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 natural 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. However, most non-Perciformes responded differently to space and/or time in the two sections. 3. Oxygen, turbidity and habitat structure (i.e. bank length, occurrence of undercut bank, richness in litter, vegetation and substratum) were important, as was the position of the sampling site relatively to the main channel in the downstream tributaries, in explaining the variation of density in space in both sections. Both habitat complexity and distance from the main channel protect young fish against unpredictable flow releases downstream from the Petit Saut dam. 4. Hydrological events played an important role in the temporal variation in densities of many fish taxa. The density of most early life and many juvenile stages (mostly Characiformes) was positively related to hydrological events. 5. Some fish taxa had reproductive habits which were relatively independent of abiotic factors, such as flow variability, and the density of their progeny did not vary with time. 6. The nursery areas of more than 45% of species in the Sinnamary River have been degraded by flow regulation.