Effects of habitat and season on competitive interactions between roach (Rutilus rutilus) and perch (Perca fluviatilis).

The competitive interactions between two distantly related fish species, roach (Rutilus rutilus) and perch (Perca fluviatilis), were studied in enclosures in two habitats. In the open water habitat dominated by small planktonic prey perch grew more slowly and altered its diet from zooplankton to benthic macroinvertebrates when in the presence of roach. No effect, however, of perch on roach was observed. This was probably caused by the higher foraging efficiency of roach on zooplankton (laboratory experiments) and to the presence of a resource utilizable only by roach, bluegreen algae/detritus. In the littoral habitat no interspecific effects were detected even though resources were more limiting. This was probably a consequence of the higher densities of chironomids, on which perch fed more efficiently than roach, and of the presence of bluegreen algae/detritus. Roach, which consumed a larger spectrum of food types than perch, as a result was less affected by competition. The distributions of the species in the lake corresponded to what could be predicted from experiments. The degree of resource limitation varied with season and was highest in summer, while no resource limitation was observed in spring. This variation in resource limitation is probably caused by the increased metabolic demands of the fish in summer.