The Response of Young Roach Rutilus rutilus to Seasonal Changes in Abundance of Microcrustacean Prey: A Field Demonstration of Switching

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Abstract
Microcrustacean populations in the water column of Alderfen Broad, a small lake in Norfolk, U.K., were sampled regularly between May 1979 and November 1982. Planktonic Cladocera dominated the abundant summertime community whilst during the late autumn and early spring the low density community was dominated by copepods. Non-planktonic cladocerans formed only a small component of the community encountered in mid-water. Young roach (40-65 mm) fed predominantly on microcrustaceans. Their diet was assessed on 25 occasions through the four year period. During late spring and summer, diet was dominated by planktonic cladocerans, whereas during the remainder of the year non-planktonic Cladocera were the most important group. The relationship between relative contribution to the diet of planktonic Cladocera and their abundance in the lake is sigmoidal. The switch between a diet principally of planktonic to one principally of non-planktonic cladocerans occurred as density of the former fell below approximately 40 individuals per litre. We propose that this field example of ''switching'' arises as a result of a change by the predators from feeding in mid-water to feeding near and in the bottom sediment.