Habitat use by 0+ fish in an old-engineered river reach (Lower Rhône, France): relative importance of habitat heterogeneity and hydrological variability

The study focused on the distribution and macrohabitat use by 0+ juvenile fishes in a southern section of the Rhone River, characterized by the presence of artificial backwaters. Using Point Abundance Sampling electrofishing, 3711 juveniles (0+) belonging to 21 species were caught during a 3 year survey of fish recruitment in 10 dike fields distributed along a hydrosedimentary gradient, mainly defined by the threshold discharges of submersion of each of the studied sites. The between-sites distribution of juveniles was firstly governed by the riparian habitat diversity and secondarily by the frequency of isolation of the sites during the recruitment period. Two main species groups were identified distinguishing between highly diversified semi-lotic sites (Leuciscus cephalus, Chondrostoma nasus, Gobio gobio, Rutilus rutilus, Blicca bjoerkna), and lentic sites characterized by introduced nest-guarder species (Lepomis gibbosus, Pseudorasbora parva. Ictalurus melas, Micropterus salmoides). Between-year changes in 0+ assemblages were only significantly correlated with the variation in the frequencies of submersion of the sites. Although the spatial use of these backwaters by juveniles was relatively stable among years, high discharge in the main channel and flood events tended to reduce the recruitment diversity in the semi-lotic nursery sites. Finally, long-term hydrological variability is the main factor that governs the riparian habitat diversity of backwaters and their spatial heterogeneity can be considered as a descriptor of the potential 0+ fish assemblage diversity of each site. But the short term hydrological variability (corresponding to the fish recruitment period), together with temperature fluctuations, acts as a filter and shapes in a different manner the 0+ fish assemblages obtained each year.