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Sexually mature white perch (Morone americana), brought into Nebraska in the spring of 1964, spawned successfully in hatchery ponds. Some of the progeny were stocked in several of the highly alkaline lakes in the Nebraska Sandhills while others were inadvertently introduced into a turbid flood control reservoir in southeastern Nebraska. The species became established in the reservoir where it made very rapid growth and achieved sexual maturity one to two years sooner than in its natural range. Over a three year period the white perch completely displaced the native black bullhead as the dominant species in the reservoir. This dominance is being maintained and the white perch population is becoming stunted. During periods of reservoir discharge in the spring large numbers of white perch escape through the spillway. The stream on which the reservoir is located is tributary to the Platte River which subsequently joins the Missouri River. There appears to be no barrier to the movement of white perch through this stream system to the Missouri as they have been captured throughout the system up to a point only 18 stream miles from the juncture of the Platte and Missouri rivers. It appears likely that many white perch have successfully made the journey. The future prospects of the species in the Missouri River should be interesting.

Changes in zooplankton diversity and density in response to reservoir aging in Pawnee Reservoir were investigated. Zooplankton samples collected from April 1992 through April 1993, were compared to a similar study conducted after initial impoundment by Helzer (1971Helzer ( ), in 1968Helzer ( -1970. Since this initial study, increases in turbidity and resulting changes in biotic interactions significantly altered the zooplankton community. A significant increase in total zooplankton density and a decrease in species richness were observed between study periods. Density increased from 24.6 to 95.4 individuals L-t, while the number of taxa declined from fourteen to ten. During this time period, Cyclops vemalis became the dominant zooplankter in the reservoir. The density of this predatory copepod increased significantly, from SO.1 1-l in 1968-1970, to 44.3 1-r in 1992-1993, which accounted for most of the increase in total zooplankton density. Though a greater spring maximum of another dominant, Bosmina spp. was found during the 1992-1993 study period, the annual density of this cladoceran was not significantly different between study periods. Similar trends for Daphnia ambigua and D. pan&a were also observed, as greater spring maxima levels were attained, however overall annual densities were not significantly different. The dominance of C. vernalis (46% of annual density) and Bosmina spp. (33%), indicate that these two zooplankters were tolerant of changes in physical conditions resulting from reservoir aging and biotic interactions that followed in the reservoir during the 22 years between study periods.

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