It is a widely accepted assumption that fish predation controls structure and functioning of aquatic food webs. In the past, however, a large part of effort was concentrated on lakes and reservoirs. Thus, the knowledge about stream ecosystems is much more rudimentary than that for lakes in this respect. The aim of this thesis, therefore, was to describe and assess the effects of fish predation in natural stream ecosystems. For that purpose a reach scale field experiment was set up using an experimental stretch with benthivorous fish and a fishless reference stretch. A wide range of effects of the fish predators on their stream invertebrates prey was studied. To discriminate between lethal and sublethal predation effects, measuring the physiological status of the organisms seemed promising. However, before it was possible to decide whether or not environmental stress, such as predation, might affect the physiological status, the internal control as well as the seasonal and species-specific variability of the energy amount stored had to be assessed. Thus, the concentration and seasonal dynamics of the major energy storage components triglycerides and glycogen were measured in two species of mayflies (Rhithrogena semicolorata and Ephemera danica) with contrasting life cycle strategies. E. danica is a burrowing, semivoltine collector-gatherer, R. semicolorata is univoltine and scrapes periphyton from stones. Although triglycerides are the major energy reserve in both species throughout the whole larval development (> 84 % of total energy storage) their seasonal dynamic differed considerably. In R. semicolorata the triglyceride concentration declined during the last weeks prior to emergence in both sexes. The same pattern was found in female larvae of E. danica, but not in male E. danica. It is suggested that females use triglycerides in the last larval stages for egg maturation, which is completed in the last larval instar. In male E. danica the triglyceride concentrations remained high until emergence, presumably due to their high energy demands as adults for their swarming flights and mating. The difference in seasonal variation of triglycerides between E. danica and R. semicolorata shows the influence of environmental factors on the dynamics of storage components. E. danica lived in a very stable environment (within the substratum). Therefore the dynamic of energy storage components was optimised with respect to maximal reproduction. R. semicolorata on the other hand, suffered from hostile environmental factors such as predation or food limitation due to low periphyton biomass after leaf sprout and following light limitation in spring. Consequently, the concentration of storage components decreased during spring. One conclusion from this study was that the measurement of storage components might reveal sublethal predation effects. However, season and sex of the organisms are important factors as well and have to be considered in the sampling design. To analyse sublethal predation effects behavioural changes due to the presence of benthivorous fish were measured. Drift as a low-energy cost means of migration may enable stream invertebrates to leave risky habitats or may even be a direct escape reaction after a predator encounter. While the control of drift activity by predators has received considerable interest from many researchers, it remains still unclear whether predators reduce or increase drift activity. Drift activity of stream invertebrates was influenced significantly by the presence or absence the two benthivorous fish species gudgeon (Gobio gobio) and stone loach (Barbatula barbatula). Contrary to previous studies gudgeon and stone loach reduced invertebrate drift density and drift activity of Baetis rhodani rather than inducing higher night-time drift. Further, species composition of the invertebrate drift differed significantly between the two stretches. A further conclusion from this study is therefore that drift is not generally a mechanism of active escape from benthos-feeding fish, as previously assumed. In addition, the reduced drift activity in the fish stretch might result in a compensation of the consumptive losses due to fish predation. Thus, in this study design the effects of fish predation on invertebrate community might be underestimated. To detect predation effects on the food web structure the reactions of the grazing mayfly Rhithrogena semicolorata and the shredding amphipod Gammarus pulex to strong predation by benthivorous fish were compared. It has been hypothesised that shredders are generally less vulnerable to fish predation and therefore less likely to be predation-controlled than grazers, because the latter are visible to the predators during their feeding on stone surfaces, while shredders may hide between leaves during foraging. Biomass of G. pulex was significantly reduced in the fish stretch while that of R. semicolorata was not. Since approximately 91 % of the annual production of G. pulex but only 12 % of R. semicolorata production was consumed by benthivorous gudgeon, the observed difference of G. pulex biomass between the fish and reference reach is likely due