Fish

Fish studies conducted by Stephen Forbes in 1920 document that four-fifths of all fish species recorded for the entire state are known to occur in the Illinois River Basin (USFWS 1963). A decline of fish populations begins in the early 1900s and never recovers. In the summer of 1912, the river near Morris is “practically destitute of fishes, and the few taken were in close proximity to the Mazon slough.” Some of the bullheads captured are “fungused or in otherwise unwholesome condition” (Mills1966). Conditions begin to change only after treatment of sewage begins in the 1930s. Even with these improvements, however, studies show a loss of 18 fish species from the Illinois River since 1908 (Mills 1966).

Different species react differently to polluted conditions. For example, fishes of the Centrarchidae family, such as bluegill (Lepomis macrochirus) and largemouth bass (Micropterus salmoides), have similar habitat requirements and are generally intolerant of polluted conditions. Green sunfish (Lepomis cyanellus) (Figure 37), however, are usually indicative of a stressed environment. The common carp and goldfish are also tolerant of polluted waters (Lerczak 1995).

Figure 37: Green Sunfish (Michigan Interactive)

It is believed that toxic substances in the sediments can contribute to the incidence of abnormalities in benthic fish - those that forage at the bottom of the river. Pelagic species - those that inhabit the water column - are less affected by these contaminants (Lerczak 1995).