| I studied the dynamics of the fish communities of the middle reach of the Brazos River and its floodplain oxbow lakes by quantifying temporal and spatial variation in abundance of juvenile and adult fishes. I collected fish and limnological samples quarterly from each oxbow and the main river channel from 1993 to 1996.;My study had two major components. First, spatiotemporal dynamics of fish assemblages in relation to environmental variables were examined. Using seine and gillnet, I collected a total of 64,911 individual fish representing 49 species and 14 families. Seine samples comprised 98% of the total numerical abundance and yielded highest species richness. Species richness was highest in summer months. Multivariate analyses indicated that water depth, Secchi depth, dissolved-oxygen concentration (DO), and temperature were important physicochemical factors determining habitat stability in the Brazos River floodplain. Of these, water depth probably was the most important determinant of habitat dynamics. Using multivariate ordination, I found that physical factors (i.e. depth, Secchi depth, and temperature) had higher associations than biotic factors with fish assemblage structure in Brazos River ecosystems.;In the second component, I examined age, growth, and condition of the two most abundant fish species, white crappie and gizzard shad, in relation to environmental variables. I collected a total of 2,846 gizzard shad and 7,574 white crappie during the study. White crappie and gizzard shad produced strong YOY classes, especially during two flood years. Brazos river oxbow lakes were classified as hyper-eutrophic systems due to high chlorophyll a concentration. Primary productivity significantly correlated with densities of zooplankton, and juvenile and adult gizzard shad. The relative weight index indicated optimal environmental conditions for both gizzard shad and white crappie. Overall, growth in oxbow lakes was rapid, particularly among age-0 and age-1 fish. Probably due to inhibitory effects of high water temperature and low DO, age-0 and age-1 gizzard shad growth was low during summer and high during spring and fall. Growth rates for both species apparently were lower during two flood years, although patterns also could have been influenced by size-dependent predation and exchanges of individuals between oxbows and the river channel. |
