Winter Habitats Used by Largemouth Bass in the Illinois River, a Large River-Floodplain Ecosystem

Abstract

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.