Invasive silver carp (Hypophthalmichthys molitrix) populations have expanded greatly in the Upper Mississippi River System (UMRS) since their introduction in the early 1970s. We conducted a Chapman-modified, continuous Schnabel mark-recapture population and biomass estimate for silver carp (106-901 mm) in the La Grange reach, Illinois River during 2007-2008. We estimated a total of 328,192 (95% CI 231,226-484,474) silver carp (2,544 per river km 1,792-3,756) comprising 705 (95% CI; 496-1,040) metric tons of biomass (5.5 metric tons per river km 3.8-8.1). Long Term Resource Monitoring Program (LTRMP) data from the La Grange reach showed an exponential increase in silver carp catches since 1998, with an intrinsic rate of increase approaching 84%. In 2008, silver carp comprised about 51% of the total LTRMP annual fish collection. To our knowledge, this large river reach may contain the greatest ambient densities of wild silver carp in the world. Our findings provide a target for reduction efforts and also emphasize the importance of the La Grange reach as a source population for potential expansion of the species to the Laurentian Great Lakes.

25 sites on the Illinois River waterway and one site in Reach 26 of the Mississippi River were electrofished to monitor fish communities. A total of 8,171 fishes representing 62 species (plus one hybrid) from 15 families were collected during 24.68 hours of sampling. Collections made in 2008 indicated continued high catches of gizzard shad, emerald shiner, and bluegill throughout most of the Illinois River waterway. Two new fish species were collected for the first time during project F-101-R. Blackside darter and longnose dace were each collected in the upper river. Single specimens of blackside darter were collected from Bull's Island Bend (RM 241.5) in Starved Rock Reach and Waupecan Island (RM 260.6) in Marseilles Reach. Two specimens of blackside darter were collected from Treat's Island (RM 279.8) in Dresden Reach. Two specimens of longnose dace were also collected at Waupecan Island. Several fish species were collected for the first time within a given river reach in 2008. A single specimen of bighead carp was collected for the first time at Brickhouse Slough on the Mississippi River. A single specimen of redear sunfish was collected at Crater-Willow Islands (RM30.0) in Alton Reach. Two new species were collected in La Grange Reach; a single pumpkinseed was collected at Pekin (RM 155.1) and two specimens of bowfin were collected at Lower Bath Chute (RM 107.1). Two specimens of blackstripe topminnow were collected for the first time in Peoria Reach at Hennepin Island (RM 207.6). Along with blackside darter and longnose dace, two additional fish species were collected in Marseilles Reach. A single specimen of mud darter and two specimens of brook silverside were also collected at Waupecan Island. A single logperch was collected for the first time in Dresden Reach at the Mouth of the DuPage River (RM 277.4). Gizzard shad were the most abundant species collected throughout the waterway in 2008 with 1,802 fish collected comprising 22.1% of the total catch. The sample from Lambie's Boat Harbor (RM 170.3, Peoria Reach) yielded the highest collection of total fish (1,375, 16.8% of the total collection), while the sample from Turkey Island (RM 148.0) produced the lowest total fish (58, 0.07% of the total collection). Fish species richness at sites ranged from 27 at Clark Island (RM 215.3, Peoria Reach) to 12 species at Moore's Towhead (RM 75.3, Alton Reach) and Turkey Island. Fish species richness of the lower, middle, and upper waterway was 24, 49, and 42, respectively. Cyprinid catches continued to remain relatively high in the upper waterway, with bluntnose minnow being the most abundant (429 total fish), making up 17.9% of the total upper waterway catch. Bluntnose minnow, emerald shiner and spotfin shiner together totaled 1,223 fish comprising 51.1% of the upper waterway catch. Important sport fish species such as bluegill, largemouth bass, and channel catfish were collected in all six waterway reaches in 2008. Bluegill catch per unit effort in number of fish collected per hour (CPUE N ) ranged from 169.50 in ...

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.