The perch (Perca fluviatilis)-roach (Rutilus rutilus) system in European lakes is among the best known freshwater fish communities. Still, the ecology of the youngest stages of these two species has been much less investigated especially at the experimental level. Although major members of European fish communities overall, perch and roach are not native to the Iberian Peninsula, but have recently been introduced into a few localities in Catalonia. The two general objectives of the proposal include: i) experimental studies in lake enclosures on competitive interactions between young-of-the-year roach and zerch; and ii) the assessment of exotic fish impact on Spanish freshwater assemblages. The well-developed conceptual framework for the perch-roach interaction will provide an exceptional opportunity for predicting and understanding changes due to the invasion by these new species on the Iberian Peninsula.