Date of this Version
Published by the U.S. Fish and Wildlife Service (2011) 55 pages.
The Niobrara River has a natural hydrograph and temperature regime with the lower 32 km protected under the National Wild and Scenic River system managed by the National Park Service. The largest threat to this river is decreased instream flows due to water withdrawals for agriculture. The Niobrara River a large tributary to the Missouri River may positively influence fish production. However, no information exists regarding phenology of fish spawning or what abiotic factors may influence spawning. Our objectives were to examine the taxonomic composition and the spatial and temporal patterns of the larval fish assemblage in relation to environmental variables in the lower Niobrara River. Larval fish sampling occurred weekly from April to August in 2008 and May to August 2009 with drift nets set in the Niobrara River at two sites: the mouth and 63 kilometers upstream. Each year, larval fish first appeared in the drift during the second week of May and were collected until the third week of August. Larval river carpsuckers Carpiodes carpio were the most abundant species in the drift during early-June, followed by red shiners Notropis lutrensis and sand shiners Notropis stramineus in late-June to mid-August, with Lepomis spp. appearing during late-June to late-July. No diel cycle in occurrence of larval fish in the drift was observed and likely resulted from the naturally high turbidity (mean nephelometric turbidity unit [NTU] > 74). Larval fish densities were 24% higher in 2009 compared to 2008. Spatially, the greatest numbers of larval fish for most fish species were collected at our upstream site located immediately downstream of Spencer Dam. Differences in environmental variables were found among sites and years as mean water temperature, velocity, and turbidity were higher and dissolved oxygen was lowest at the mouth site in 2009. The results of canonical correspondence analysis found red shiners and sand shiners were associated with high water temperatures with low stable flows found late summer. Larval flathead chubs Platygobio gracilis and common carp Cyprinus carpio abundance was positively related to dissolved oxygen and water velocity and negatively with water temperature. River carpsuckers were associated with high water velocities and moderate water temperatures while Lepomis spp. were positively associated with high conductivity and high water temperatures. Fish species that successfully spawned in the lower Niobrara River are adapted to extreme temperatures, high variability in discharge, turbidity, and sediment load. Based on the importance of abiotic factors affecting larval fish abundances, a reduction in in-stream flows would likely jeopardize native fish populations and eliminate some productivity of fish in this river.