For centuries, Midwestern rivers have been degraded by urban development, agricultural and navigational practices, and invasive species. These disturbances have clearly changed the structure and function of rivers by introducing pollutants, altering animal communities and habitats, and disconnecting river channels from floodplains. Although social and political actions have improved water quality during the past 50 years, lasting changes to river infrastructure still persist. Despite decades of restoration and mitigation efforts, most Midwestern rivers still suffer the persistent effects of humankind’s conquering legacy. 

Human-produced modifications continue to threaten floodplain rivers. Dams and channels built for navigation and levees constructed for agriculture and urban development can also affect the life cycle of floodplain river fishes. Levees restrict connections to the floodplain, one of the biggest challenges facing Midwestern rivers today. Because of the development of floodplain areas and the persistent sedimentation plaguing many Midwestern rivers, many fishes that depend on floodplain connectivity and backwater quality are in serious trouble.

For example, nest-building centrarchids (sunfishes like bass and bluegill) need a stable backwater habitat for spawning and overwintering. In the Illinois River watershed, the upper and lower river, separated by the Great Bend at Hennepin, have different amounts of backwater habitat available to centrarchids and other fishes. For example, The La Grange reach in the lower river has 5.5 times more backwater habitat than the Dresden reach in the upper river. Moreover, during the 11 years from 2005 through 2015, water levels in the lower river exceeded flood stage 28.3 percent of the time, over four times more than in the upper river where water levels exceeded flood stage only 6.8 percent of the time. Thus, lower-river fish should have greater access to more habitat areas than upper-river fish.

Figure 1. Cassidy Miles, IRBS technician, holding two big largemouth bass collected from the Dresden pool of the upper Illinois River.

Considering these differences in available habitat, we may think centrarchid populations in the lower river would be healthier with more habitat areas, and greater water-level fluctuations should allow greater access to backwaters. However, recent sampling by staff at the Illinois River Biological Station (IRBS) indicates that despite having a smaller amount of backwater habitat available to them, centrarchid populations in the upper river are plentiful. Alternatively, centrarchid populations in the lower river are much worse, even meager by comparison. Why are there such marked differences in these populations? We believe some differences relate to their life cycle, such as individual growth rates.

To begin addressing this question, we began a pilot study in the spring of 2014. We collected largemouth bass (Fig 1, Micropterus salmoides) from the Dresden reach of the upper Illinois River using pulsed-DC electrofishing. We extracted sagittal otoliths, a fish’s inner-ear bones, which we used to estimate ages and growth rates. We used statistical models of growth rates that are influenced by several environmental factors. Our models indicated that individual largemouth bass growth was greater in years with higher river water levels. 

With this notion, we started a larger multi-year research project at IRBS to investigate differences in life cycles for three popular centrarchid sportfish between the upper and lower Illinois River. We collected bluegill (Lepomis macrochirus), black crappie (Pomoxis nigromaculatus), and largemouth bass from Dresden (in the upper river) and LaGrange (in the lower river) reaches during spring 2015 and 2016. We extracted sagittal otoliths, which we used to calculate growth rate and maximum age, and also calculated a standardized growth index (SGI).

For all three species, the growth rate was greater in the lower river, but maximum size was greater in the upper river. This indicates that fish are growing faster in the lower river, but don’t live as long as those in the upper river. We may expect growth to be greater in the lower river, with a slightly warmer climate farther south. However, questions remain: what factors pertaining to water and climate are affecting growth rates? Are those factors different between the two locations?

	Figure 2. Best multiple-regression models describing growth of bluegill (left), black crappie (middle), and largemouth bass (right) in the upper and lower Illinois River. GDD = growing degree days, FD = flood days. The first factor listed for each species (with r2) was the best single factor; the second factor listed (with R2) was the best complementary factor not from the same suite of variables (hydrological, climatological, and biological).

Our statistical models show that connectivity (via flooding) can have both positive and negative effects on the growth of these fishes. Connectivity can provide greater access to beneficial foods in flooded terrestrial areas, a refuge from exhausting river flows, and more valuable prey who also benefit from increased connectivity or flow by increasing reproductive success. Negative effects of connectivity include decreased foraging opportunities because of cloudy water, increased competition because of population growth, and increased periods of cold water.

Ultimately, the lower river has more available backwater habitat, greater connectivity with the main part of the river, and faster growth, but still has low-quality centrarchid populations. We believe that poor backwater quality causes this effect. Sedimentation is a well-documented and pervasive problem in many Midwestern rivers, especially in the lower Illinois River. We believe that by improving our understanding of how sedimentation affects backwater quality, we can better inform future management of potential backwater restoration efforts to improve spawning and nursery and overwintering habitats for centrarchids and other fishes.