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.
