downstream direction are: the increased abundance of food organisms 

 such as mayflies, snails, and fingernail clams in the Lower Illinois as 

 compared to the Middle and Upper Illinois; the relatively higher oxygen 

 levels in the Alton Pool as compared to the upstream pools; and the 

 proximity to the relatively unpolluted Upper Mississippi River. 



Black bullhead were abundant at one station, Ballard Island Chute, 

 Marseilles Pool (mile 247.8-248.2), which apparently provides preferred 

 habitat for this species. 



Northern pike, yellow perch, and walleye were once abundant in the 

 river, but are now rare or limited in their distribution. Yellow perch 

 populations have declined probably as the result of the dissappearance 

 of beds of aquatic plants and disappearance of clean sand or pebble sub- 

 strates perch use for spawning. 



Gizzard shad and carp were generally abundant throughout the river, 



Goldfish showed a trend of increasing abundance in the upstream 

 direction, toward Chicago. Goldfish can tolerate the low dissolved 

 oxygen levels which are found in the Upper Illinois River. Moreover, 

 there is an absence of predators in the upper river, such as largemouth 

 bass, which are known to feed on goldfish. The fish populations of the 

 polluted Des Plaines River show the classic response of a community of 

 organisms to pollutional stress. The number of species is reduced as 

 pollution eliminates the intolerant organisms. Populations of the re- 

 maining tolerant species often expand, because of reduced competition 

 and predation. 



The incidence of disease and deformities among all the fish, ob- 

 served to increase in the upstream direction, is probably related to 

 chemicals and pathogenic organisms in effluents from the Chicago area. 

 "Knothead" has been discussed earlier, under carp. Species other than 

 carp, such as bigmouth buffalo sometimes exhibit knothead, and the in- 

 cidence appears to increase upstream as for carp. Virtually all the 

 goldfish taken from the Des Plaines in 1973 had "popeye" (swollen, pro- 

 truding eyes) or missing eyes. The incidence of sores on the fins and 

 body, tumors, frayed fins, and abnormal spinal curvature was higher 



40 



