Satiation-dependent, intra-cohort variations in prey size selection of young roach (Rutilus rutilus)

Feeding rate and behaviour of juvenile roach in a non-depleting patch consisting of Daphnia magna of two size classes (optimal and much smaller) were studied in 20-min experiments. The medium size group (standard length 26-28 mm), large (28-32 mm), and small (24-26 mm) fish differed significantly in the proportion of small prey eaten. During a short initial period of burst feeding (1-5 min) only large prey were selected by all fish. The switch from large to small prey occurred much earlier in small than in large roach. During the initial period small fish had higher feeding rates. In small fish, small prey made up about 30% of the food biomass consumed during the whole 20-min feeding period, whereas in large fish small prey played a minor role (about 2%). Within the medium-sized group, one subgroup of fish, characterized by early switching to small prey, showed a feeding behaviour similar to small fish, whereas the other subgroup, which switched later, behaved like large fish. Neither the apparent prey size model nor optimal foraging theory, even in its dynamic version, were supported by the data. It is suggested that vigilance against predators changes the initial energy-maximizing strategy in hungry fish to a time-minimizing strategy in partially satiated fish. Intra-cohort variations in prey size selection might be related to social status of the fish. Social status may affect prey size selection and feeding rate, and thus result in different growth rates and size differentiation.