. The Canadian field-naturalist. 424 The Canadian Field-Naturalist Vol. 97 ers. Juveniles were designated as those specimens of mm L or greater (Jones et al. 1978). Sixty larvae and 170juveniles were examined. Spec- imens were selected to represent sample sites, dates, and sizes as completely as possible. Larval fish were cleared with 3% KOH and glycerine to make gut material visible through the digestive tract, which is relatively undifferentiated until 20 mm L. Total gut analysis was necessary up to mm FL, after which only stomach contents were analyzed. Dry weight biomass of the g
. The Canadian field-naturalist. 424 The Canadian Field-Naturalist Vol. 97 ers. Juveniles were designated as those specimens of mm L or greater (Jones et al. 1978). Sixty larvae and 170juveniles were examined. Spec- imens were selected to represent sample sites, dates, and sizes as completely as possible. Larval fish were cleared with 3% KOH and glycerine to make gut material visible through the digestive tract, which is relatively undifferentiated until 20 mm L. Total gut analysis was necessary up to mm FL, after which only stomach contents were analyzed. Dry weight biomass of the gut contents was esti- mated from mean dry weight of the predominant food organisms. These were vacuum dried for 24 hours in a Fisher Isotemp vacuum oven at 70°C. Dry weight was determined with a Cahn electrobalance. Six groups were determined for each food organism, when possi- ble, the number of organisms per group varying with their availability. Cladocerans and copepods were obtained from the plankton samples. Dipteran and ephemeropteran insects, ostracods, and amphipods were obtained from juvenile stomachs; in order to avoid discrepancies in mass due to digestion, these organisms were only taken from the anteriormost por- tion of the stomach. Only intact specimens were used. The dry weight of rotifers was not estimated. We could not find intact specimens, and their size ren- dered them relatively insignificant to the diet. The cladocerans Bosmina spp. and Daphnia reiro- curva and the copepod Diaptomus minutus were the primary food items of young Alewives until early August (Figure 2). The three species represent by far the largest proportion of the zooplankton in the water column (Figure 1). Except for 23 May, rotifers were present in the diet from 20 May to 27 June in reason- able numbers. Calculations of feeding indices (Berg 1979) suggest that rotifers and copepod nauplii are strongly selected at lower lengths of larvae and juve- niles, but negatively selected by larger
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