Importance of flood dynamics versus intrinsic physical habitat in structuring fish communities: Evidence from two adjacent engineered floodplains on the Sacramento River, California

We examined the factors structuring fish communities at two adjacent engineered floodplain systems on the Sacramento River, California: Yolo and Sutter bypasses. We intensively sampled fishes at each location during January-June 2002 and 2004 by rotary screw trap, collecting a total of 126,635 fish comprised of 29 species. Nonmetric multidimensional scaling indicated that distinct fish communities persisted between the locations during our study, despite nearly identical hydrographs and water temperature regimes. Regression models evaluated with an information-theoretic approach also indicated that location was an important factor explaining the abundances of selected species. Overall, Yolo Bypass had more species and a greater proportion of native species than did Sutter Bypass. Sutter Bypass had a greater proportion of species classified as freshwater, while Yolo Bypass had a greater proportion of species classified as either estuarine or anadromous. We believe these results are related to substantial differences in the underlying physical habitat within the floodplains, which are primarily associated with connectivity to the adjacent river. Although the dynamic flooding that occurs at both locations appeared unable to override the underlying physical habitat differences in structuring the overall fish communities, it was important in controlling the abundances of two prominent native species, Chinook salmon Oncorhynchus tshawytscha and splittail Pogonichthys macrolepidotus, which represented 79% of all individuals collected; splittails spawn on the inundated floodplains, and age-0 individuals of both species use these areas as rearing habitat. Our results have important restoration implications in that they illustrate the importance of both flood pulse dynamics and underlying physical habitat associated with connectivity in structuring river-floodplain fish communities.