The role of the shoreline ecotonal zone in spawning success and early life history of dominant fish species in the lowland Sulejow Reservoir

The roach preferred branches of spruce and juniper as artificial spawning substratum rather than pine and birch. Perch preferred birch over conifers. Thus altering the availability of spawning substrata in the littoral zone should be effective in controlling the density and structure of juvenile roach and perch communities in reservoirs. Density dependent processes affects the early life history of fishes. Surveys in the Sulejow Reservoir in 1994 and 1997 provide an example of how increased availability of prey fish in the littoral zone may influence a predator population. During early summer age-0 pikeperch formed two discrete groups, one in the littoral and the other in the pelagic zone. In the littoral pikeperch became piscivorous at c. 3 cm on the numerous juvenile cyprinids, and grew relatively fast (5.2->9 cm by mid-July 1994). Those in the pelagic where prey fish were scarcer fed chiefly on zooplankton and grew relatively slowly (4.2-6.6 cm by mid-July 1994). From the resulting bimodal length distribution the upper modal group (littoral) had a higher probability of surviving their first winter. These data suggest a critical role for the ecotonal habitats in the dynamics of the trophic cascade, and show possibilities for controlling symptoms of eutrophication.