494 



895 & '96 to 1897 & '98 

 909 & '10 



River 



Quiver Lake 



2.16 1.93 



5.07 6.13 



Thompson's Lake 



7.80 



7.77 



In the three full years of the earlier period whose data are used 

 in this discussion, the plankton of Quiver Lake water was lowest, 

 that of the river was but little higher, and that of Thompson's Lake 

 was three and a half times the latter. In 1909-10 the yield of the 

 river was lowest, that of Quiver Lake was 21 per cent, larger, and 

 Thompson's Lake yielded 27 per cent, more than Quiver, the change 

 being due to a great increase in the river yield and a still greater 

 increase in that of Quiver Lake, w^hile the yield of Thompson's Lake 

 remained substantially unchanged. 



If to these facts concerning the increase, in recent years, in the 

 percentage of plankton contained in the waters of the Illinois, we add 

 those concerning an increase in the average volume and area of tlie 

 waters themselves, w^e shall see that their total plankton product must 

 have been many times multiplied, and that the fisheries of the stream 

 -should feel the effects of this greater abundance of this important 

 element of fish food; provided, it must be added, that the plankton 

 supply is really at any time a limiting element in the production of 

 fishes, such that we may amend the aphorism given on another page, 

 to the form : "The more plankton, the more fish." It will, however, 

 be a long time, in the writers' judgment, before the whole economy 

 of fish production in our streams is so thoroughly understood that 

 such a statement will be warranted. At present we can only say that 

 there was produced in the waters and backw^aters of the Illinois, in 

 1909-1910, an available and accessible amount of fish food sufficient 

 for a much greater population of fishes than the stream had pre- 

 viously supported, and that if no such population is maintained, the 

 reasons for the fact must be sought in some other direction. 



Causes of an Increased Puankton 



No change has recently occurred in the Illinois River system, or 

 in the basin of the Illinois, to account for the increased productivity 

 of its waters except the one already repeatedly referred to — the open- 

 ing of the sanitary canal connecting the Illinois and the Chicago 

 rivers at the beginning of 1900. The effects of this occurrence on 

 the plant and animal products of the stream may conceivably have 

 been produced in one or more of these three principal methods: (a) 



