| Summary: | Food habits, feeding selectivity, digestion rates, and daily rations were determined for stocked rainbow trout in a Western Manitoba winterkill lake. Amphipods, cladocerans, and chironomid larvae were the most important food items with amphipods comprising more than 90% by weight of the food consumed from August to October in 1970 and 1971. Rainbow trout were diurnal in their feeding habits and fed more or less continuously throughout the day and early evening. Peak stomach fullness occurred in late afternoon. Food consumption was reduced or absent from about midnight to 6:00 a.m. The type of food organisms eaten at different times of the day did not change appreciably. Evidence suggested that feeding was limited primarily to the limnetic zone. Laboratory and field experiments on feeding selectivity indicated that prey availability was the most important factor in determining which foods were eaten while predator preference was of minor significance. Chironomid larvae, although apparently preferred by the trout, were negatively selected because of their inaccessibility. The more easily captured amphipods were positively selected. Terrestrial invertebrates, plant material, and many littoral organisms were avoided entirely. The relationship between state of digestion and time approximated a negative exponential. The number of hours required for 95% digestion at 5, 12, 17, and 22C was 54, 16, 14, and 9 respectively. Digestion rates were independent of meal size, food type, and fish size for the range of food items and fish weights used in the experiments. Several methods of determining daily ration were evaluated. Estimates based on the balanced equation of Winberg (1956) were considerably higher than those derived from data on digestion rates and diel feeding intensities. Food consumption rates of 3.8% to 5.9% of body weight per day were calculated by the former method for July, August, September, 1971.
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