Largemouth bass response to habitat and water quality rehabilitation in a backwater of the upper Mississippi River

Abstract

Severe sedimentation since lock and dam construction in the 1930s has reduced water depth in Upper and Lower Brownˈs lakes, a backwater complex in Pool 13 of the upper Mississippi River, and resulted in periods of chronic anoxia. This backwater complex was rehabilitated by construction of a deflection levee, installation of a water control structure, and excavation of canals through the area. Water quality variables inside and outside the project area, movement of radio-tagged largemouth bass in response to changing oxygen concentrations, and creel statistics were used to evaluate the success of the improvements. Turbidity was significantly less in the Brownˈs Lake complex than in the main channel. Oxygen concentrations were allowed to deteriorate to 3 ppm before the water control structures were opened during the winter; within 7 d, oxygen concentrations as high as 10 ppm were found in the top strata in most of the Brownˈs Lake complex. Chemical and thermal stratification observed in the dredge canal water column were caused by colder (32°F), highly oxygenated water from the main channel moving over denser, warmer (36–38°F) water in the dredge canals. Water in the dredge canals remained stratified until ice-out, with colder, oxygenated water in the surface stratum; warmer, but anoxic, water in the bottom stratum; and a mixture (3–7 ppm oxygen and 35–36°F) in the middle stratum. Fourteen radio-tagged largemouth bass Micropterus salmoides were located in the Brownˈs Lake complex in December before oxygen concentrations began to decline, Concurrent with oxygen declines, most radio-tagged fish exited the complex through a slough connected to the main channel and returned when the water control structure was opened and oxygen concentrations increased. Some radio-tagged largemouth bass moved 4 mi under ice to return to the complex. Estimated angler effort and catch increased 58 and 117%, respectively, in the Lower Brownˈs Lake–Lainesville Slough complex following rehabilitation. A 10-fold increase in angler effort and catch was estimated for Upper Brownˈs Lake after the project was completed.