1. Life-history omnivory or size-induced mixed competition–predation systems have under many conditions theoretically been shown to be fragile, whereas at the same time existing empirical data suggest such systems to be common in nature.
2. In a whole lake experiment covering 17 years, we analysed the effects of the introduction of the intraguild prey roach (Rutilus rutilus) on the population size and individual performance of the intraguild predator perch (Perca fluviatilis) and on resource levels in two low productivity systems.
3. A strong long-term effect of roach on the zooplankton resource but not on the macroinvertebrate resource was present. Competitive effects of roach on perch were observed in one of the lakes the first years after the introduction, but at the end of the study no competitive effect of roach on either size class of perch was observed in any of the two lakes. In contrast, a positive predatory effect reflected in improved growth rates of older perch was present.
4. The lack of a support for a competitive effect of roach on small perch raises the question of the importance of mixed competition–predation interactions in life-history omnivorous systems and the problem of comparing descriptive data on feeding relationships with theoretical predictions based on interaction modules.