WINTER HABITATS USED BY LARGEMOUTH BASS IN LA GRANGE
REACH OF THE ILLINOIS RIVER.

Kevin S. Irons1, Paul T. Raibley1, Timothy M. O'Hara1, K. Douglas Blodgett1 and
Richard E. Sparks2. 

1 Illinois Natural History Survey, LTRMP La Grange Field Station, 704 N.
Schrader Ave., Havana, IL 62644
2 Illinois Natural History Survey, Forbes Biological Station, River Research Lab,
P.O. Box 590, Havana, IL 62644

During the winter of 1993-94, we characterized habitats used by 17 radio-tagged
largemouth bass in La Grange Reach of the Illinois River.  Radio-tagged
largemouth bass wintered in backwaters and off-channel coves, ditches and
marinas from November through February.  Electrofishing mean CPUEs 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 powerplants and water temperatures in all nine
study areas were warmer than the main channel during winter.  Current velocities
averaged 2 cm/sec in the study areas and 19 cm/sec in the main channel.  We did
not document any radio-tagged fish moving 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, winterkilled near one study area when they
became trapped in a backwater after river levels declined.  Large numbers of age-0
fish wintering in the study areas and other off-channel habitats are susceptible to
similar winterkill events which may deplete these cohorts.  As sedimentation
continues to fill in backwaters, river levels are critical to 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 attempts to maintain stable winter river levels.    

Keywords: largemouth bass, winter, radio-tagged, Illinois River


Kevin S. Irons
Illinois Natural History Survey
La Grange Field Station
704 N. Schrader Ave.
Havana, IL 62644
Phone: (309) 543-6000
Fax: (309) 543-2105

Preference: Platform presentation, will not present poster