during low-flow periods. When impounded, the river is said to be in 

 pool. All stations could not be fished every year because of high- 

 water levels; no stations were sampled in 1971 and 1972. 



Fish Population Sampling 



Fish populations were sampled by means of electrof ishing. Fish 

 were stunned by an electric current produced by a 230-volt, 180 cycles/ 

 sec, AC generator (Homelite 9HY-1) , and transmitted through the water 

 via 3 cables suspended from booms in the front of an 18-ft aluminum boat. 

 Electrof ishing was conducted in 15-minute segments, and a total of 

 60 minutes was spent electrof ishing at most stations. In small chutes, 

 or where an abundance of fish was collected quickly, only 30 minutes was 

 spent electrofishing. 



The stunned fish were dipped from the water and placed in plastic 

 garbage cans containing water. Fish were identified, counted, weighed, 

 checked for disease, and returned to the river. The few fish that died 

 were buried on shore. 



There are problems associated with any sampling technique, including 

 electrofishing. In a turbid river such as the Illinois, fish must be 

 within a few inches of the surface to be seen and netted. Bottom- 

 dwelling species such as catfish and bullhead do not always surface 

 when shocked, and they are underrepresented in electrofishing catches. 

 Gars, such as the short-nose gar, arid bigmouth buffalo are not as vul- 

 nerable to electric shock as other species, such as sunfish. Gizzard 

 shad are often only momentarily stunned and occur in such large numbers 

 that it is impossible to net them all before they recover. Since the 

 electrofishing program was designed to sample populations of adult fish, 

 dip nets of %-in. mesh size were used; hence, small fish such as minnows 

 often were not retained in the nets. In addition, electrofishing was 

 conducted only in areas that were connected to the river at all times of 

 the year, so that species occurring primarily in lateral lakes which 

 were either permanently or intermittently cut off from the river were 

 not represented or were underrepresented in electrofishing collections. 



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