Effects of river bed restructuring on fish and benthos of a fifth order stream, Melk, Austria

Studies conducted on 15 sections of seven different epipotamal streams between 1981 and 1984 established the impact of river bed structures on fish communities. Reduced spatial heterogeneity due to river straightening resulted in decreasing numbers of fish species, stock density and biomass. The variance of maximum depths (VMD) used as a measure of habitat structure showed a highly significant correlation with the number (NFS) and diversity of fish species (DFS). In 1987 these models were used for the evaluation of a planned restructuring project on a channelized section of the Melk River, which was realized in 1988. During a three year investigation of the response of the fish fauna and benthic community to changed hydraulic and morphometric conditions within the restructured river bed section the validity of the VMD/NFS model could be verified. The number offish species increased from 10 to 19. Fish density and biomass as well as annual production of 0+ fish increased three-fold. In contrast the DFS did not reach expectations, suggesting that more time is needed to establish a balanced fish community. The newly created riparian zones (e.g. shallow water lagoons, pioneer vegetation of gravel banks, etc.) provided important refuge regions during flooding, places for hatching or emerging, and nursery grounds for younger benthic instars and fish fry, indicating the positive effects of a complex water-land ecotone. Owing to the increasing habitat variability, including riffle-pool sequences, more and diverse specific benthic communities with many epipotamalic species developed. The number of benthic invertebrate taxa increased from 202 in the channelized section to 273 in the restructured area (in total 347 taxa were recorded). The benthic drift decreased significantly in the restructured river section, suggesting unfavourable conditions for many benthic invertebrates in the straightened section. Terrestrial invertebrates, however, occasionally entering the water body, showed a ten-fold increase in drift in the channelized reaches.