Simplified map of the Illinois River showing the three major river segments and locations of navigation dams and metropolitan areas.
Though the Illinois River is a unique biological and recreational resource in Illinois, it is typical among large midwestern rivers in the way that its natural river-floodplain habitats have been degraded over the past century. Sources of stress have included draining and leveeing of floodplain wetlands for agriculture, pollution from treated and untreated sewage and industrial wastes, excessive siltation, overharvesting of aquatic resources, and manipulation of water levels to maintain a commercial navigation channel. By the 1950s, although a certain degree of pollution control had been in place for decades, Illinois River fish communities reflected a degraded system.
In 1957, the Illinois Natural History Survey began an annual fish population monitoring program of the entire Illinois River that continues to be updated yearly. Fish populations, monitored in this program, are sampled by electrofishing. A major goal of this program is to analyze the long-term data to separate and identify real trends from short-term variability. To simplify data summaries, three river segments are recognized based on similarities in fish communities and the amount of aquatic habitat per unit length of the river: the lower river (80 miles in length beginning at the Mississippi River); the middle (151 miles); and the upper (50 miles), including a section of the Des Plaines River near where it joins with the Kankakee River (see map) to form the Illinois.
Simplified map of the Illinois River showing the three major river segments
and locations of navigation dams and metropolitan areas.
Rather than considering all species collected, many of which were taken in small numbers, analysis of fish communities was further simplified by listing fish species in descending order of abundance for each river segment until 95 percent of the total catch was accounted for. Because the common carp (Cyprinus carpio) is more tolerant of degraded conditions than the bluegill (Lepomis macro-chirus), these species were used as biological indicators to track the long-term environmental state of the river. In general and risking oversimplification, high catches of carp and few bluegill are expected in a degraded system; the converse may be the case in a healthy system. With better habitat conditions and a more diverse fish community, carp might be expected to decline in abundance due to increased predation on their young and increased competition with other species.
Analysis of more than 30 years of data has shown that Illinois River fish communities have significantly changed since the 1960s. On average, 11 species accounted for 95 percent of the fish collected on the lower river in both the 1960s and 1990s; however, in the 1960s, carp tended to dominate catches, averaging 42 fish per hour of collection, while bluegill averaged 8 per hour. In the 1990s, carp dropped to an average of 8 per hour, and bluegill increased to 34 per hour and dominated catches from the lower river.
Flathead Catfish (Pylodictis olivaris) held by INHS researcher Thad
Cook.
(Photo by LTRMP staff at Havana.)
On the middle and upper river, 95 percent lists averaged 9 and 6 species in the 1960s, respectively, and 14 species on both segments in the 1990s. Carp declined from averages of 57 per hour on the middle river and 35 per hour on the upper river in the 1960s to 14 per hour and 6 per hour, respectively, in the 1990s. Bluegill increased from averages of 8 per hour on the middle river and less than 1 per hour on the upper river in the 1960s to 27 per hour and 12 per hour, respectively, in the 1990s. In recent years carp rarely dominated catches in any river segment, and fish communities were more evenly distributed among a greater number of species than in the 1960s. But because carp can be very large, they continue to be a major proportion of the total catch weight. The most noteworthy changes have occurred on the upper river, historically the most degraded segment due to its nearness to Chicago area pollution sources. For example, in 1962 and 1963, just four species made up the 95 percent lists: gizzard shad (Dorosoma cepedianum), emerald shiner (Notropis atherinoides), carp, and goldfish (Carassius auratus)--a species with pollution tolerance similar to carp. In contrast, 10 species made up the 95 percent list for 1995; species on this list included the bluegill, largemouth bass (Micropterus salmoides), orangespotted sunfish (Lepomis humilis), and several species of small minnows. The carp was not on the 95 percent list, and although the goldfish was not collected, the carp x goldfish hybrid was collected at a rate of less than 1 per hour.
Electrofishing rig being operated by Havana LTRMP staff.
(Photo by INHS
staff at Havana.)
Improved water quality (e.g., higher dissolved oxygen concentrations) resulting from better pollution control since the 1960s is probably the best explanation for the observed changes in fish communities, despite continuing habitat deterioration from siltation. Additional stress to the system may result from increases in commercial navigation traffic and from reproducing populations of new exotic species: grass carp (Ctenopharyngodon idella), zebra mussel (Dreissena polymorpha), and a minute crustacean called Daphnia lumholtzi. In fact, the U.S. Army Corps of Engineers is currently conducting a feasibility study for expanding the river's navigation capacity. The fish population monitoring program will be useful for tracking further changes in fish communities and for evaluating the effects of new system-wide stresses.
Thomas V. Lerczak, Center for Aquatic Ecology
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