Factors affecting habitat use by fish species in the Platte River, Nebraska

The effects of flow regulation on fish populations have received much attention because changes in flow directly modify physical conditions and provide unstable habitats for fishes. I examined factors which affect habitat use by the fish community in the Platte River, Nebraska from 1987 to 1995 by using field observations, field experiments, and laboratory experiments. Factors responsible for changes in habitat use include physicochemical factors, spatial and temporal variation, habitat availability, and biotic interactions such as competition and predation. Fish were collected using 2 x 4 m electrofishing grids from ten sites in the Platte River from Elm Creek, Nebraska downstream to the Missouri River. A total of 9,245 fish, representing 38 species, from 2,185 grids was collected during 1992-1993. Cyprinids dominated the collections, represented by 14 species and 8,700 specimens. Red shiner Cyprinella lutrensis, river shiner Notropis blennius, and sand shiner Notropis stramineus were common at all stations. Fish were collected monthly to determine diel and seasonal patterns of composition and abundance. More species were taken during day times than at night in spring. In contrast, more species were taken at night than in day times in summer. In fall, equal numbers of species were caught during day and night. Comparisons of fish densities showed significant differences among spring, summer, and fall. Major taxa were significantly more abundant in spring, but river shiner and western silvery minnow (Hybognathus argyritis) were more abundant in fall. We developed a simple laboratory stream to evaluate the effects of competition and predation on habitat use by red shiner, sand shiner, and plains killifish. The presence of predators altered the spatial distribution of its prey, but the effects were not consistent among the three studies. Habitat use shifts among co-occurring species in a laboratory may lead to competitive interactions. Analyses of species abundance showed that strong positive associations usually include pairs of species with similar characteristics and habitat requirements among co-occurring species. Weak negative associations indicate the limited importance of predator-prey relationships. This suggests physical habitat characteristics are more important than biotic factors structuring fish communities in the Platte River