Seasonal Investigation of Native Fishes and Their Habitats in Missouri River and Yellowstone River Backwaters

Interactions between the Missouri River and its floodplain have been severely degraded due to channelization and impoundment. Ecologists have assumed that backwaters are a critical habitat component for certain life stages of native fishes; however, documented relationships are limited. During this study, I monitored fishes, invertebrates, and habitats to help determine the importance of backwaters to native fishes of various life stages, and to assess the changes that occur in habitat characteristics and invertebrate densities during periods with differential connectivity. Seven assemblages of fishes, identified with catch-per-unit-effort data, had at least some relationship with the backwaters. The assemblages included two groups of residential fishes, two groups of transient fishes, and single groups of backwater spawners, lotic obligates, and age- 0 drifters. The most prominent of these assemblages were the primary residents and the backwater spawners. Species such as black bullhead Ameiurus melas and white crappie Pomoxis annularis were residential and abundant in the backwater communities. Age- 0 bigmouth Ictiobus cyprinellus and smallmouth buffalo l. bubalus were also abundant in autumn after being spawned in the backwaters during the flood pulse and utilizing the backwaters as nursery habitat. All life stages of other species, such as walleye Stizostedion vitreum and sauger S. canadense, were more transient and appeared sporadically. Some native larval fishes, such as blue sucker Cycleptus elongatus and burbot Lota Iota, drifted into and utilized backwater habitats, but not necessarily during peak flows. Other species, although present near the backwater connection in the Missouri River, did not appear to directly utilize the backwater habitats and were more obligated to the flowing water habitats. Stable nitrogen and carbon analyses, along with food habits data, were used to assess community structure and energy flow. Chironomidae and Corixidae were an important link between the producers and secondary consumers. In autumn, the age-0 fish community also assimilated energy through a zooplankton pathway and then served as prey, transferring nutrients to the tertiary consumers. In general, detritus appeared to be an important energy source in the spring, but became secondary to primary production energy resources during the summer and autumn month; hence, primary production is also an important carbon source in the backwater systems. Although backwater habitats can be very productive, fishes such as the flathead chub Platygobio gracilis and sicklefin chub Macrhybopsis meeki did not heavily utilize the backwaters proper, but likely benefitted from backwater prey production that flushed into the channel during connection periods. The lateral dimension in river ecology has several processes that promote the health of populations, nutrient cycles, and the entire ecosystem. These results lend support to the importance of backwater habitats to numerous fish species and to the entire Missouri River system; however, further information on several topics, such as connectivity duration and prey flushing, would help clarify the importance of backwater habitats to several native species of concern