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The effect of restoring connectivity for fish by the construction of 11 fish ladders in the Pielach and Melk rivers, both tributaries to the Danube in Austria, was monitored using electric fishing and fish traps between 1999 and 2004. In order to assess the efficiency of connectivity rehabilitation measures pre- and post-project data combining electric fishing and trap catch data were analyzed by means of three fish-based assessment methods: a MUlti-Level concept for a Fish-based, river-type-specific Assessment of ecological integrity (MULFA), the Fish Index Austria (FIA) and the European Fish Index (EFI). The effect of adding qualitative trap catch data to electric fishing data on metrics and indices was also tested and the magnitude of the effect was related to the distance of the sites from the river mouth. The results clearly demonstrated the significant contribution of connectivity rehabilitation measures to the ecological integrity of rivers like the River Pielach where morphological conditions are good, whereas remaining channelization still limits the success of connectivity measures in the River Melk. Trap catch data were found to represent an essential source of additional information to assess the efficiency of connectivity measures shortly after their implementation. The negative correlations of the magnitude of the effect of different indices and metrics with the distance of assessment sites from river mouths obviously underline the importance of the river Danube as a source for the re-colonization process. While the indices tested were found to have limited ability to reflect short-term response of fish assemblages to continuum rehabilitation, guild metrics were able to detect improvements of the ecological status shortly after the implementation of connectivity measures. Six metrics showed significant differences between pre and post-project data reflecting the expected increase of the ecological integrity: (1) Fish Region Index (FRI; FIA, MULFA), (2) number of subdominant species and (3) number of flow-guilds (FIA), (4) number of type specific species (MULFA), (5) number of benthic species and (6) number of potamodromous species (EFI); the FRI differences were only significant when trap catch data were added. The EFI indicated a decline of ecological integrity through increases in the density of omnivorous species and the relative number of tolerant species as well as a decrease in the relative number of intolerant species. Significantly decreasing responses with the distance from the river mouth were documented by the EFI and MULFA-index, the FRI (FIA, MULFA), total biomass and for the number of type specific species (MULFA). [ABSTRACT FROM AUTHOR]

A total of 10 649 larval and juvenile fishes of 24 species were caught in the drift at Marchfeldkanal, a man-made side branch of the Danube River near Vienna, Austria, with tubenose gobyProterorhinus marmoratusbeing the dominant species. Distinct differences in inter- and intraspecific drift patterns among different sampling stations along the course of the channel were found. The percentage of the rheophilic and rheoparous ecological guild was highest at the inlet of the channel, where it is directly fed with water from the Danube. For individual species, significant differences in drift densities among sites were found for tubenose goby, barbelBarbus barbus, ideLeuciscus idus, roachRutilus rutilusand for the two most abundant percids, the pike-perchSander luciopercaand zingelZingel zingelcombined. The occurrence of larval and juvenile fishes in the drift was related to certain developmental stages and differed between species and sites. Most species(common breamAbramis brama, bleakAlburnus alburnus, gudgeon speciesGobiospp., chubLeuciscus cephalus, ide and roach) occurred with highest densities at the earliest developmental larval stage, but some species( e.g. common bream and roach) were also found abundantly in drift at later developmental stages. Application of Ivlev‘s index of electivity as a drift index describing the propensity of the different species to drift, yielded the highest indices for the gudgeon species, common bream and bleak and the lowest for perchPerca fluviatilis, Prussian carpCarassius auratus gibelioand ruddScardinius erythrophthalmus. [ABSTRACT FROM AUTHOR]

Seasonal and diurnal patterns of larval and juvenile fish drift were investigated in the Marchfeldkanal, a man-made side branch of the Danube River near Vienna, Austria. A clear seasonal pattern with peak densities in mid-June was found. Species composition varied over time, showed a site specific pattern and was dominated by tubenose gobyProterorhinus marmoratus. Water temperature was the main factor responsible for the increase of drift densities until the median drift date and repeated occurrence of early larval stages in drift indicated repeated spawning for many species. Significant differences in drift densities between different time periods of the day(day, dusk, night and dawn) were found for common breamAbramis brama, barbelBarbus barbus, chubLeuciscus cephalus, tubenose goby and roachRutilus rutilus. The highest drift rates occurred at night(2200–0400 hours), with 86% of all larvae drifting during the hours of darkness. Fish larvae of different lengths drifting at different phases of the day were found for common bream, bleakAlburnus alburnusand chub, with largest larvae drifting during dusk(chub) and day(bleak and common bream). For bleak, all gudgeon speciesGobiospp., tubenose goby, roach and for all cyprinid species combined, one 2 h night sample was found to be sufficient to predict the total 24 h drift. [ABSTRACT FROM AUTHOR]