Nutritional Energetics Of Spinibarbus Sinensis Fed Diets Containing Different Protein Levels

Two series of experiments were conducted on the Spinibarbus sinensis, an endemic freshwater fish of China. In experiment I, six isocaloric experimental diets containing different protein levels (20.5%, 26.5%, 34.2%, 41.0%, 49.9%, and 55.9%) were formulated, using white fish meal as the protein source. Each diet was randomly assigned to four groups of 12 fish for 10 weeks at 27.5℃in a circulated filtered rearing system. The effects of dietary protein levels on growth performance were investigated to find the optimal dietary protein level for S. sinensis. In experimentâ…¡, the effects of dietary protein levels on standard metabolic rate and specific dynamic action (SDA) were investigated in juvenile S. sinensis fed for 10 weeks in experimentâ… . Based on the data from the two series experiments, the nitrogen budgets and the energy budgets at the different dietary protein levels were compiled to approach the influence of dietary protein on growth in juvenile S. sinensis in terms of energetic mechanism.The results were as follows:1. As dietary protein increased gradually from 20.5% to 41.0%, the feed intake ratio of dry matter (FI_dm) and the feed intake ratio of energy (FI_e) decreased primarily, and then slightly increased. The maximum value of FI_dm (16.07 g kg^-1d^-1) and FI_e (0.25 MJ kg^-1d^-1) were attained by 20.5% group, and the minimum value of FI_dm (13.55 g kg^-1d^-1) and FI_e (0.21 MJ kg^-1d^-1) were attained by 41.0% group. As dietary protein increased from 20.5% to 55.9%, apparent digestibility of dry matter significantly decreased from 73.12% to 55.02% (p<0.05), whereas, there was no significant difference in the apparent digestibility of protein among all treatments.2. Specific growth rates of weight (SGR_w) significantly increased primarily as dietary protein increased (p<0.05), and then attained a plateau. From 20.5% to 41.0% groups, SGR_w increased from 0.60% d^-1 to 0.97% d^-1, but decreased to 0.96% d^-1 in 55.9% group. As dietary protein increased from 20.5% to 55.9%, the feed efficiency (FE) increased primarily, and then attained a plateau, and the protein efficiency ratio (PER) decreased from 178.23% to 116.60%, and the protein productive value (PPV) decreased from 23.92% to 18.62%。3. There was no significant difference in ash contents among all treatments. As dietary protein increased the body protein contents and moisture contents increased significantly (p<0.05), whereas body lipid and energy density decreased significantly (p<0.05). Analysis by correlation showed that dietary protein levels was positively correlated to body protein contents (r=0.678, p<0.01), and negatively correlated to body lipid and energy density (i=-0.880, p<0.01; r=-0.828, p<0.01).4. The standard metabolic rates (Rs) of all groups ranged from 106.8 mgO₂ kg^-1g^-1 to 109.1 mgO₂ kg^-1g^-1 and no significant difference was found among groups. The energy loss of specific dynamic action (SDAe) increased as the meal energy increased in all groups, and the rate of SDAe increased with the increasing meal energy was the fastest in 41.0% group. The relationships between energy intake and SDAe in all groups could be described as:20.5% group:SDA_E =33.630+0.047C (n=13,r=0.834,p<0.05)26.5% group:SDA_E=33.843+0.046C (n=14,r=0.915,p<0.05)34.2% group:SDA_E=35.893+0.061C (n=13,r=0.963,p<0.05)41.0% group:SDA_E=37.203+0.091C(n=13,r=0.971,p<0.05)49.9% group:SDA_E=32.485+0.066C (n=14,r =0.896,p<0.05)55.9% group: SDA_E=35.373+0.063C (n=14,r=0.809,p<0.05)5. The SDA coefficient, SDA peak value, SDA duration and factorial scope of SDA were significantly influenced by dietary protein levels (p<0.05). In 41.0% group the values of SDA coefficient, SDA peak value, and factorial scope of SDA were all significantly higher than other groups. Among 34.2%, 49.9%, and 55.8% groups, there were no significant difference among SDA coefficient, SDA peak value, factorial scope of SDA, but these four gorups were all higher than 20.5% and 26.5% groups in all indexes (p<0.05).6. The nitrogen retained in body (G_N%) in 41.0% group was significantly higher than those in other groups (p<0.05), but the nitrogen loss of this group in excreta (U_N%) was lower (p<0.05). Values of the nitrogen loss in feces (F_N%) among all groups have no significant difference. The nitrogen budget equation was 100C_N = 13.48F_N + 64.77U_N+ 21.75G_N. Analysis by partial correlation showed that G_N% was positively correlated to SGR_w (r =0.795,p<0.01). Similarly G_N% were positively correlated with PER (r =0.862, p<0.01).7. As dietary protein increased from 20.49% to 34.20%, the energy allocation to growth (G%) increased from 26.15% to 27.56%. However as the dietary protein level increased above 41.0%, G% significantly decreased (p<0.05). As dietary protein increased from 20.5% to 55.9%, the energy loss in metabolism (R%) decreased significantly from 61.43% to 56.94% (p<0.05). On the contrary, the energy loss in feces (F%) increased from 8.24% to 10.89% and the energy loss in excreta (E%) increased from 5.00% to 10.43% (p<0.05).The energy budgets of all groups were:20.5% group: 100C=26.15G +61.43R+8.24F +4.19U26.5% group: 100C=26.85G+59.59 R+8.33 F +5.22 U34.2% group: 100 C = 27.56 G + 58.08 R+ 8.00 F + 6.37 U41.0% group: 100C = 27.10G + 58.51 R+8.48F + 5.91 U49.9% group: 100 C = 24.40 G + 57.10 R + 10.37 F + 8.13 U55.9% group: 100 C = 23.02 G +56.94 R+10.89 F +9.41 UThe average energy budget equation was 100C = 25.85G +58.61R + 9.05F+6.54 U.The average proportions of G and M in A were described by the equation: 100A = 30.57 G + 69.43 M. The conclusions suggested in this study were as follows:1. The optimal dietary protein requirement for the growth of juvenile S. sinensis at 27.5℃would be 39.6-42.2%.2. The lower SGR_w might be induced by poor FI_dm and apparent digestibility which make the fish get scarce dietary protein from diets.3. It suggests that dietary protein has no influence on standard metabolic rate of S. sinensis. The SDA coefficient of juvenile S. sinensis, 5.2-9.9%, are slightly lower than some other species. The SDA coefficients are positively correlated to SGRw, which is in accordance with Jobling's theory. Below the optimum dietary protein level, SDA coefficient increases as dietary protein increases. But above that level, SDA coefficient would decrease as dietary protein increases.4. The energy allocated to growth ranges from 23.02% to 27.56% (G%) and accounts for 28.76%-32.18% assimilated energy (G/A%), which is lower than some other fish species. It suggests that the lower energy allocation to growth is another reason for poor growth performance of S. sinensis.5. There is higher proportion of energy resource allocated to metabolism and lower proportion to growth in S. sinensis than piscivorous fishes. This pattern of energy allocation suggests an optimal energetic strategy adopted by this omnivorous species.