Branched Oak Lake and Pawnee Reservoir are flood-control reservoirs located in the Salt Creek watershed of southeastern Nebraska, and are popular sites for water-based recreation. Angler use of these reservoirs has declined in recent years and the Nebraska Game and Parks Commission is interested in restoring angler use to historic levels. The observed declines in angler use in both reservoirs have been associated with shifts in the fish communities and declines in the quality of the sport fisheries.
White perch comprised a large portion of the fish biomass in both reservoirs following their unintentional introduction. An important step towards the improvement of the sport fisheries in these reservoirs was reduction of white perch population sizes and their maintenance at low abundances. Previous attempts to reduce white perch population sizes by stocking predatory fish were unsuccessful, likely because gizzard shad present in both reservoirs provided predatory fish with preferred, alternative prey. Physical reductions (removal of large portions of the populations) were needed in the white perch and gizzard shad populations for predators to have any chance of controlling and maintaining white perch abundances at low levels.
GoalsManagers needed baseline information on the abundances and spatial distributions of white perch and gizzard shad to assist in implementing effective actions for removing large proportions of these populations. Also, the effects of management actions need to be described to improve future management actions. The goals of this project were to provide this baseline information and to describe the effects of a control effort. The three specific objectives were:
1. Quantify white perch and gizzard shad abundances;
2. Describe the seasonal spatial distributions of white perch and gizzard shad;
3. Describe the effects of the 2013 low-dose rotenone application on white perch and gizzard shad populations in Pawnee Reservoir.
In Branched Oak Lake, we estimated that there were 1.14-1.26 million white perch along with 0.77-1.04 million gizzard shad. In Pawnee Reservoir, we estimated that there were 1.49-1.69 million white perch along with 0.59-0.68 million gizzard shad. White perch exhibited seasonal movements, particularly in Branched Oak Lake. During spring, white perch abundance was greatest in the lower portions of the reservoir. During summer, white perch dispersed into the upper reaches of the reservoir. In the fall, white perch shifted back to the lower reaches of the reservoir. We did not detect a consistent pattern in gizzard shad spatial distributions.
In an attempt to reduce white perch and gizzard shad abundances in Pawnee Reservoir, managers applied a low-dose of rotenone (9 µg/L target concentration) during November 2013. This application nearly extirpated gizzard shad from Pawnee Reservoir and reduced white perch abundance by 83%. This reduction in white perch abundance was accompanied by a 47% increase in white perch mean length from September 2013 to September 2014.
Principal Investigator(s)-Kevin L. Pope, NE CFWRU
-Chris Chizinski, NE CFWRU
Graduate Student(s)-Lucas Kowalewski, M.S. (2014)
- Nathan Stewart. M.S.(2015)