The behaviour of young-of-the-year (YOY) perch Perca fluviatilis as a dominant species in the assemblage of fry in the pelagic of Slapy Reservoir (Czech Republic), was studied during late May and mid-June 2002 using acoustic methods and complementary net catches. During the day, perch fry were present simultaneously in littoral, epipelagic and bathypelagic habitats. Bathypelagic perch fry, forming a scattering layer, migrated vertically each day between the epilimnion and hypolimnion, with an amplitude of 11Á0 m in May and 12Á5 m in June. At dusk, the migratory bathypelagic fry mixed in the epilimnion with non-migrating epipelagic fry and spent the night close to the thermocline (abundance maximum at 3-4 m in May, 0-2 m in June). In June, shoaling behaviour by some of the bathypelagic perch fry was also observed: the shoaling fry remained higher in the water column than the non-shoaling fry. Both depths of the scattering layer and the depths of the fry shoals were strongly controlled by the light intensity. The contribution of the bathypelagic part of the population to the total numbers of pelagic perch fry decreased from 28Á1% in May to 4Á7% in June, while the density of all pelagic perch fry increased (c. 96 000 individuals ha À1 in May and 142 000 individuals ha À1 in June). In May, the bathypelagic (average total length, L T , 11Á9 mm) and epipelagic (average L T 14Á6 mm) perch fry differed in size while, in June, the epipelagic fry were divided into two distinct size groups. The more abundant group, of small epipelagic perch fry (average L T 14Á6 mm), was similar in size to the bathypelagic fry (average L T 14Á6 mm) while the less abundant group, of larger epipelagic fry (average L T 34Á4 mm), was similar in size to littoral perch fry (average L T 35Á0 mm). The results suggest that in perch fry three different survival strategies with different risks can be used in the same locality, time and year. # 2005 The Fisheries Society of the British Isles

–  Spatial distribution of the fish community in the deep canyon‐shaped Římov Reservoir, Czech Republic, was studied using overnight gillnetting fishing in 1999–2007. Effects of depth, distance from the dam to the tributary and habitat type (benthic and pelagic) on fish community structure, catch per unit of effort (CPUE), biomass per unit of effort (BPUE) and average weight were tested. Fish were recorded in all sampled depths and parts of the reservoir. Redundancy analysis revealed that effects of three environmental variables were significant and most variability was explained by depth, then by distance from the dam and habitat type. CPUE and BPUE of all species decreased with depth and responses of individual species to depth were similar for juvenile and adult fish. Number of species, CPUE and BPUE of all species except perch increased heading towards the tributary and peaked close to or at the tributary part of the reservoir. Responses of juvenile fish to distance from the dam differed from that of adult fish. Structure of fish community differed in benthic and pelagic habitats with species preferably occupying epipelagic (bleak, asp, rudd and juvenile bleak, roach and bream) or littoral waters (perch, pikeperch, ruffe, roach, bream and juvenile percids). This study showed that fish distribution in the reservoir followed distinct patterns, which were probably shaped by a combination of physiological constraints plus trade‐off between food resources and competition.

The distribution and diet of age 0+ fish were studied in the deep canyon-shaped Římov Reservoir (Czech Republic), which is characterized by a longitudinal trophic gradient. During late summer of two years, 0+ fish were sampled from inshore and offshore habitats along the longitudinal reservoir axis. Offshore catches of 0+ fish from the surface layer were dominated by roach (Rutilus rutilus), bream (Abramis brama) and perch (Perca fluviatilis), whereas in the deeper open water perch predominated. Inshore catches of 0+ fish were constituted mainly by perch and roach. The proportions of roach in the inshore catches were highest at the upper and most eutrophic part of the reservoir, whereas the proportions of perch in the inshore catches were higher at the downstream areas. Total catches of both inshore and offshore 0+ fish increased upstream in the reservoir. Offshore 0+ perch were of consistently smaller size than inshore 0+ perch. Inshore 0+ perch had significantly smaller size at the upstream reservoir part than at the downstream, more lacustrine regions. The diet of both inshore and offshore 0+ fish consisted predominantly of crustacean zooplankton. Perch diet was generally dominated both by cladocerans and copepods, whereas roach diet consisted chiefly of cladocerans.

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Adverse effects of 23 bypass‐ or mill stream‐type small hydropower stations (SHPSs) have been studied by traditional benthological and ichthyological techniques. Four locations, if available, were distinguished for each SHPS: the natural flow sections (NFS) above and below the SHPS area, the mill stream or diversion channel bringing water to the turbine, and the reduced discharge section (RDS) of the former stream course. Most SHPSs took a high percentage of the stream discharge (turbine intake capacity was usually bigger than average yearly discharge of the stream at a given point). Despite this, the decrease in the stream‐wetted area was relatively low (usually 0–30%). The changes of benthic communities did not show any distinct trend explicable by the SHPS operation. Mechanical damage of fish by turbines was found rather rarely with SHPSs. The diverting weir (which usually distributes the discharge between the RDS and the mill stream), in combination with the abstraction of water for SHPSs, was an important migration barrier for resident fish in 30% of SHPSs. Water abstraction causes succession from large‐bodied fish species (adult brown trout, chub, dace, grayling, etc.) towards small‐bodied fish (trout fry, minnow, bullhead, stone loach, gudgeon). Average individual weight and fish biomass decreased four times in RDSs. Fish biomass losses exhibit significant negative relationships to the ratio of turbine intake capacity and stream discharge. Several mitigation measures are suggested. © 1997 John Wiley & Sons, Ltd.

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The inshore fish community of the Ř ı´mov Reservoir in the Czech Republic was evaluated over 21 years using shore seining at night. The development of the fish community was divided into two separate phases: a highly dynamic and unstable phase dominated by perch, Perca fluviatilis L., was replaced by an extremely stable cyprinid phase dominated by roach, Rutilus rutilus (L.), and bream, Abramis brama (L.). The abundance of both these species oscillated during the cyprinid phase, but with decreasing amplitude. The proportion of piscivorous fish species such as asp, Aspius aspius (L.), pike, Esox lucius L., and pikeperch, Sander lucioperca (L.), increased slightly with time but remained low. The biomass of large Cladocera was negatively correlated with fish biomass only

Abstract. Many species of chironomids undergo their immature development in habitats that rapidly change in suitability, such as rain pools, phytotelmata, freshly filled ponds or soil layers that seasonally dry out. Strategies for the utilization of these habitats can be divided into two groups: i) physiological or behavioral adaptations of larvae, which enable them to survive unsuitable conditions (in situ resistance) or ii) repeated recolonization of temporarily suitable habitats. In situ resistance, includes desiccation or frost resis tance, often in combination with cocoon building and migration of larvae into the sediment. Generally, the species that use the recolonization strategy tend to be better at migration and have a higher fertility and shorter development. Recolonization may include only temporary habitats or temporary habitats and some more stable habitats as well.