During the winter of 1993–1994, we characterized habitats used by 17 radio‐tagged largemouth bass Micropterus salmoides in La Grange Reach of the Illinois River, a large river–floodplain ecosystem that has been significantly altered from its natural state. Radio‐tagged largemouth bass wintered in backwaters, off‐channel coves, ditches, and marinas from November through February. Electrofishing mean catch rates were higher in the study areas during winter than in other seasons, indicating fish were more concentrated in these areas during winter. Five of nine study areas received thermal inputs from springs or power plants, but water temperatures in all nine areas were warmer than the main channel during winter. Current velocities averaged 2 cm/s in the study areas and 19 cm/s in the main channel. We did not document movement of radio‐tagged fish into the main channel during winter. River levels played a major role in determining the suitability of wintering habitats. When river levels declined, temperatures increased and dissolved oxygen levels decreased in smaller areas with thermal inputs. Thousands of fish, including juvenile and adult largemouth bass, suffered winterkill near one study area when they became trapped in a backwater after river levels declined. As sedimentation continues to fill in backwaters, river levels become ever more critical for maintaining adequate depths and stable water quality for wintering fish. Perhaps the best strategy for providing adequate wintering habitat for Illinois River fish would be a combination of habitat rehabilitation and the maintenance of stable winter river levels by using navigation dams.
Little is known about the population dynamics of largemouth bass Micropterus salmoides in temperate large river–floodplain ecosystems. However, the hydrological regimes in these systems are often similar to those of large reservoirs where fluctuating water levels during spawning have been shown to affect largemouth bass population dynamics. Most backwater lakes of the Illinois River have soft, silty substrates. These substrates are not conducive to nest building, so spawning centrarchids may use annual spring floods to access inundated terrestrial vegetation and previously dry, compacted substrates on the floodplain. We used electrofishing catch data from La Grange Reach of the Illinois River (1990–1995) to assess how spring and early‐summer river levels influence the cohort strength of largemouth bass. Strong cohorts were produced during years with high spring floods (1990, 1993, and 1995) when largemouth bass could access the floodplain for spawning and nursery habitat. In both 1990 and 1993, fish had access to the floodplain for extended periods during spring and summer, which resulted in exceptionally strong cohorts. These cohorts probably benefitted from abundant forage and cover on the floodplain, which bolstered survival rates. Weak cohorts were produced in 1992 and 1994 when river levels were low and fluctuating during spawning and in 1991 when river levels declined dramatically during spawning. Length distributions of other centrarchids mirrored those of largemouth bass, indicating that river levels probably affected other nest‐building species in La Grange Reach.
The UMRS encompasses the commercially navigable reaches of the Upper Mississippi River, as well as the Illinois River and navigable portions of the Kaskaskia, Black, St. Croix, and Minnesota Rivers. Congress has declared the UMRS to be both a nationally significant ecosystem and a nationally significant commercial navigation system. The mission of the L TRMP is to provide decision makers with information for maintaining the UMRS as a sustainable large river ecosystem given its multiple-use character. The long-term goals of the Program are to understand the system, determine resource trends and effects, develop management alternatives, manage information, and develop useful products. Data (factual record) and information (usable interpretation of data) are the primary products of the LTRMP. Data on water quality, vegetation, aquatic macroinvertebrates, and fish are collected using a network of six field stations on the Upper Mississippi and Illinois Rivers. Analysis, interpretation, and the reporting of information are conducted at the six field stations and at the Environmental Management Technical Center, the operational center of the LTRMP. Informational products of the LTRMP include professional presentations, reports, and publications in the open and peer-reviewed scientific literature. This document is an annual status report for 1993, containing a synthesis of data from fish populations and communities in the Upper Mississippi River System. This report satisfies, for 1993, Task 2.2.8.4, Evaluate and Summarize Annual Results under Goal 2, Monitor and Evaluate the Condition of the Upper Mississippi River Ecosystem as specified in the Operating Plan for the Long Term Resource Monitoring Program (USFWS 1993). This report was developed with funding provided by the Long Term Resource Monitoring Program. The purposes of this annual synthesis report are to provide (1) a system wide summary of data in standardized tables and figures, and (2) initial identification and interpretation of observed spatial and temporal patterns. The primary data summarized in this report are available from the Environmental Management Technical Center.
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