to a lethal predation effect. However, no sublethal predation effects such as reduced concentration of storage components (triglycerides, glycogen) or reduced reproductive success were observed for both species. Hence, in contrast to the initial hypothesis, in the studied stream the shredder was top-down-controlled, while the grazer was not. It is concluded that top-down control depends on the ecological characteristics of a specific predator-prey pair rather than on trophic guild of the prey. To assess the predation effects on the life history of merolimnic insects and its consequences on fecundity the larval development and emergence of R. semicolorata was studied. It was possible to show lethal and sublethal effects of predation by benthivorous fish (Gobio gobio, Barbatula barbatula). Predation consequently resulted in changes of larval development and population fitness. The presence of two benthivorous fish species (gudgeon and stone loach) led to slower larval development and a delayed emergence. However, no differences in the adult size and fecundity between the fish reach and the reference were observed. Nevertheless, the longer time spent in the larval phase resulted in a higher mortality and therefore in a lower mean population fitness. The presence of gudgeon alone, however, did not seem to influence larval development, growth or time of emergence and consequently fecundity. Further, strong lethal impact of gudgeon could not be detected. Thus, the population fitness measured as the product of adult density and egg number was not reduced by gudgeon alone. It is assumed that the stronger lethal impact in the combined fish experiment is caused mainly by stone loach because the proportion of mayfly consumption by stone loach to mayfly production shortly before emergence was higher than the proportion related to gudgeon. Thus another conclusion is that 1) the impact of predation seems to differ for the fish species and 2) lethal effects have a stronger impact on the population survival than life history changes. Combining the results mentioned above leads to the assumption that predation by benthivorous fish has the potential to shape invertebrate communities and food webs in streams. It was possible to show reductions of benthic densities and mean population fitness. The strength of trophic interactions seemed to be specific for the single predator-prey pairs here. Finally, it can be stated that contrary to previous assumptions consumption of the fish predators seemed to be more important for the prey populations than sublethal predation effects.

It is a widely accepted assumption that fish predation controls structure and functioning of aquatic food webs. In the past, however, a large part of effort was concentrated on lakes and reservoirs. Thus, the knowledge about stream ecosystems is much more rudimentary than that for lakes in this respect. The aim of this thesis, therefore, was to describe and assess the effects of fish predation in natural stream ecosystems. For that purpose a reach scale field experiment was set up using an experimental stretch with benthivorous fish and a fishless reference stretch. A wide range of effects of the fish predators on their stream invertebrates prey was studied. To discriminate between lethal and sublethal predation effects, measuring the physiological status of the organisms seemed promising. However, before it was possible to decide whether or not environmental stress, such as predation, might affect the physiological status, the internal control as well as the seasonal and species-specific variability of the energy amount stored had to be assessed. Thus, the concentration and seasonal dynamics of the major energy storage components triglycerides and glycogen were measured in two species of mayflies (Rhithrogena semicolorata and Ephemera danica) with contrasting life cycle strategies. E. danica is a burrowing, semivoltine collector-gatherer, R. semicolorata is univoltine and scrapes periphyton from stones. Although triglycerides are the major energy reserve in both species throughout the whole larval development (> 84 % of total energy storage) their seasonal dynamic differed considerably. In R. semicolorata the triglyceride concentration declined during the last weeks prior to emergence in both sexes. The same pattern was found in female larvae of E. danica, but not in male E. danica. It is suggested that females use triglycerides in the last larval stages for egg maturation, which is completed in the last larval instar. In male E. danica the triglyceride concentrations remained high until emergence, presumably due to their high energy demands as adults for their swarming flights and mating. The difference in seasonal variation of triglycerides between E. danica and R. semicolorata shows the influence of environmental factors on the dynamics of storage components. E. danica lived in a very stable environment (within the substratum). Therefore the dynamic of energy storage components was optimised with respect to maximal reproduction. R. semicolorata on the other hand, suffered from hostile environmental factors such as predation or food limitation due to