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.

Click to increase image sizeClick to decrease image sizeKey words: dimorphism among sexesindividual growthPerca fluviatilisRutilus rutilus

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.

In a novel biomanipulation experiment salmonids were used as a tool to improve water quality. The manipulation was initiated in spring 2000 as a response to non-point sources of phosphorus in a drinking water reservoir in Saxony, Germany. Salmonids (brown trout, Salmo trutta forma lacustris) were chosen as predators as the reservoir has a large hypolimnic water body and surface temperatures rarely exceed 20 °C. The vertical distributions of prey fish and brown trout were analysed with a fleet of vertical gill nets set in the pelagic zone of the reservoir. Consumption of brown trout was estimated by means of a bioenergetic model and the diet analyses of the trout. While the dominant planktivore (roach, Rutilus rutilus) was caught almost exclusively in the epilimnion during the stratification period trout were caught mainly below a depth of 10 m. Diet analysis revealed that the trout performed vertical migrations to consume food in the epilimnic layer, as an important food component were adult terrestrial and aquatic insects. The amount of fish in the food increased strongly with the size of the brown trout. The consumption estimate suggested that the trout had consumed 2–3% of the total roach stock during the study period (May–November 2000) of the first year of biomanipulation. We conclude that in general salmonids are suitable for food-web manipulation in deep reservoirs, but the stocked fish should be as large as possible (> 300 mm) and the proportion of large trout (> 500 mm) should be as high as possible.

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.

Several hypotheses concerning the dominance of percids under mesotrophic lake conditions were tested in five lakes of intermediate trophic state, situated in northeastern Germany. In contrast to prediction, the fish communities in four lakes were dominated by cyprinids (roach, Rutilus rutilus and bream, Abramis brama), while percids (Eurasian perch, Perca fluviatilis) dominated in one lake only. Percid biomass was not positively related to macrophyte cover, and age-0 perch exhibited no switch from zooplankton to benthic prey during summer in all lakes studied. Perch of the size-class 100–150 mm were partly piscivorous in the lakes not dominated by perch, while they were more benthivorous in the other. Perch larger than 150 mm were piscivorous in all lakes, though those in the lake dominated by perch had to a large extent consumed benthic invertebrates in spring. Roach utilized macrophytes instead of bluegreen algae as an important food resource and otherwise were mainly benthivorous. Thus, important predictions concerning the structure of fish communities in mesotrophic Eurasian lakes could not be verified by this study.

The structure and diet of the roach (Rutilus rutilus) population in the hypertrophic Bautzen Reservoir was examined from April to November 1998. Under the long-term impact of high predation pressure by piscivorous fish, a very heterogeneous population structure of roach had developed. Only a few age classes were dominant while other age classes were nearly absent. The proportion of males decreased with increasing age to 4% of the total abundance of one age class, which nevertheless seemed to have no negative effect on reproductive success. Food analysis revealed that the diet consisted of a high proportion of algae and macrophytes. The collapse of the Daphnia galeata population in early summer 1998 forced the roach to switch to benthic food resources [macroinvertebrates and fish: chironomids, molluscs and ruffe (Gymnocephalus cernuus)] in early June. Total consumption of age-2 and age-4 roach, the two most dominant year classes, was calculated by a bioenergetics model. Additionally, consumption of age-0 roach was estimated by assuming a fixed daily food consumption rate. The results suggest that daily consumption by these age groups, which never exceeded 0.2% of total biomass of the D. galeata population, had a negligible impact on the population of daphnids in Bautzen Reservoir during the period studied.

We conducted comprehensive field investigations to characterise the trophic status and food web structure of Großer Vätersee (GV) and Kleiner Vätersee (KV), two stratified lakes in the Baltic lake region of northeastern Germany. The lakes will be used as manipulation (GV) and reference (KV) ecosystems in food web studies. Both lakes are mesotrophic to slightly eutrophic with mean epilimnetic total phosphorus (P) concentrations of 24 and 21 μg L–1 in GV and KV, respectively. Mean phytoplankton biomass was 0.9 and 0.5 cm3 m–3 in GV and KV. During summer, the algal community of both lakes was dominated by cyanobacterial picoplankton and Ceratium spp. Filamentous cyanobacteria (mostly Planktothrix sp.) prevailed in a deep chlorophyll maximum found within the chemocline of both lakes. Bacterial biomass in the epilimnion of GV usually ranged between 1 and 3 cm3 m–3. Mean ciliate abundance was 13 ind. mL –1 in GV and 14 ind. mL–1 in KV. While Oligotricha dominated in GV, Scuticociliata were found most frequently in KV. Mean annual crustacean biomass was high in both lakes (GV 2.5 cm3 m–3, KV 1.2 cm3 m–3); Eudiaptomus gracilis, E. graciloides and Daphnia spp. were the most prominent species. Total fish biomass of both lakes was dominated by roach (GV 52%; KV 57%) and perch (GV 24%; KV 34%) Vaucheria dichotoma and Chara spp. dominated the macrophyte community in GV; Chara spp. dominated in KV. The slightly more eutrophic character of GV was most likely a result of greater P accumulation in the sediments due to external loading, which resulted in greater hypolimnetic P concentrations throughout the summer in GV. However, on an annual basis these differences were only occasionally reflected at higher trophic levels. The differences in fish biomass, however, might have been a consequence of higher structural diversity in GV provided by submersed macrophytes rather than its more eutrophic character. We conclude that the trophic characteristics and food web structure of Großer Vätersee and Kleiner Vätersee are similar enough to be suitable as manipulation and reference lakes in food web experiments.

Three measures (gape, tip and tooth row) of pharyngeal bones from 13 Eurasian cyprinid fish species caught in north‐eastern Germany were used to establish species specific bone measurelength relationships for reconstruction of prey fish length. Left and right bones were symmetrical in the case of gape, tip and length of tooth row in 69, 46 and 8%, respectively. Gape and tip showed significantly lower maximum values of relative error than the tooth row measure. A comparison of bone measures established for roach Rutilus rutilus (L.) and bream Abramis brama (L.) from north‐eastern and from southern Germany showed significant differences. Left and right bone measure‐length relationships are not generally identical. Gape and tip measures allow more accurate back‐calculation of length than tooth row measures and locality specific relationships should be established.

Research on predator-prey relationships in the Littoral zone of Lake Constance showed that eels (Anguilla anguilla (L.)) were the most numerous piscivorous predators in the shallow water zones up to 3-metres depth in 1992. From July on fish was the most important component of the diet of eels. Perch (Perca fluviatilis L.), burbot (Lota lota (L.)) and bream (Abramis brama (L.)) were thwe most frequently consumed fish. As 61% of all identifiable fish the eels had consumed were perch, an attempt was made to estimate the impact of eel predation on the young-of-the-year (y-o-y) of the perch population. Consumption by the total eel population never exceeded the amount of perch fry consumed by adult, cannibalistic perch estimated in other studies, but it seems possible that eel predation could have an adverse influence on weak year classes. Further reoligotrophication of the lake might lead to even higher fish consumption by the eels due to declining benthic production and consequent increased predation pressure.