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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

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–  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 fish communities of 84 Central and Eastern European reservoirs were sub-divided according to their species compositions into six fish faunal types that are identical with the successional stages of reservoir ichthyocenoses. The six types are:(1) Briefly existing fish faunas in which riverine species (especially salmonids) predominate. Found in 4070 of reservoirs.(2) Faunas characteristic of the reservoir initial filling period and with extraordinarily high percentage occurrence of northern pike. Found in 6% of reservoirs. (3) Faunas in which perch (Perca fluviatilis) is predominant usually with one particular year class strongly represented (a 'cycling' population). Found in 9% of reservoirs. (4) A transient fish fauna which is dominated by perch plus cyprinid fish. Found in 8% of reservoirs, with 20-50% of the fauna being perch and the rest represented by the predominant cyprinid species of type 5. (5) A fauna dominated by cyprinids, usually by Rutilus rutilus, Abramis brama and/or BUcca bjoerkna together with a non-cycling perch population of less than 20%. This faunal type is the most frequently occurring one in Central and Eastern European reservoirs (61 % of cases). (6) The remaining reservoirs (12%) contain fish faunas that are dominated by coregonids or Clupeonella or Carassius or Ctenopharyngodon or Hypophthalmichthys or Aristichthys or Cyprinus carpio or Pelecus cultratus.The characteristics and validity of these fish faunal types are discussed.

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.

The large-scale spatial distribution of fish was investigated within a morphometrically simple canyon-shaped reservoir with a single major tributary and a longitudinal trophic gradient (Římov Reservoir, Czech Republic). Samples of fish were taken by Nordic survey gill nets (several mesh sizes from 8 to 70 mm knot to knot) installed as surface nets at several offshore areas located along the longitudinal axis of the reservoir. Surveys were carried out in late summer during 1999-2003. An obvious distribution gradient of fish was revealed along the longitudinal axis of the Římov Reservoir. The total relative fish abundance and biomass (catch per unit effort) decreased considerably from the upstream end of the reservoir toward the dam. Roach (Rutilus rutilus), bleak (Alburnus alburnus) and bream (Abramis brama) comprised the bulk of catches at all areas. Enhanced dominance of bream was observed in the fish assemblage at the uppermost, more eutrophic area of the reservoir. The highest number of fish species and the highest abundance of young-of-the-year fish were also observed in the tributary area. In the downstream part of the reservoir, gill net surveys along the vertical depth profiles indicated that offshore fish occupied mostly the epilimnion. Extreme flood events affected the Římov Reservoir, however, it seemed they had no significant impact on the gradients described.

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