. The Canadian field-naturalist. 1983 Gregory, Brown and Daborn: Food Habits of Alewives 425 100-1 90' 80 ct 70' 60' 50- Q- 40- 30' 20' 10' MAY. / â¢ââ¢COPEPODA 0---ODAPHNIA' ®â®BOSMINA â â¢DIPTERA AND EPHEMEROPTERA JUNE JULY AUGUST Figure 2. Changes in weight percent of food ingested by young-of-the-year Alewives {A losa pseudoharengus) in Lake AinsHe, 1979. Discussion Brooks and Dodson's (1965) theory that the Ale- wife is an obligate planktivore selecting larger prey sizes as length increases, was generally true until August. At this time, large numbers of adult chiro- nomids and mayflies
. The Canadian field-naturalist. 1983 Gregory, Brown and Daborn: Food Habits of Alewives 425 100-1 90' 80 ct 70' 60' 50- Q- 40- 30' 20' 10' MAY. / â¢ââ¢COPEPODA 0---ODAPHNIA' ®â®BOSMINA â â¢DIPTERA AND EPHEMEROPTERA JUNE JULY AUGUST Figure 2. Changes in weight percent of food ingested by young-of-the-year Alewives {A losa pseudoharengus) in Lake AinsHe, 1979. Discussion Brooks and Dodson's (1965) theory that the Ale- wife is an obligate planktivore selecting larger prey sizes as length increases, was generally true until August. At this time, large numbers of adult chiro- nomids and mayflies were incorporated into the diet. The period of heavy reliance on insects corresponds with the period of lowest zooplankton density. Sim- ilarly, Pritchard (1929), Odell (1934), Norden (1968), Hutchinson (1971), Watt and Duerden (1974), and Kohler and Ney (1980) supply data suggesting that young Alewives become more omnivorous with increasing size. Larvae in May were taking 1 or 2 food types whereas juveniles in August were taking 14 to 18. Brooks and Dodson (1965) have suggested selective predation as a cause of cyclomorphosis, the cyclical seasonal change in shape exhibited by some Daphnia populations; Daphnia relrocurva inhabiting lakes with landlocked populations of Alewives is often strongly cyclomorphic. Despite intense predation on Daphnia retrocurva by young Lake Ainslie Alewives until August (when it disappeared from plankton samples), it underwent neither changes in body size or form. The low zooplankton densities exhibited during August would undoubtedly have an effect on a plank- tivorous fish. It has been suggested that low food resources could be a cause of downstream migration of Alewive juveniles (Richkus 1975; Vigerstad and Cobb 1978). We have no evidence of early August migration from Lake Ainslie in 1979, although there seems to have been a general movement of juveniles toward the outlet of the lake at this time (O'Neill 1980). Low plankton densi
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