567 



There is thus, in general, a correspondence between plank- 

 ton production and the product of the fisheries in that the di- 

 rection of movement in both is usually the same. They rise or 

 fall together. If we compare the changes of the product of the 

 fisheries with those of the sum of plankton production in chan- 

 nel and backwaters, as given in the table on page 565, we see 

 that the direction of the change is the same in both from year to 

 year in every instance in 1894-1898. If similar comparisons 

 are made of the product of the fisheries and plankton produc- 

 tion in channel or backwaters alone, or in total discharge, we 

 find that in three cases out of four the direction of the change 

 is the same in both. It is also generally true that years in 

 which plankton production is below the average are also ones 

 in which the product of the fisheries falls below the mean. 



Plankton production at Havana, provided a similar plank- 

 ton content is maintained until the run-off reaches the mouth 

 of the river, would result in an average discharge of 67,750 

 cubic meters of plankton, equivalent in weight to somewhat 

 more than 149,050,000 pounds, or 15 times the annual produc- 

 tion of fish. To this wastage of organic matter, which in great 

 part is permanently lost to the drainage basin of the Illinois, 

 should be added the unutilized nitrogen and other food elements 

 in suspension and solution which escape with the run-off, es- 

 pecially of flood waters (see Table X.). 



How shall this waste be prevented and the plankton be 

 turned into marketable fish ? The problem is a complex one, 

 but the results of this investigation should contribute towards 

 its solution. The first step will be to impound the richly fer- 

 tilized flood waters and thus to afford time for the utilization 

 of their food elements by the developing plankton, which by 

 various chains of food relations is joined to marketable fish. 

 An illustration of the productive possibilities of impounded 

 Spoon River floods is seen in Phelps Lake, our richest plankton 

 station, and also the home of great numbers of young fish. 

 Thompson's Lake, another impounding backwater, not only 

 breeds an abundant plankton but contributes no insignificant 



