1. Previous comparative analyses of fish communities in European lakes have mainly focused on the response of community composition to eutrophication. In addition, frequently only one or two lake habitats have been sampled. 2. Here, we present fish community data from 67 lakes in north-east Germany. Fish abundance was estimated in littoral, benthic and pelagic habitats from which a composite parameter indicating lake-wide relative species abundances was derived. This parameter was used in group comparisons and non-metric ordination procedures to explore, among 40 lake habitat descriptors sampled, those most important in structuring community composition. 3. Fish community composition was mainly determined by maximum and mean depth, chlorophyll a content and lake volume. The impact of anthropogenic alterations of shore structure and human-use intensity of lakes were of minor importance. The dominant fish species were vendace Coregonus albula, perch Perca fluviatilis, smelt Osmerus eperlanus and several cyprinids (roach Rutilus rutilus, bream Abramis brama, white bream Abramis bjoerkna and bleak Alburnus alburnus). 4. A response of relative species abundance to lake productivity could be demonstrated for small perch, ruffe and bream. However, when the relationship between lake morphology and productivity was controlled for, differences in species abundances were not longer attributable to differences in productivity, but to maximum lake depth. 5. This suggests that there are two distinct fish community types in Germany, namely the cold-water community with vendace and perch inhabiting deeper lakes, and the warmwater cyprinid community inhabiting more shallow lakes. The previously established conceptual model of a community succession from salmonids through perch to cyprinids with increasing eutrophication is hence not continous, but includes a switch between two lake and fish community types.

Summary 1.With the implementation of the European Water Framework Directive, the need for studies on European lake fish communities has increased to include lake type-specific fish community features. Although several standardized fish sampling methodologies are available, most previous fish community studies lack a simultaneous consideration of the littoral, benthic and pelagic habitats of lakes. 2.To compare habitat-specific fish communities, we sampled 67 lakes in the northeastern German lowlands using Norden multimesh gillnets in the benthic and pelagic habitats, and electrofishing in the littoral zone. 3. Standardized catches and diversity of the fish community differed among the three habitats sampled. Species richness and Shannon diversity were higher in benthic and littoral habitats compared with pelagic habitats. Overall, the benthic habitat had the most homogeneous catches and contained the most diverse fish community. 4. Cluster analysis and subsequent indicator value analyses produced substantially different optimum cluster numbers for the three habitat-specific fish communities. Based on the significant differences in fish community composition among the habitats, a simultaneous consideration of numerical fish catches from all habitats was performed using standardized fish abundances. 5. The cluster analysis of the combined abundances resulted in three groups of lakes that were indicated by three fish species. Morphological descriptors (volume, area, maximum depth, mean depth) and descriptors related to the trophic situation (conductivity, total phosphorus) clearly distinguished the three lake groups. 6. All three habitats showed distinct characteristics with respect to either species diversity or relative species' abundances. Our results do not support a conceptual model for all lakes of a gradual succession of fish communities as a result of eutrophication. 7. Synthesis and applications. Only simultaneous consideration of all lake habitats will fulfil the requirements of the Water Framework Directive for evaluating the ecological integrity of lakes. A pre-separation into at least two community types according to lake morphology is necessary before the deviation of the present fish community relative to a reference state can be determined.

1. Classification of European lake fish assemblages can be based on fish-assemblage structure or morphological, geographical, physical and chemical lake attributes. However, substantial gaps in knowledge exist with respect to the correspondence between both classification approaches. 2. Here, we compiled fish assemblage data from 165 lakes situated in the European 'Central Plains' ecoregion. Cluster analysis of fish abundances was performed to compare fish assemblage types of the entire ecoregion with those from previous country-specific studies. Nonparametric group comparisons, classification trees and partial canonical ordinations were used to infer the correspondence between fish assemblage types and morphology, geographical position and nutrient concentration of the lakes. 3. Three distinct fish assemblages were revealed: vendace (Coregonus albula), ruffe (Gymnocephalus cernuus) and roach (Rutilus rutilus) lake types. Both latitude and lake depth were the best determinants of lake type, but total phosphorus (TP) concentrations were also important. Vendace lakes were deep and had low TP concentrations, whereas the shallower ruffe and roach lakes had higher TP values. Roach lakes were more frequent in the north-west area of the ecoregion, whereas ruffe lakes were more often found south of the Baltic Sea. 4. Controlling for the influence of nutrient concentration showed that lake morphology and geographical position were important determinants of fish assemblages. However, the variance explained was low (<20%), implying that biological interactions may also be important in forming the lake-specific fish assemblages. 5. The results suggest that fish assemblages differ between deep and shallow lakes, and between the north-west and south-east locations within the Central Plains ecoregion. Accordingly, establishment of depth-related lake morphotypes is needed, and the European ecoregions recommended to be used in evaluation systems according to the Water Framework Directive seem to be too coarse to reflect the subtle differences of fish species richness along geographical gradients.

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Recaptures of adult, hatchery-reared, brown trout, Salmo trutta L., and fishing time from anglers were used to evaluate the benefits of stocking programmes with repeated releases of adult brown trout. The recapture rate varied between 17% and 29%. The time between stocking and capture (referred to as residence time) varied between 1 and 160 days (median 3-49 days). Between 67% and 84% of trout caught in the river were recently released fish. Fishing effort increased after stocking, thereby increasing the impact of angling on wild stocks. Stocking with adult brown trout decreases the impact of angling on wild trout only if the time spent fishing by all anglers is kept stable. Furthermore, because of the short residence time of stocked trout, long-term impacts through competition for space and food, or genetic impact through introgression, are limited.angling, brown trout, catch per unit effort, movement, recapture rates, stocking.

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Morphotypes for 67 lakes in the German lowlands were derived, based on maximum depth and mixis type. A threshold of 11 m maximum depth was identified to be the best level to discriminate shallow from deep lake morphotypes. The fish communities in these two morphotypes were significantly different. Indicator species analyses based on fish biomasses found vendace Coregonus albula in deep lakes and ruffe Gymnocephalus cernuus, bream Abramis brama, white bream Abramis bjoerkna, roach Rutilus rutilus, pikeperch Sander lucioperca and small perch Perca fluviatilis in shallow lakes to be the most representative species of their communities. Lake productivity was closely related to biomass and in part abundance of the type-indicator species, with vendace declining with increasing chlorophyll a concentration in the deep lakes, whereas biomass of pikeperch, bream, white bream and ruffe increased and biomass of small perch decreased with increasing chlorophyll a. These results indicate that assessment of ecological integrity of lakes by their fish fauna is generally possible, if lakes are initially separated according to a depth-related morphotype before the assessment, and if eutrophication is considered to be the main anthropogenic degradation.

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