484 



of fishes as to increase the fisheries products of the stream. It was 

 also a matter of interest and importance to learn the effects, both 

 direct and indirect, of the increased inflow of sewage by way of the 

 sanitary canal and the Des Plaines, upon the fishes and mollusks of 

 the Illinois — an inquiry calling for chemical examinations of the 

 waters of the stream and systematic collections from it at various 

 points on its course, under various conditions, and at different times 

 of the year. 



Character and Chronology of Field Operations 



The work planned upon these lines has taken three directions : 

 the first year was given mainly to a study of the plankton of the river 

 and of the principal bottom-land lakes, made on the same grounds 

 and by the same methods and equipment as those of the period from 

 1894- 1899; the second year was devoted especially to chemical de- 

 terminations of the gases of the waters and of the bottom sediments 

 of the upper river, from its origin to Chillicothe, and to parallel 

 collections of the minuter plankton — the so-called microplankton — 

 of the stream made at the same times and places as the chemical 

 studies ; and in the third year, similar chemical and biological de- 

 terminations were made, with principal attention, however, to fishes 

 and mussels and to the other plant and animal life of the bottom and 

 the shores. 



Advantage has also been taken of opportunities given us by the 

 State Fish Commission to collect the plankton of the Mississippi and 

 Ohio rivers for comparison with that of the Illinois, and especially 

 to study the effect of protracted drouth and continuous low water 

 on plankton production in the streams themselves. Much time was 

 given during the spring seasons of 1910 and 191 1 to field studies of 

 the habits of spawning fishes and the times and places where their 

 eggs were laid, and to the fate of the eggs and the rate of growth 

 of the iry. These observations were intended especially to give us 

 a better knowledge of the proper limits of a closed season for the 

 protection of the more valuable fishes, and to show us also what 

 measures are necessary to keep the numbers of the young up to the 

 limits set by the available food supply.* 



The collection of materials for a comparative study of the plank- 

 ton of our principal rivers was actually begun in 1902, when virtually 

 continuous collections were made from the steamer "Illinois" of the 



*For a summary of results, see Bull. 111. State Lab. Nat. Hist.. Vol. IX, 

 articles VII and VIII, March, 1913. 



