SUMMARY 1. Silver carp, Hypophthalmichthys molitrix (Val.), feeds on both phyto‐ and zooplankton and has been used in lake biomanipulation studies to suppress algal biomass. Because reports on the effects of silver carp on lake food webs have been contradictory, we conducted an enclosure experiment to test how a moderate biomass of the fish (10 g wet weight m−3) affects phytoplankton and crustacean zooplankton in a mesotrophic temperate reservoir. 2. Phytoplankton biomass <30 μm and particulate organic carbon (POC) <30 μm were significantly higher in enclosures with silver carp than in enclosures without fish, whereas Secchi depth was lower. Total copepod biomass declined strongly in both treatments during the experiment, but it was significantly higher in fish‐free enclosures. Daphnid biomass was also consistently higher in enclosures without fish, although this effect was not significant. However, the presence of fish led to a fast and significant decrease in the size at maturity of Daphnia galeata Sars. Thus, the moderate biomass of silver carp had a stronger negative effect on cladoceran zooplankton than on phytoplankton. 3. Based on these results and those of previous studies, we conclude that silver carp should be used for biomanipulation only if the primary aim is to reduce nuisance blooms of large phytoplankton species (e.g. cyanobacteria) that cannot be effectively controlled by large herbivorous zooplankton. Therefore, stocking of silver carp appears to be most appropriate in tropical lakes that are highly productive and naturally lack large cladoceran zooplankton.

Turbidity can strongly influence predation success of visually oriented fish, especially piscivores such as adult Eurasian perch (Perca fluviatilis). This purely carnivorous species usually becomes a facultative piscivore after two discrete food niche shifts. Perch biomass has been observed to decrease in lakes along the productivity gradient, and then be replaced by cyprinids in non-manipulated eutrophic systems. Until now, this change has been mainly attributed to the competitive superiority of cyprinids for zooplankton prey during the juvenile phase of perch, while the piscivorous phase--as a possible factor influencing the recruitment success of perch--has been neglected. As the abundance of suitably sized prey fish should not be limiting in highly productive systems, we hypothesise that the switch from benthivorous feeding to preying on fish is inhibited by the reduced visibility in eutrophic lakes. We tested this hypothesis in laboratory experiments, where perch were fed two size classes of juvenile cyprinids at different phytoplankton- and bentonite-induced turbidity levels. Predation success was significantly influenced by turbidity level and turbidity source, but not by prey size. These experimental results suggest for the first time that piscivory of Eurasian perch is negatively influenced by different sources of turbidity, and hence low visibility might delay the onset of the food niche shift to fish prey.

Habitat and food resource use of perch and roach in a deep mesotrophic reservoir: enough space to avoid competition?Un resumen en espan˜ol se incluye detra´s del texto principal de este artı´culo. Kahl U, Radke RJ. Habitat and food resource use of perch and roach in a deep mesotrophic reservoir: enough space to avoid competition? Ecology of Freshwater Fish 2006: 15: 48-56. Ó Blackwell Munksgaard, 2005Abstract -The aim of this study was to test assumptions about the interaction of perch (Perca fluviatilis L.) and roach (Rutilus rutilus L.) concerning their food and habitat use in a deep and mesotrophic reservoir. From 1998 to 2001 the fish were caught with gillnets to perform diet analyses. Additionally, vertical gillnets were set to study vertical spatial distribution of both species. The results indicate that in a large and deep reservoir perch and roach are able to avoid competition by separating their niches first in the spatial dimension and then in the dimension of food utilisation. These separations result in low inter-and intraspecific competition of both species. We conclude that these niche separations can widen the juvenile competitive bottleneck to such an extent that the perch are not forced to compete with their older conspecifics during their first year of life, enabling perch to become large and consequently piscivorous.

Water level fluctuations in a drinking water reservoir (Saidenbach Reservoir, Saxony, Germany) and length-frequency data of roach (Rutilus rutilus) derived from gillnet catches were studied over a period of eight succeeding years and subsequently analysed with respect to level change and cohort establishment. The results indicate a strong impact of water level on the year class strength of roach. Constantly high water level with large areas of inundated plants led to a very strong roach year class. Conversely, decreasing water level shortly after the spawning period was found to result in a total loss of the new roach year class. Any other possible factors influencing year class strength of roach (such as food, temperature, winter duration) may only play a role if the water level after spawning remained relatively stable. Consequently, water quantity management in reservoirs can be used as a biomanipulation tool to reduce the planktivorous fish stock and to improve water quality, which should be an interesting option especially in drinking water reservoirs.