ABUNDANCE AND ASSEMBLAGE STRUCTURE OF FISH LARVAE IN THE LOWER MISSOURI RIVER AND ITS TRIBUTARIES

Most of the backwater habitat in the floodplain of the lower Missouri River has been eliminated, but the lower reaches of tributaries have f low and depth characteristics similar to backwaters. We sampled the as semblages of fish larvae and age-0 juveniles in the lower 8 km of four tributaries and in the Missouri River main channel to determine the e xtent to which riverine species use these backwaters for early life hi story stages. We also sought to determine which tributary characterist ics were associated with use by riverine species. Densities of 8 of th e 10 fish taxa analyzed were greater in tributaries than in the Missou ri River in 1987 and 1988. The composition of the larval fish fauna va ried: carpsuckers Carpiodes spp., goldeye Hiodon alosoides, freshwater drum Aplodinotus grunniens, and common carp Cyprinus carpio occurred in large rivers; whereas mostly gizzard shad Dorsosoma cepedianum, nea rctic cyprinids (mostly emerald shiner Notropis atherinoides and red s hiner Cyprinella lutrensis), and sunfishes Lepomis spp. occurred in sm all tributaries. Assemblages in large tributaries included large-river and small-tributary taxa. The gradient in assemblage composition, fro m small-tributary fauna to large-river fauna, corresponded to a gradie nt in channel morphometry, from shallow waters and coarse substrate to greater channel depths and fine sediments. The complex pattern of flo w between the Missouri River and its tributaries and the diversity of tributary morphometry were associated with, and may be critical to, pr oductivity and the maintenance of a diverse fish fauna.