Larval Fish Assemblages Express the Potential Effects of River Regulation

River regulation has altered flows and temperature which may affect native fish reproduction.  We compared larval fish assemblages in a free flowing and regulated tributary of the Lower Missouri River. Drift net larval sampling was conducted  weekly from March 20 to July 3, 2013 at one transect 14.5 river kilometers (rkm) from the Missouri River confluence in the Osage River and 3.2 rkm from the Missouri River confluence in the Gasconade River. Osage River river regulation may have increased the contribution of reservoir species and limited the reproduction of species that depend on natural discharge and temperature (i.e., catostomids and percids). The three most abundant families from the Osage River (i.e., centrarchids, clupeids, and percids) exhibited maximum CPUE on the same day when the Osage River experienced its lowest discharge (314 cms) and oddly when the Gasconade River was at its greatest discharge (640 cms). Clupeids (gizzard shad Dorosoma cepedianum) in the Osage River had the greatest CPUE of any family (65,049 fish/m³) and contributed 64 times the second most contributed family (percids) from the Osage River. Our results suggest river regulation may affect the timing and abundance of larval fishes, but this relationship may differ by taxa.