Article VII. — Some Obscnrntions on the Oxygen Requiretnents of 

 Fishes in the Illinois River. By David H. Thompson. 



Introduction 



The amount of oxygen dissolved in the water is one of the most im- 

 portant factors hmiting the fish life of the Illinois River. Every year 

 low dissolved oxygen concentrations over a large portion of the river 

 profoundly affect the food, distribution, and even the existence of the 

 fishes. In the conservation of the Illinois River fishery it becomes neces- 

 sary to evaluate the oxj'gen requirements of the more common fishes. 

 The Illinois Natural History Survey and the State Water Survey have 

 for many years conducted a biological and chemical survey of the Illi- 

 nois River. As a result, the general oxygen conditions are rather com- 

 pletely known for the various parts of the river during the different sea- 

 sons of the year, and the distribution of fishes is well enough understood 

 to show the general relation to the range of oxygen concentrations. The 

 present paper is the result of an attempt to determine more precisely the 

 minimal oxygen requirements of the common fishes and some of the 

 effects of low dissolved oxygen concentrations, especially in winter. 



Dr. R. E. Greenfield and Mr. A. L. Sotier, of the State Water Sur- 

 vey, made a series of dissolved oxygen determinations on the river dur- 

 ing the latter part of January, 1925. They reported that the fishes were 

 crowding into the "spring holes" in Peoria Lake, evidently on account of 

 unsuitable conditions in parts of the lake which were frozen over. The 

 writer, accompanied part of the time by Mr. Sotier, spent the latter part 

 of January and all of February gathering data on the fish life of the river 

 and making dissolved oxygen determinations in places where fishes were 

 being taken, where fishes were dying, and where fishes usually had been 

 taken but were absent at the time on account of special conditions of the 

 water. Much credit is due a number of fishermen for collateral infor- 

 mation and for their co-operation in the field work. 



Some valuable determinations of dissolved oxygen in the channel 

 were made by the State Water Survey on January 23 and 24 and on Feb- 

 bruary (i and 7, 1925, which are published here for the first time. 



The General Relation of Ice and Dissolved Oxygen 



It is a common observation that fish often die in the winter under the 

 ice. This has been noticed most often where the ice has completely 

 covered the water for a considerable period. It is also commonly said by 



