Habitat and food resource use of perch and roach in a deep mesotrophic reservoir: enough space to avoid competition?

The aim of this study was to test assumptions about the interaction of perch ( Perca fluviatilis L.) and roach ( Rutilus rutilus L.) concerning their food and habitat use in a deep and mesotrophic reservoir. From 1998 to 2001 the fish were caught with gillnets to perform diet analyses. Additionally, vertical gillnets were set to study vertical spatial distribution of both species. The results indicate that in a large and deep reservoir perch and roach are able to avoid competition by separating their niches first in the spatial dimension and then in the dimension of food utilisation. These separations result in low inter‐ and intraspecific competition of both species. We conclude that these niche separations can widen the juvenile competitive bottleneck to such an extent that the perch are not forced to compete with their older conspecifics during their first year of life, enabling perch to become large and consequently piscivorous.