Key environmental variables affecting the ichthyofaunal composition of groyne fields in the middle Elbe River, Germany

During its development into an important waterway the middle Elbe River underwent many anthropogenic influences, all of which strongly impacted the fish fauna assemblage. Both the river morphology and hydrology were affected by groyne field construction, with consequential changes of fish habitats along river banks. Variables in the ichthyofaunal composition of middle Elbe River groyne field areas and types were analyzed for their environmental influence based on data using a random point-abundance-sampling-strategy with an electrofishing device. Altogether 22 fish species and hybrids of cyprinids from eight families, mostly cyprinids and percids, were found. Habitat guilds were dominated by eurytopic fish species (40%), followed by the oligorheophilic guild (24%). The reproductive guilds were represented mainly by phyto-lithophils (36%) and phytophils (28%). The feeding guilds were dominated by benthivorous species (32%), followed by omnivorous and piscivorous species (each 24%). Most frequent were roach (Rutilus rutilus, 32.6%), perch (Perca fluviatilis, 29.7%), and ide (Leuciscus idus, 25.6%). The canonical correspondence analysis indicated that substrate, slope, and vegetation cover were the most important factors affecting the entire fish assemblage, including all age groups.The types of groyne fields were found to be mostly of minor importance with regard to differences in the ichthyofaunal structure. Fish assemblage at groyne heads and surrounding areas were compared to those in the siltation and static flow areas, whereby rheophilic fish species occurred most often at the groyne head and surrounding areas. The greatest temporal changes within the young-of-the-year (YOY) fish fauna were during the summer months June to August, where habitat shifts among the groyne field areas were a result of changes in YOY habitat requirements throughout the year.Hence, this study indicates that groyne fields are used as alternative habitats by a high number of different fish species and life stages and could therefore contribute to sustain a species-reach fish fauna in regulated lowland rivers.