Spatio-temporal distribution of young fish in tributaries of natural and flow-regulated sections of a neotropical river in French Guiana

1. We investigated which environmental parameters control the variation in density, in space and time, of young stages of fish in tributaries of a nat ural and a flow-regulated section of the Sinnamary River, French Guiana. 2. The density of the progeny in most taxa varied in space and/or time. How ever, most non-Perciformes responded differently to space and/or time in th e two sections. 3. Oxygen, turbidity and habitat structure (i.e. bank length, occurrence of undercut bank, richness in Litter, vegetation and substratum) were importa nt, as was the position of the sampling site relatively to the main channel in the downstream tributaries, in explaining the variation of density in s pace in both sections. Both habitat complexity and distance from the main c hannel protect young fish against unpredictable flow releases downstream fr om the Petit Saut dam. 4. Hydrological events played an important role in the temporal variation i n densities of many fish taxa. The density of most early life and many juve nile stages (mostly Characiformes) was positively related to hydrological e vents. 5. Some fish taxa had reproductive habits which were relatively independent of abiotic factors, such as flow variability, and the density of their pro geny did not vary with time. 6. The nursery areas of more than 45% of species in the Sinnamary River hav e been degraded by flow regulation.