Robert J. Radke

Effects of phytoplankton-induced turbidity on predation success of piscivorous Eurasian perch (Perca fluviatilis): possible implications for fish community structure in lakes

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.

Trophic Characterization, Pelagic Food Web Structure and Comparison of Two Mesotrophic Lakes in Brandenburg (Germany)

We conducted comprehensive field investigations to characterise the trophic status and food web structure of Grosser Vatersee (GV) and Kleiner Vatersee (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 eu rophic 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 cm 3 m - 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 cm 3 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 cm 3 m -3 , KV 1.2 cm 3 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 Vatersee and Kleiner Vatersee are similar enough to be suitable as manipulation and reference lakes in food web experiments.

Food-web manipulation of drinking water reservoirs with salmonids: vertical distribution of prey and predator

Abstract 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.

The Roach Population in the Hypertrophic Bautzen Reservoir: Structure, Diet and Impact on Daphnia galeata

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.

Is bigger better? – Sex-specific growth of perch and roach

The transition from immaturity to sexual maturity in fish has consequences in relation to energy allocation. An immature individual allocates the energy supplied in the food to survival and growth. After reaching sexual maturity the individual has to allocate the energy to survival, growth and additionally to reproduction. Because o f the large costs o f reproduction, growth is most likely impaired after the first maturation o f an individual. Consequently, the age at first maturation is a trade-off between the increased fecundity of older fishes and the reduced chance of survival to that age. The pattem of energy allocation should maximise the number of offspring produced in the lifetime of an individual. Females make larger investrnents in gonad development than males. Thus they often mature slightly later. Since growth and reproduction are complementary processes, an earlier maturation should result in slower somatic growth. Due to the common phenomena of earlier-maturing males (TESCH 1955, DIANA 1995, BOYLE & CECH 1996), one can hypothesise that they fall behind in growth compared with females, Ieading to sexual growth dimorphism. The aim o f this study was to investigate sex-specific and individual differences in growth of perch and roach and possible consequences of an earlier maturation of the males.