Effects of Intra- and Interspecific Competition on Dynamics and Size Structure of a Perch Perca Fluviatilis and a Roach Rutilus rutilus Population

Food overlap between roach Rutilus rutilus and perch Perca fluviatilis in a shallow eutrophic lake was low. Roach consumed largely detritus and algae not utilized by perch, but the competitive effect of the roach population on the perch population was substantial since the population size of roach was ten times that of perch. Competitive interactions between the species is also supported by the fact that the roach population increased at the same time that the perch population decreased. There was a small, but significant change in the size structure of the perch population. It is argued that increased interspecific competition results in a food niche contraction due to a decrease in individual growth rate and average size of the population. The same will be true when intraspecific competition increases in lakes with low habitat heterogeneity. Roach is more abundant than perch in eutrophic lakes whereas the opposite is the case of oligotrophic lakes. This is suggested to be related to availability of detritus and algae, the size distribution of food resources and the distribution of food resources with respect to habitat.