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 sampling method in September 1998 in the Lower Saone River, during a very low discharge period when current velocity was not the structuring factor. Patterns exhibited by the four most abundant species were investigated at different spatial scales of habitat heterogeneity using logistic regression analysis. At the higher scale (distinction between side-arms and channel), significant responses were always characterized by higher probabilities of presence in side-arms. Three of the four species showed patterns at the mesohabitat scale (the different macrophytes beds). At the microhabitat scale (the sampling point), the role of aquatic macrophytes was investigated by distinguishing the main species. In order to investigate the relationship between juvenile fish abundance and food habitat characteristics, phyto- and zooplankton were sampled in each mesohabitat, in both channel and side-arms. The type of vegetation appeared to be crucial in the determination of plankton abundance, but plankton abundance was not sufficient to explain patterns of habitat use in the fishes. Morphological characteristics of the different macrophytes studied were contrasted. Then we hypothesized that such differences resulted in different habitat quality (i.e. protection against predators) and may have affected the habitat use of juvenile fishes.