Flow and habitat effects on juvenile fish abundance in natural and altered flow regimes

Conserving biological resources native to river systems increasingly depends on howflow-regulated segments of these rivers managed. Improving management will require a betterunderstanding of linkages between river biota and temporal variability of flow and instreamhabitat. However, few studies have quantified responses of native fish populations to multiyear(\u3e2year) patterns of hydrologic or habitat variability in flow-regulated systems. To provide thesedata, we quantified young of year (YOY) fish abundance during four years in relation to hydrologicand habitat variability in two segments of Tallapoosa River in the southeastern United States.One segment had an unregulated flow regime, whereas the other was flow-regulated by peakloadgenerating hydropower dam. We sampled fishes annually and explored how continuouslyrecorded flow data and physical habitat simulation models (PHABSIM) for spring (April-June) andsummer (July-August) preceding each sample explained fish abundances. Patterns of YOY abundance in relation to habitat availability (median area) and habitat persistence (longest periodwith habitat area continuously above the long-term median area) differed between unregulatedand flow-regulated sites. At the unregulated site, YOY abundances were most frequentlycorrelated with availability of shallow-slow habitat in summer (10 species) and persistence ofshallow-slow and shallow-fast habitat in spring (9 species). Additionally abundances werenegatively correlated with 1 h maximum flow in summer (five species). At the flow regulated site,YOY abundances were more frequently correlated with persistence of shallow-water habitat (fourspecies in spring; six species in summer) than with habitat availability of shallow-water habitatscomparable to the unregulated site. However habitat persistence was severely reduced by flowfluctuations resulting from pulsed water releases for peak-load power generation. Habitatpersistence, comparable to levels in the unregulated site, only occurred during summer when lowrainfall or other factors occasionally curtailed power generation. As a consequence, summerspawning species numerically dominated the fish assemblage at the flow-regulated site; five ofsix spring-spawning species occurring at both study sites were significantly less abundance at theflow regulated site. Persistence of native fishes in flow regulated systems depends in part, onseasonal occurrence of stable habitat conditions that facilitate reproduction and YOY survival. http://dx.doi.org/10.1890/1051-0761(2001)011[0179:FAHEOJ]2.0.CO;