Transparency 



The Secchi disk visibility measurements indicated that generally, 

 the Upper Illinois was more transparent than the lower portions of the 

 river. The anomalously low measurements at Ballard Island Chute on the 

 Upper Illinois can be attributed to the effects of wave action on the 

 shallow bottom and to phytoplankton. During the period 1963 to 1966, 

 similar observations were recorded by a Jackson turbidimeter: turbidity 

 readings were higher in the lower three pools than in the upper three 

 pools (Starrett, 1971). The softer mud bottoms and influx of silt from 

 agricultural activities, particularly below Hennepin (mile 207.5), con- 

 tribute to the higher turbidity of the Lower Illinois. 



Electrofishing Results, (1959-1974) 



A total of 67 species of fishes were taken from the Illinois River 

 by electrofishing from 1959 to 1974 (Table 3) . The list includes 16 

 commercial species, 16 sport species, 8 species considered as both 

 commercial and sport fish, and 27 other species. This categorization 

 of the species is based upon lists contained in 1974-1975 Illinois 

 Fishing Information , published by the Illinois Department of Conserva- 

 tion (1974), and upon the author's personal knowledge of the species 

 sought by sport fishermen along the Illinois River. For example, the 

 Department of Conservation publication does not specifically mention 

 bullheads as sport fish, yet there are sport fishermen who seek bull- 

 heads. (As with many common names used by fishermen, the name "bull- 

 head," actually includes several species: the black, yellow, and brown 

 bullheads. ) 



During approximately the same period covered by the electrofishing 

 survey, a total of 101 fish species have been taken by Illinois Natural 

 History Survey crews employing various types of sampling gear, or by 

 commercial fishermen. Not all 101 species were taken by electrofishing, 

 due to sampling biases, which were discussed in Part II. 



Scientific collections of fishes of the Illinois River were made 

 well before the electrofishing survey commenced. This background data 



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