No general percid dominance at mesotrophic lake conditions: insights from the quantification of predator-prey interactions

Abstract

Contrasting to predictions, roach ( Rutilus rutilus) are more abundant than perch ( Perca fluviatilis) in a number of mesotrophic lakes in the Baltic lake region of northeastern Germany. To reveal underlying mechanisms, roach and perch habitat use were analysed on a diel and seasonal scale, and piscivorous fish predation was quantified in one of those lakes, Lake Großer Vätersee (Brandenburg). For the latter point, piscivorous and non-piscivorous fish population sizes were assessed by multiple mark-recapture experiments and piscivorous consumption was quantified by bioenergetics modelling. Piscivorous fish biomass accounted for about 30% of total fish biomass, and top-down control of third trophic level biomass was intense. Fish habitat shifts closely linked the littoral and pelagic habitat, and were an integral component of the fish species interactions. While pike predation was mainly restricted to the littoral zone, piscivorous perch foraged in the open water during daytime. Roach ≥ age-1 counteracted the pelagic predation risk by diel horizontal migrations. So they could prey on pelagic zooplankton, and at the same time kept their predation risk in the pelagic zone as low as possible. In the littoral zone, roach extensively used non-animal resources not accessible to perch. Non-piscivorous perch were almost exclusively restricted to the littoral habitat; only age-0 perch had an initial pelagic stage. They didn't experience a juvenile competitive bottleneck. Overall, perch were relatively more affected by piscivorous fish predation than roach. This might be decisive for roach dominance under mesotrophic conditions. Additionally, Orconectes limosus, an introduced crayfish species, played a decisive role both as alternative prey for piscivorous fish and as a potential structuring force in the littoral habitat.