Summary The assessment of fish densities using point abundance sampling by electro‐fishing requires information about the size of the sample area. For electro‐fishing the effective fishing range depends on biological effects such as species and length of fish as well as physical effects like conductivity of water or substrate type. The present study investigates systematically the impact of conductivity and substrate type on the extension of the electrical field of a battery‐powered electro‐fishing gear (DEKA 3000, Marsberg, Germany), modified for larval and juvenile fishes. Threshold values for galvanotaxis were examined for juvenile fishes of five species in terms of current densities. Based on 71 experiments a general function relating body length to current density threshold values was developed. Optimal electrical current flow periods of 10 s were determined. For three different substrate types (gravel, sand, mud) a formula has been developed to quantify biological and physical effects on the effective fishing range. Each equation included information on the length of fish and the ambient conductivity. An increase in the effective fishing range of about 10% every 0.1 mS cm−1 was established. Reduction of the fishing range over muddy substrate was about 20–30% compared with coarse gravel or sand. This study provides a sufficient tool to calculate area‐related densities of larval and juvenile fishes in different habitat types of a large river system using point abundance sampling by electro‐fishing. Finally, calculated fish densities were evaluated by different types of fishing gear.

Summary From the years 1997 to 2000, fish assemblages from 15 groyne fields and training walls (a current guiding dyke, protecting the groyne field) of the Middle Elbe River (Germany) were examined by electrofishing. The aim of the study was to detect abiotic environmental key variables which determine preadult and adult fish assemblage in typical riverbank structures using multivariate ordination techniques (canonical correspondence analysis) and univariate methods (logistic regression analysis). Habitat preferences of preadult and adult stages of selected fish species were also estimated. Fish sampling was carried out at 190 river stretches; a further 1615 samples were taken by the point‐abundance sampling method. In total, 21 732 preadult and adult individuals belonging to 30 fish species were caught; perch, eel, ide, roach, chub, gudgeon, bleak and white bream were the most frequent. Eurytopic species clearly dominated the fish assemblage with 72% total abundance, followed by rheophilic species (26%). Limnophilic fish species (rudd, crucian carp, stickleback and tench) were relatively rare (2% of total abundance), with frequencies of occurrence between 1 and 10%. The highest species diversities were estimated in structurally diverse training walls and groyne fields with defective groynes, whereas low species diversity was found to occur in poorly structured, strongly silted groyne fields with intact groynes. Additionally, typical seasonal and annual changes of the fish assemblage were observed. From the mesoscalic point of view, the structure of the preadult and adult fish assemblage was affected by both spatial (type of groyne field, river stretch) and temporal factors (year, season). Regarding the microhabitat, hydromorphological parameters (slope, current velocity, water level, predominant and secondary substrate) followed by physicochemical water values [dissolved oxygen (DO), temperature and pH‐value] are of significance for the structure of the fish community. Furthermore, hiding places (shelter) are of subordinate but significant importance for the fish assemblage in the poorly structured main channel of the Elbe River. Importance of hydromorphological factors on the microhabitat scale was confirmed by multiple logistic regression in 12 of 14 preference models, conducted for different ontogenetic stages of roach, bream, white bream, bleak, ide, chub, gudgeon, asp, perch and eel. Habitat preferences of adult bleak and preadult roach were first determined by physicochemical water values and seasonal influences. Univariate models were generated to describe specific habitat preferences of different species and age‐stages regarding the most important environmental variables (depth, velocity, predominant substrate, slope and shelter). Intraspecific differences were noticed between preadult and adult chub regarding preferences for water velocity, or between preadult and adult perch concerning slope of shore. Interspecific differences were observed, e.g. between adult roach and white bream, with regard to the ...

No abstract

Summary Groynes are the dominant river engineering structures along the lowland section of large European rivers such as the Rhine, Danube and the Elbe. More than 6000 groynes structure the 400 km stretch of the potamal of the Elbe River. After 1945, destruction of the groynes increased through ice and flood events in the eastern part of Germany. In the past ten years, groyne reconstruction was accompanied by a controversial discussion in the context of the ecological integrity of the Elbe River. With the modular habitat model (MHM) a tool was developed to evaluate the suitability and to balance the availability of fish habitats in groyne fields of different conditions. The morphodynamic module produced a digital terrain model and a spatial model of flow velocity for each groyne field separately. Based on point abundance sampling by electro‐fishing, models of habitat preference were developed for different life history stages by logistic regression. Statistical models predicting the preference of fish‐environment relationships (Leuciscus idus) at different life history stages. The models were discriminated and validated by receiver operating characteristic (ROC) curves. The link between the statistical and the spatial model was realised in the suitability module. The suitability of microhabitats is expressed in classes for each species and life history stage separately. Habitat availability is balanced on the level of mesohabitat, e.g. different types of groyne fields. The temporal dynamics of habitat availability are analysed by considering different levels of discharge. For the stage ‘juvenile A’ and preadult the habitat suitability is better in fields downstream of destroyed groynes. For ‘juvenile B’ and adult stages of the ide, groyne fields in general constitute low habitat suitability. Differences in spatial availability are higher than the differences in temporal habitat availability.

No abstract