| Crucian Carp(Carassius auratus), an important economic fish, are widely cultured in China because of its good taste, high-quality meat and high market value. Given the increasing demand and world supply fluctuations of fish meal(FM) for aquaculture, it has become very urgent to search for FM substitutes in aquafeeds. In this regard, microalgae appears as a promising alternative aimed to enhance the nutritive value of conventional feeds, and to be used, at least partially, as a substitute for FM for its richness in protein, essential amino acids(EAAs) and polyunsaturated fatty acids(PUFAs). Thus, the present study was conducted to evaluate:(1) the effect of FM replacement by Chlorella meal on growth performance, feed utilization, enzyme activities of hepatic lipid metabolism in crucaian carp; and(2) the effect of cellulase supplementation in Chlorella diets on growth and metabolism of crucain carp; and(3) the nutritional value of Chlorella meal combined with soybean meal or rapeseed meal in crucian carp diets. The main results were as follows: 1. Effect of substituting FM with Chlorella meal on growth, feed utilization and hepatic lipid metabolism in crucian carp.Five isonitrogenous(CP, 38%) diets were formulated to substitue FM for Chlorella meal with 0(control), 21.8%(RM 21.8), 43.6%(RM43.6), 65.5%(RM65.5) and 87.3%(RM87.3), respectively. Each experimental diet was randomly fed to triplicate groups with 20 crucian carp 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 intraperitoneal fat ratio(IPR) cross groups, while hepatosomatic index(HSI) and viscera somatic index(VSI) increased with the increased dietary Chlorella. The protein contents of muscle and liver decreased with the elevated dietary Chlorella level, while the highest hepatic lipid contents were observed in RM43.6 and RM65.5 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). 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. 2. Effects of dietary cellulase addition on growth performance, nutrient digestibility and digestive enzyme activities of juvenile crucian carp.The present trail was conducted to determine the effects of dietary cellulase supplementation on improving the nutritive value of Chlorella for juvenile crucian carp. Five isonitrogenous and isoenergetic experimental diets were formulated to contain 0.0(control), 0.5, 1.0, 1.5 and 2.0 g kg-1 cellulase, respectively. Each experimental diet was randomly assigned to triplicate groups with 25 juvenile fish per fiberglass tank for 8 weeks. The results showed that WGR, SGR(specific growth rate), FI(feed intake) and the trypsin activity in the anterior intestine increased with increasing dietary cellulase to 1.5 g kg-1 and then declined with further addition. However, the m RNA expression levels of Mrf4 and Myf5, the apparent digestibility coefficients for dry matter, protein, energy and the majority of amino acids, the activity of lipase in the anterior intestine were highest in fish fed with 1.0 g kg-1 cellulase diet, and then tended to decline with further cellulase supplementation. In conclusion, the optimal dietary cellulase supplementation level was 1.0-1.5 g kg-1, which can improve growth performance, digestive activities and nutrient digestibility in crucian carp. 3. Effect of replacing fish meal by a mixture of soybean meal and Chlorella meal in crucian carp diets.Five isonitrogenous and isoenergetic experimental diets were formulated to replace 0(SCM0), 25(SCM25), 50(SCM50), 75(SCM75), and 100%(SCM100) of protein from fish meal with a mixture of soybean meal and Chlorella meal(at a ratio of 3: 2). Each experimental diet was randomly assigned to triplicate groups with 25 juvenile fish per fiberglass tank for 6 weeks. With increasing substitution levels, WGR, SGR, FI, PER and digestive enzymatic activities(amylase, trypsin and lipase) in intestine and ADCs for dry matter, protein, lipid, energy and most amino acids showed a decreased trend, but these values in SCM25 group showed no significant difference with the control group(P > 0.05). FCR exhibited the opposite trend with PER. Dietary SCM substitution significantly improved the crude protein and lipid contents(P < 0.05), but did not affect the dry matter and ash contents of liver(P > 0.05). In conclusion, our results suggested that 25% of FM can be replaced with SCM without adverse growth performance and feed utilization, but the higher substitution levels induced some negative effects on growth and metabolism of crucian carp. 4. Effect of replacing fish meal by a mixture of protein sources(rapeseed meal and Chlorella meal) in crucian carp diets.Five isonitrogenous and isoenergetic experimental diets were formulated to replace 0(RCM0), 25(RCM25), 50(RCM50), 75(RCM75), and 100%(RCM100) of protein from fish meal with a mixture of rapeseed meal and Chlorella meal(at a ratio of 1: 1). Each experimental diet was randomly assigned to triplicate groups with 25 juvenile fish per fiberglass tank for 6 weeks. With increasing substitution levels, WGR, SGR, FI and PER increased but FCR decreased. Dietary protein sources substitution improved lipid content of muscle(P < 0.05), but had no significant effect on other proximate composition of muscle and liver(P > 0.05). The apparent digestibility coefficients of dry matter, protein, lipid, energy and most amino acids increased with increasing substitution levels, which showed the similar trend with digestive enzymatic activities(amylase, trypsin and lipase) in intestine. In conclusion, the results from this experiment indicated a positive effect on growth and feed utilization by the use of the mixture of protein sources(rapeseed meal and Chlorella meal), and FM could be completely replaced by the mixed protein sources in crucian carp diets. |
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