Food of roach (Rutilus rutilus) and ide (Leusiscus idus): significance of diet shift for interspecific competition in omnivorous fishes.

Feeding relationships between roach and ide from two sites in a mesotrophic lake SE Norway are presented and discussed. When animal food supply was scarce, both fish species increased their consumption of macrophytes; roach sevenfold, and ide threefold. Along a typical littoral vegetation gradient, ide fed among helophytes, while roach fed in the zone outside. This different habitat selection was reflected in the most important plants consumed (ide: Equisetum fluviatile, roach: Characeae), and confirmed by gill net catches. During the vegetative season, roach avoided areas with dense vegetation. In shallow areas beyond the littoral, the most important food plant for both fish species was Potamogeton perfoliatus, which constituted 80% of the total food consumed (dry weight basis) in the roach and 35% in the ide. The diet shift to plants seemed to be strongly influenced by the supply of animal food and the intensity of intra-and interspecific competition.