Influence of decreasing water temperature and shortening of the light phase on macronutrient self-selection by rainbow trout Oncorhynchus mykiss and common carp Cyprinus carpio.

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Macronutrient self-selection patterns in rainbow trout and common carp, which were maintained in groups, were examined when both water temperature and duration of light phase gradually decreased. Three semipurified macronutrient diets composed mainly of protein (casein and gelatin; crude protein (CP)=65%), fat (pollock oil and soybean oil; crude fat (CF)=38%), or digestible carbohydrate (dextrin and gelatinized starch; crude starch (CS)=57%) were offered to five groups of trout (20 fish/group, 51 g/fish) and carp (15 fish/group, 36 g/fish) using three self-feeders. A standard diet (ST) containing the three macronutrients at an appropriate ratio (CP=47%, CF=14%, CS=25%) was also offered to three groups each by a single self-feeder. After an initial 3 weeks of experiencing a long photoperiod (14 h light: 10 h dark cycle) and high water temperature (trout, 17°C; carp, 25°C), the light phase and temperature were gradually decreased in the following 3 weeks to 10 h light: 14 h dark, and 8°C for trout and 13°C for carp, and feeding was continued for a further 3 weeks under low temperature and short photoperiod conditions. Growth and protein utilization parameters between the ST diet self-fed groups and macronutrient diet self-selection groups were not different (P>0.05) for both trout and carp. Trout preferred the high-protein diet (HP) to the high-fat and the high-carbohydrate diets (P<0.05) regardless of the changes in water temperature and photoperiod. Carp also preferred the HP diet (P<0.01) under the high temperature and long photoperiod conditions; however, this preference gradually disappeared with lower temperatures and shorter photoperiods. These results suggest that the most appropriate macronutrient ratios in carp diets change with environmental conditions.

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