Spatial and temporal patterns and the influence of abiotic factors on larval fish catches in the lower Niobrara River, Nebraska

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] \u3e 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

Movement Patterns of Age-1 and Age-7 Pallid Sturgeon Within the Missouri River During Record 2011 Discharges Downstream of Fort Randall Dam

During the summer of 2011, the Missouri River system experienced the largest discharge levels ever recorded from Fort Randall Dam, (\u3e 4,247 m3s-1 sustained from 25 June to 31 July of 2011), which was approximately four fold above normal. Our objective was to determine movement patterns of pallid sturgeon Scaphirhynchus albus in the inter-reservoir riverine reach between Fort Randall and Gavins Point dams, South Dakota, during this record discharge period. Fifty-one hatchery reared pallid sturgeon implanted with ultrasonic transmitters from two size classes, large (age-6 at tagging; n = 24) and small (age-1; n = 27), were at large during the flood. Small age-1 fish ranged from 352 – 389 mm fork length (FL; mean = 369 mm) and 165 – 235 g (mean = 193 g), while fish in the large size class ranged from 564 – 723 mm (mean = 652) and 700 – 1,600 g (mean = 1,154 g). After discharge increased in late May, 67% of large pallid sturgeon were relocated at least once with 10 fish relocated multiple times, whereas only 11% of small fish were relocated. Mean net movement between six relocation surveys for both pallid sturgeon size groups pooled was positively correlated with discharge (r2 = 0.77, p = 0.07). Mean net movement of the large size class (r2 = 0.82, p = 0.04) was positively correlated to changes in net discharge, with two fish moving in the main channel to within 6.5 river km of Fort Randall Dam when flows exceeded 4,000 m3s-1. Small pallid sturgeon also demonstrated upstream movement with increasing discharge but to a lesser extent. No fish were found on the inundated floodplain or during one survey in late May of Lewis and Clark Lake. Overall, pallid sturgeon remained in the main river channel and generally moved upstream during record discharges out of Fort Randall Dam; however other factors such a temperature and food availability may have also contributed to their movements