Fish occurring in the outlets of water basins reduce the abundance of zooplankton. The study was performed at the outlet sections of the lake and waste stabilization pond of a sewage treatment plant. The aim of the study was to determine which zooplankton is chosen more often by the roach (Rutilus rutilus), those drifting from the waste stabilization pond or from the lake. The zooplankton from the pond was dominated by Daphnia pulex while zooplankton from the lake was dominated by small planktonic rotifers. We observed that the larger the plankter-victim’s size, the faster the reduction of its number. The fish were more likely to feed on zooplankton drifting from the waste stabilization pond than from the lake. It was influenced by D. pulex individuals, attractive for fish due to their largest body size among the analyzed zooplankton. The significance of riverine zooplankton in the downstream food web may render this data even more important.

Water transparency is an important factor affecting fish availability (underwater visibility) for diving birds. The diet of Great Cormorants Phalacrocorax carbo in relation to water transparency (range 1.4–4.0 m) was studied by pellet analyses at the submontane reservoir Dobczyce, Poland, from June to November. Although water transparency proved to be related to the birds' distribution, in the range of turbidities studied, no relationship was found with either fish species and fish size taken. Of 14 species in the diet, Roach Rutilus rutilus was dominant in all monthly samples (35–91% in 2002, 56–82% in 2004). Numbers of Great Cormorants and water transparency (range 0.4–1.4 m) were studied in the turbid lowland reservoir Goczałkowice during the migration period in autumn (August–November 2011 and 2012). Observations here suggest that the effect of water transparency on food uptake and habitat choice was only apparent below 0.6 m Secchi depth. The number of foraging Great Cormorants was not only affected by Secchi depth, but by a complex of factors (year, month, place, Secchi depth and water depth). We argue that, especially when the number of Great Cormorants is low, only highly detailed measurements of these factors (both spatially and as time series) can show the complex relationship between prey distribution, environmental conditions and the predator's behaviour.

This paper focuses on the role of invasive, omnivorous/carnivorous amphipods in the food web of lakes and evaluates their significance as food items for fishes in two major lakes and reservoirs of the northwestern region of Russia: Lake Ladoga and Rybinsk Reservoir. Fish samples from both water bodies were collected, there age determined and their stomach content analysed to assess the role of the introduced amphipod in the diet. Fish species mainly analysed were: perch Perca fluviatilis, ruffe Gymnocephalus cernuus, burbot Lota lota, roach Rutilus rutilus, vendace Coregonus albula and kilka Clupeonella cultriventris. Analysis of original and literature data showed that the energy flow from zoobenthos to fish was modified by the Baikalian amphipod Gmelinoides fasciatus, introduced in the 1960s-80s into many lakes and reservoirs of Russia. The invader established successfully in different aquatic systems, rapidly reaching high abundances. Examples from Lake Ladoga and the Rybinsk Reservoir confirmed that the species is an important food item for littoral benthivorous fishes (ruffe, perch, gobies, burbot), especially at early ontogenetic stages and also for several pelagic fish (vendace, kilka). Although these fish species are considered low food quality, they enrich the food supply for large predatory fish species. In Lake Ladoga, G. fasciatus influenced negatively the abundance and biomass of native benthic invertebrates (including Gammarus lacustris and Asellus aquaticus) which were previously important prey items for littoral fishes: in the beginning of introduction up to 24100 m⁻² and 33.5 g m⁻²; after introduction 6200-7800 m⁻² and 16.5 g m⁻², respectively. At the same time, the invader uses food sources (chironomids and oligochaetes) that would otherwise not be available to fishes. Therefore, the introduction of G. fasciatus into lakes and reservoirs of European part of Russia led to a greater share of the benthic productivity available to fishes, thereby allowing the intensive use of new ecosystem components and increasing the overall production in these systems.