| Different diets may impact fish body composition, digestive enzyme activity, immune serum constituents, lipid metabolism, and other aspects of aquaculture production. Biological diets, such as rotifers, Artemia, and Daphnia, are important for the growth performance of juvenile fish, but their production can be limiting in meeting the demands of intensive farming. Therefore, artificial feeds, as an alternative to biological diets, are considered more practical.In this study, juvenile Coilia nasus (initial body weight 2.78±0.09 g) were randomly divided into three groups, with three groups with 3replicates of 500 fish in each group. Each group of fish was fed with one of three different diets (diet 1, soft pellet; diet 2, slow-sinking pellet; diet 3, Daphnia) for 60 d.In order to explore the effects of diet on the growth performance of juvenile Coilia nasus we measured differences in growth parameters, serum biochemical indicators, related enzyme activities, the morphology of the digestive system, muscle composition, and A6 fatty acyl desaturase (FAD6) gene expression. The results are summarized below:1. The different diets had a significant impact on growth performance in Coilia nasus, but indicators of immune system function remained unchanged. There were significant differences among the three groups in total body mass, specific growth rate, and rate of mass gain (P< 0.05). Fish fed with diet 2 had a significantly higher fullness coefficient than those fed with diet 1 (P< 0.05). Fish fed with diet 3 exhibited a significantly higher viscerosomatic index than those fed with diet 1. Growth traits (length, length to mass ratio, length to width ratio, and hepatopancreas somatic indices) and serum biochemical indices (total protein, albumin, blood glucose, cholesterol, and triglycerides) in all three treatments were not significantly different (P>0.05).2. The different diets resulted in significant differences in lipase enzyme activity in Coilia nasus, but no significant effect on pepsin or amylase activities was observed (P> 0.05). Fish fed with diets 2 and 3 exhibited significantly higher lipase activity than those fed with diet 1 (P<0.05), while fish fed with diet 3 exhibited significantly lower lipoprotein lipase, carnitine palmitoyltransferase-II, and acetyl-CoA carboxylase activity than those fed with diets 1 and 2. However, carnitine palmitoyltransferase-I activity was significantly different among all groups (P< 0.05).3. Diet can influence the morphology and organizational structure of the digestive system in fish under intensive culture. Hepatic cell lipid droplets were not obvious, and nuclei appeared to have a uniform distribution, in fish fed diet 1.In contrast, fish fed diet 2 had elevated hepatic cell lipid content, and nuclei were compressed and distributed towards the periphery of the cell. Similarly, fish fed diet 3 had obviously increased hepatic cell lipid content, and cytoplasm was reduced with nuclei that were not distributed uniformly. However, the histological features of the intestines were similar between the three groups.4. The experimental diets had differential effects on muscle composition in Coilia nasus. The content of dry matter and crude fat in fish fed diets 1 and 2 were significantly higher than those fed diet 3 (P<0.05). Crude fat content was approximately three times higher among fish fed diets 1 and 2, but moisture content was significantly lower than in fish fed diet 3 (P< 0.05). Ash content was significantly different among the three groups (P<0.05). Saturated fatty acid (SFA) and monounsaturated fatty acid (MUFA) contents in diet 3 were significantly lower than the others. However, polyunsaturated fatty acid (PUFA) content, n-3 unsaturated fatty acid, n-6 unsaturated fatty acid content, and EPA+DHA volume in diet 3were significantly higher than the others. Three groups of Coilia nasus juveniles had keep amino acid balance. Essential amino acid content of muscle in fish fed diets 1,2, and 3 was calculated to be 19.9 g/100 g,19.1 g/100 g, and 30.6 g/100 g, respectively. Content of L-glutamic acid (Glu) was highest in all three groups, followed by aspartic acid (Asp).5. The experimental diets had a significant effect on Fad6 gene expression in the various tissues of Coilia nasus, with each group displaying differing patterns of expression:diet 1 (brain> liver> gill> intestine> kidney> heart> muscle), diet 2 (liver> intestinal> brain> gill> heart> muscle> kidney), diet 3 (brain> intestinal> liver> heart> gill> muscle> kidney). The expression of Fad6 was higher in brain than in liver in fish fed diets 1 and 3 (P< 0.05), and was higher in liver than in brain in fish fed diet 2 (P< 0.05). Furthermore, the level of expression in the muscle, heart, and intestine in fish fed diets 2 and 3 were significantly higher (P< 0.05) than in fish fed diet 1. The level of expression of Fad6 in the brain of fish fed diet 3 was significantly higher (P< 0.01) than in fish fed diets 1 or 2, whereas the level of expression in the liver of fish fed diet 2 was significantly higher (P< 0.05) than in fish fed diets 1 or 3.The above results demonstrate that different diets have different effects on Coilia nasus. Live foods are more effective than artificial diets in the promotion of nutrient absorption and accumulation of nutrition in Coilia nasus. However, artificial diets offer more flexibility, and improvements in dietary lipid and protein content should result in improved growth performance. In addition, no significant changes were observed in serum biochemical factors of Coilia nasus in any of the diet groups, suggesting that immune function was stable and adaptable. Comprehensive analysis of digestive enzymes, growth performance, serum biochemical indices, and fat metabolism indicate that captive juvenile Coilia nasus can be raised on artificially prepared foods. Improvements in the quality of artificial diets will be necessary to avoid the associated problems with fish quality. |
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