6% carbohydrate(P<0.05). HSI, condition factor (CF) and liver glycogen content of fish significantly increased with the increasing dietary carbohydrate (P<0.05). The increasing dietary carbohydrate significantly increased the level of plasma glucose, which reached to the peak in fish fed diet with 34.3% carbohydrate, however, increased first and then decreased the levels of plasma total cholesterol and triglyceride of juvenile (P<0.05). The activities of intestinal tract amylase and hepatic PK were significantly stimulated by the increasing dietary carbohydrate, which reached to the highest value in fish fed diet with 21.6% and 28.8% carbohydrate, respectively (P<0.05). HK, PFK, FBPase and PEPCK activities of fish were independent of dietary carbohydrate contents (.P>0.05). Based on second-order polynomial regression analysis of SGR against dietary carbohydrate content, the optimal level of dietary carbohydrate (isonitrogenous and isolipidic diet) for juvenile large yellow croaker (26.0±0.03g) was estimated to be 19.02%. |
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