Habitat occupancy patterns of juvenile fishes in a large lowland river: interactions with macrophytes

Juvenile fish were sampled by electric fishing using the point abundance sa mpling method in September 1998 in the Lower Saone River, during a very low discharge period when current velocity was not the structuring factor. Pat terns exhibited by the four most abundant species were investigated at diff erent spatial scales of habitat heterogeneity using logistic regression ana lysis. At the higher scale (distinction between side-arms and channel), sig nificant responses were always characterized by higher probabilities of pre sence in side-arms. Three of the four species showed pat terns at the mesoh abitat scale (the different macrophytes beds). At the microhabitat scale (t he sampling point), the role of aquatic macrophytes was investigated by dis tinguishing the main species. In order to investigate the relationship betw een juvenile fish abundance and food habitat. characteristics, phyto- and z ooplankton were sampled in each mesohabitat, in both channel and side-arms. The type of vegetation appeared to be crucial in the determination of plan kton abundance, but plankton abundance was not sufficient to explain patter ns of habitat use in the fishes. Morphological characteristics of the diffe rent macrophytes studied were contrasted. Then we hypothesized that such di fferences resulted in different habitat quality (i.e. protection against pr edators) and may have affected the habitat use of juvenile fishes.