Flow and habitat effects on juvenile fish abundance in natural and alteredflow regimes

Conserving biological resources native to large river systems increasingly depends on how flow-regulated segments of these rivers are managed. Improvi ng manage ment will require a better understanding of linkages between rive r biota and temporal variability of flow and instream habitat. However, few studies have quantified responses of native fish populations to multiyear (>2 yr) patterns of hydrologic or habitat variability in flow-regulated sys tems. To provide these data, we quantified young-of-year (YOY) fish abundan ce during four years in relation to hydrologic and habitat variability in t wo segments of the Tallapoosa River in the southeastern United States. One segment had an unregulated how regime, whereas the other was flow-regulated by a peak-load generating hydropower dam. We sampled fishes annually and e xplored how continuously recorded flow data and physical habitat simulation models (PHABSIM) for spring (April-June) and summer (July-August) precedin g each sample explained fish abundances. Patterns of YOY abundance in relat ion to habitat availability (median area) and habitat persistence (longest period with habitat area continuously above the long-term median area) diff ered between unregulated and flow-regulated sites. At the unregulated site, YOY abundances were most frequently correlated with availability of shallo w-slow habitat in summer (10 species) and persistence of shallow-slow and s hallow-fast habitat in spring (nine species). Additionally, abundances were negatively correlated with I-h maximum flow in summer (five species). At t he flow regulated site, YOY abundances were more frequently correlated with persistence of shallow-water habitats (four species in spring; six species in summer) than with habitat availability or magnitude of flow extremes. T he associations of YOY with habitat persistence at the flow-regulated site corresponded to the effects of how regulation on habitat patterns. Flow reg ulation reduced median flows during spring and summer, which resulted in me dian availability of shallow-water habitats comparable to the unregulated s ite. However, habitat persistence was severely reduced by flow fluctuations resulting from pulsed water releases for peak-load power generation. Habit at persistence, comparable to levels in the unregulated site, only occurred during summer when low rainfall or other factors occasionally curtailed po wer generation. As a consequence, summer-spawning species numerically domin ated the fish assemblage at the flow-regulated site; five of six spring-spa wning species occurring at both study sites were significantly less abundan t at the how-regulated site: Persistence of native fishes in flow-regulated systems depends, in part, on the seasonal occurrence of stable habitat con ditions that facilitate reproduction and YOY survival.