No empirical evidence for community-wide top-down control of prey fish density and size by fish predators in lakes

I tested relationships between fish predator and prey densities or size across 66 lakes of the European temperate zone, covering mean depths between 1 and 22 m and a phytoplankton biomass range between 1 and 270 mg m(-3) chlorophyll a. I hypothesized that in lakes of comparable phytoplankton biomass or depth, prey fish densities and size were lower at high predator densities or size than in systems with lower piscivory. Accordingly, I also expected a stronger trophic cascade from predatory fish to phytoplankton biomass in high-piscivory systems. None of the hypotheses were confirmed. Prey abundances and biomasses were consistently higher in high-piscivory than in low-piscivory lakes, if the effects of trophic state or lake depth on fish densities were accounted for. Chlorophyll a concentrations in lakes of similar depth did not differ between high and low piscivory, suggesting no difference in the strength of the trophic cascade. The overall size of prey was slightly higher in lakes with larger predators. The weak top-down influence of predatory fish on lower trophic levels may be attributable to the fact that the numerically dominant predator, perch, has to recruit through smaller ontogenetic stages, which dominate the prey community in most of the lakes. The availability of size refuges for prey and the overall omnivory of predators may contribute to the weak evidence for a trophic cascade.