INTERSPECIFIC AND INTRASPECIFIC SIZE CLASS COMPETITION AFFECTING RESOURCE USE AND GROWTH OF PERCH, PERCA-FLUVIATILIS

Effects of roach Rutilus rutilus density on growth rate and resource use of 1+ perch in the presence and absence of young-of-the-year (0+) perch was studied in a highly productive lake in S Sweden. Pelagic resources decreased with increasing roach density, a decrease stronger in the presence of 0+ perch. Diet of 1+ perch in the absence of 0+ perch changed from mainly pelagic cladocerans and calanoid copepods to cyclopoid copepods with increasing roach density. A similar diet shift was found for 0+ perch. In the presence of 0+ perch, 1+ perch shifted to benthic cladocerans at the highest roach densities. Growth rates of 0+ perch, 1+ perch as well as roach decreased with increasing roach density. The mass increase of 1+ perch was lower in enclosures with both roach and 0+ perch present than in enclosures with only roach present. The depression in 1+ perch growth caused by 0+ perch was higher at low roach densities than at high roach densities. The presence of size-structure in populations may alter the form of density dependence. Comparison of the growth depression of perch (both 0+ and 1+ perch) in the lake enclosure experiment with the growth depression at similar roach densities in a previous pond experiment illustrates the importance of the littoral benthic habitat, competitive refuge for perch, for the interaction between perch and roach and for ontogenetic niche shifts in perch. Interspecific competition from roach will both increase the time period during and the intensity by which size classes of perch compete. -Authors