Ecophysiological Responses In A Carnivorous Fish, The Southern Catfish (Silurus Meridionalis Chen) To Nutritional Stress Of Dietary Carbohydrate

Four experiments were conducted on the southern catfish, Silurus meridionalis an obligatory carnivorous species. In experiment I, three isonitrogenous and isoenergetic diets containing 0%, 15%, and 30% level of precooked starch were formulated, and were referred to as the control, middle and high carbohydrate (CHO) diet, respectively. At 27.5°C, the effects of dietary carbohydrate level and feeding period on growth performance and plasma glucose were tested in the fish fed for 0, 2, 4, 8, 12, and 16 weeks, respectively. In experiment II, effects of high CHO diet on growth and plasma glucose in the fish were tested at 17.5, 20, 22.5, 25, 27.5, 30, and 32.5°C, respectively. In experiment III, effects of high CHO diet on resting metabolic rate in the fish at different temperatures were studied. In experiment IV, effects of high CHO diet on SDA in the fish at different temperatures were tested.The results were as follows:1. Specific growth rate (SGR), feed efficiency (FE), and protein productive value (PPV) in this fish were not significantly depressed by dietary carbohydrate within a feeding period of 12 weeks, but it were when the experiment lasted for more than 12 weeks(P<0.05). With feeding time lasting, there was a trend that hepatic glycogen and Hepatosomatic index (HSI) in the fish fed at the middle or high CHO level increased to their peaks, and then decreased to a low level.2. Lipid productive value (LPV) of the high CHO group were more than 100%, and were significantly higher than those of both the control and middle CHO group irrespective of experimental period (P<0.05). G6PDH activities of the high CHO group were significantly higher than those of the control at the 8^th and 12^th week (P<0.05). No significant difference in LPV and G6PDH activities was found between the control and middle CHO group.3. When fed for more than 2 weeks, the plasma glucose of the middle and high CHO groups were significantly higher than those of the control (P<0.05), while no significant difference was found between the middle CHO and high CHO groups. HSI of the middle and high CHO group were significantly higher than those of the control in the 2^th, 4^th, and 8^th week (P<0.05). Hepatic glycogen contents of the middle and high CHO group were higher than those of the control in the 2^th, 4^th, 8^th, and 12^th week (P<0.05).4. At 17.5°C, SGR, FE, and PPV of the high CHO group were significantly lower than those of the control (P<0.05), while no significant difference was found at other temperatures. At 17.5 and 20°C, protein efficiency ratio (PER) of the high CHO group were significantly lower than that of the control (P<.05), while no significant difference was found at other temperatures. LPV of the high CHO group were significantly higher than that of the control at temperatures above 17.5°C (P<0.05), while no significant difference was found at 17.5°C.5. At higher temperature (≥27.5°C), resting metabolic rate in the high CHO group was not significantly different from that of the control. At moderate temperature (20-25°C), resting metabolic rate in the high CHO group was higher than that of the control significantly (P<0.05). At 17.5°C, no significant increase of rest metabolic rate was found in the high CHO group compared to the control.6. Energy expended on specific dynamic action (SDA_E, kJ.kg^-1), SDA coefficient(SDA coefficient, %), SDA peak(SDA peak, mgO₂·kg^-1.h^-1), time to the peak(T_peak, h), SDA duration(SDA_D, h), and factorial scope of both the control and high CHO group increased linearly with temperature(T) significantly(P<0.05), and the relationships could be described as:Contol: SDA_E = 27.133+1.544T (n=26, r² =0.739, p<0.01)High CHO: SDA_E = 18.792+1.698T (n=20, r² =0.720, p<0.01)Contol: SDA coefficient = 4. 096+0.223T (n=26, r² =0.726, p<0.01) High CHO: SDA_coefficient = 2.733+0.250T (n=20, r² =0.721, p<0.01)Contol: SDA_peak = -69.106+12.258T (n=26, r² =0.948, p<0.01)High CHO: SDA _peak = -52.760+10.920T (n=20, r² =0.927, p<0.01)Contol: T_Peak= 24.807-0.525T (n=26, r² =0.587, p<0.01)HighCHO: T_peak = 20.078-0.369T (n=20, r² =0.349, p<0.01)Contol: SDA_D= 113.6-1.5T (n=26, r² =0.542, p<0.01)HighCHO: SDA_D= 110.4-1.5T (n=20, r² =0.379, p<0.01)Contol: Factorial scope = 1.850+0.053T (n=26, r² =0.400, p<0.01)High CHO: Factorial scope = 1.614+0.055T (n=20, r² =0.303, p<0.01)7. At 17.5°C, SDA_E and SDA coefficient in the high CHO group were significantly lower than those of the control at either the initial or the end of a month of feeding (P<0.05), while not significantly different at other temperatures.8. At 32.5°C, SDA peak in the high CHO group was significantly lower than those of the control at the initial of the feeding experiment (P<0.05), while the similar significant difference was found at 17.5°C at the end of a month of feeding. No significant difference was found between the two groups at other temperatures.The conclusions suggested in this study were as follows:1. The results showed that the stress of dietary carbohydrate could not depress apparent growth of carnivorous fish within a certain feeding period; however, the growth performance appeared depressed when the stress period lasted longer.2. The adaptive regulation mechanisms in the carnivorous fish under the nutritional stress were presented. As a response to middle level of dietary carbohydrate stress, the fish body increases its sugar store by elevating levels of both plasma glucose and liver glycogen. Under high level of carbohydrate stress, lipid synthesis and deposition in the body would increase in addition to a response as the former.3. The similar hyperglycemia response in the southern catfish fed with the middle and high CHO diets appeared in a short feeding period, which showed a positive adaptive mechanism of increasing the concentration of glucose substrate, which could accelerate the futile cycling to dissipate excess ingested carbohydrate.4. At a lower temperature (17.5°C), the southern fish utilized dietary carbohydrate poorly and its growth performance was depressed by high CHO diet. The capabilities of carbohydrate utilization by lipogenesis, glycogenesis, and glycolysis increased with temperature gradually, as a result of which dietary carbohydrate would not depress the growth performance at higher temperatures. The study presented the test data about carnivorous fish to support the hypothesis that there would be a positive relationship between body temperature of the animal and its carbohydrate utilization.5. The poor utilization of dietary carbohydrate in the carnivorous fish should be due to its low resting metabolic rate, and excess intake of carbohydrate could result in overloaded power input, which would accumulate as hyperglycemia. With temperature increasing, metabolism of fish body would switch gradually from 'demand-limited' to 'supply-limited', which could enhance the capability of fish to adapt dietary carbohydrate stress.6. Two peaks appeared during the SDA response in the fish at a lower temperature. It was proposed in this study that the peaks of "mechanical component" and "biochemical component" of SDA could be separated when temperature was low enough. Energy expended on mechanical processes in the southern catfish accounted for a considerable portion in the whole SDA, as a result of which the two separated peaks were observed. It improved the schematic model illustrating relationship of temperature and SDA in ectotherm summarized by McCue (2006).7. It was suggested that at lower temperature, energy expended on SDA and its ratio to energy intake decreased in the fish fed with high CHO diet, which reflected as the depressed growth performance and protein efficiency ecophysiologically. The suggestion above would present a test evidence to support the theory proposed by Jobling (1985) that SDA response would be positively correlated with the rate of growth rate and protein synthesis in fish.