435 

 Summary op Table II 



Observations made at Peoria Narrows in the summer of 1923 showed 

 that fishes died over night in water having less than two parts of oxygen 

 per million. Dogfish seemed to be most sensitive to low oxygen concen- 

 trations, for individuals of that species were often fotmd dead when all 

 other species remained alive in the same hoop-net. Carp seemed to be 

 the most resistant, being often caught farther out in the channel than 

 other species, in nets where the water was deficient in oxygen. Gars 

 were more abundant than any other species in water with little or no 

 oxygen, probably because of their habit of breathing at the surface. 



The Effect of Dissolved Oxygen Concentration 

 ON THE Fish Yield of the Illinois River 



Table III gives the fish yields for the different sections of the Illi- 

 nois River in representative years covering almost two decades. The 

 peak of fish production was reached in 1908, when the Illinois River 

 produced over 19 million pounds, according to the Illinois Fish Com- 

 mission report, and almost 2i million pounds, according to the U. S. 



