Spatiotemporal environmental gradients shape early life stage fish densities, assemblage structure, and growth in river-reservoir ecosystems

Early life stage fishes are susceptible to variation in environmental gradients, which dictate growth and survival, ultimately shaping adult assemblages. Creation of river-reservoir ecosystems (RRE) by impounding rivers results in longitudinal zonation within the reservoir (i.e., riverine, transition, and lacustrine zones) and gradients owing to changes in temperature, flow, turbidity, food availability, and physical structure. We quantified relationships among density, assemblage structure, size structure and daily growth rates of early life stage (ELS) fishes and environmental conditions across zones of eight RRE in central Ohio. We observed significantly higher densities in upstream zones of RRE, correlated with increases in littoral and floodplain habitat (p < 0.001). Fish assemblage structure was similar across zones from river inflows to the dam, but more variable temporally due to seasonal differences in water temperature (p < 0.001), and spawning periodicity. Fish size increased in a downstream direction with smaller individuals in riverine and transition zones and larger individuals in lacustrine zones (p < 0.001). Average growth rates did not differ significantly among zones (p > 0.05) but varied among taxa. Our results demonstrate that high-density nursery areas with more complex habitats do not necessarily correspond with enhanced growth.