low periphyton biomass after leaf sprout and following light limitation in spring. Consequently, the concentration of storage components decreased during spring. One conclusion from this study was that the measurement of storage components might reveal sublethal predation effects. However, season and sex of the organisms are important factors as well and have to be considered in the sampling design. To analyse sublethal predation effects behavioural changes due to the presence of benthivorous fish were measured. Drift as a low-energy cost means of migration may enable stream invertebrates to leave risky habitats or may even be a direct escape reaction after a predator encounter. While the control of drift activity by predators has received considerable interest from many researchers, it remains still unclear whether predators reduce or increase drift activity. Drift activity of stream invertebrates was influenced significantly by the presence or absence the two benthivorous fish species gudgeon (Gobio gobio) and stone loach (Barbatula barbatula). Contrary to previous studies gudgeon and stone loach reduced invertebrate drift density and drift activity of Baetis rhodani rather than inducing higher night-time drift. Further, species composition of the invertebrate drift differed significantly between the two stretches. A further conclusion from this study is therefore that drift is not generally a mechanism of active escape from benthos-feeding fish, as previously assumed. In addition, the reduced drift activity in the fish stretch might result in a compensation of the consumptive losses due to fish predation. Thus, in this study design the effects of fish predation on invertebrate community might be underestimated. To detect predation effects on the food web structure the reactions of the grazing mayfly Rhithrogena semicolorata and the shredding amphipod Gammarus pulex to strong predation by benthivorous fish were compared. It has been hypothesised that shredders are generally less vulnerable to fish predation and therefore less likely to be predation-controlled than grazers, because the latter are visible to the predators during their feeding on stone surfaces, while shredders may hide between leaves during foraging. Biomass of G. pulex was significantly reduced in the fish stretch while that of R. semicolorata was not. Since approximately 91 % of the annual production of G. pulex but only 12 % of R. semicolorata production was consumed by benthivorous gudgeon, the observed difference of G. pulex biomass between the fish and reference reach is likely due to a lethal predation effect. However, no sublethal predation effects such as reduced concentration of storage components (triglycerides, glycogen) or reduced reproductive success were observed for both species. Hence, in contrast to the initial hypothesis, in the studied stream the shredder was top-down-controlled, while the grazer was not. It is concluded that top-down control depends on the ecological characteristics of a specific predator-prey pair rather than on trophic guild of the prey. To assess the predation effects on the life history of merolimnic insects and its consequences on fecundity the larval development and emergence of R. semicolorata was studied. It was possible to show lethal and sublethal effects of predation by benthivorous fish (Gobio gobio, Barbatula barbatula). Predation consequently resulted in changes of larval development and population fitness. The presence of two benthivorous fish species (gudgeon and stone loach) led to slower larval development and a delayed emergence. However, no differences in the adult size and fecundity between the fish reach and the reference were observed. Nevertheless, the longer time spent in the larval phase resulted in a higher mortality and therefore in a lower mean population fitness. The presence of gudgeon alone, however, did not seem to influence larval development, growth or time of emergence and consequently fecundity. Further, strong lethal impact of gudgeon could not be detected. Thus, the population fitness measured as the product of adult density and egg number was not reduced by gudgeon alone. It is assumed that the stronger lethal impact in the combined fish experiment is caused mainly by stone loach because the proportion of mayfly consumption by stone loach to mayfly production shortly before emergence was higher than the proportion related to gudgeon. Thus another conclusion is that 1) the impact of predation seems to differ for the fish species and 2) lethal effects have a stronger impact on the population survival than life history changes. Combining the results mentioned above leads to the assumption that predation by benthivorous fish has the potential to shape invertebrate communities and food webs in streams. It was possible to show reductions of benthic densities and mean population fitness. The strength of trophic interactions seemed to be specific for the single predator-prey pairs here. Finally, it can be stated that contrary to previous assumptions consumption of the fish predators seemed to be more important for the prey populations than sublethal predation effects.