The Utilization of Dietary Carbohydrates by Trout and Carp

Abstract

The growth responses, nutrient utilization and tissue carbohydrase activities in Rainbow trout and Mirror carp when respectively fed 0-30% and 0-45% of cassava, rice, corn, potato or dextrin in isonitrogenous diets were investigated. In trout, optimum growth and food utilization was at 20% dietary cassava or rice. Weight gain was positively correlated with dietary level of corn or potato. The cassava and rice diets produced better growth than the corn and potato diets. Dietary dextrin was less well utilized. Carbohydrate digestibility depended on the carbohydrate source, while protein digestibility did not, but was often over 75%. Carp could tolerate higher levels of carbohydrates and less protein than trout. The 45% cassava, rice or dextrin, or 30% corn led to the best weight gain and food utilization in carp. Apparent digestibility of most of the carbohydrates was over 80%. Dietary carbohydrate did not seem to suppress protein digestibility, which was always over 65%. In both species, the very low levels (<3.5%) of dietary hydrolysable carbohydrate greatly suppressed carbohydrate digestibility. No group manifested significant liver damage, or persistent hyperglycaemia. α-amylase and β-glucosidase activities increase initially with increase in dietary dextrin, and is higher in Carp than in Trout. 45% dextrin in the diets of carp produced the highest levels of α-amylase in the liver and β-glucosidase in the hind-gut. Carcass and liver composition were affected only marginally except for glucose/glycogen contents which showed positive correlation with the dietary digestible carbohydrate levels. It is concluded that digestible carbohydrates enhance growth, the extent depending on carbohydrate type and level, when fed to Rainbow trout and Mirror carp, the latter being better adapted to utilizing high levels.