565 



cently hatched and generally plankton feeders, just at the sea- 

 son of the vernal plankton pulse, which is often the maximum 

 production of the year. It may well be that the abrupt dimi- 

 nution of plankton following the May and June pulses is accel- 

 erated to a large degree by the plankton-feeding habits of the 

 fry of these fishes. 



The plankton thus enters directly into the food of most 

 young fishes and of some important adult fishes, and indirectly 

 it is the primal source of food of most fishes. A knowledge of 

 its local and seasonal distribution and of the environmental 

 conditions which favor or impede its development is fundamen- 

 tal to any scientific utilization of the present resources of this 

 stream or any future development of resources now unproductive. 



The data at hand afford an opportunity of comparing the 

 plankton production and the annual output of marketed fish. 

 The reports of the Illinois River Fisherman's Association for 

 1894-1898 give the following statistics based upon estimates 

 and partial records of leading men engaged in the fisheries. 

 They are not exact records, but the error involved is no greater 

 than that in the plankton data. 



PLANKTON AND FISHERIES. 



Hydrographic conditions control to some extent the rela- 

 tive possibilities and efficiency of fishing methods, and this 

 adds to the difficulties of comparison. 



While the relative and absolute amounts of plankton do 

 not determine the number of marketable fish taken in any 

 year, they do indicate in a measure the available primal food 

 supply, and this is a factor in determining the growth, and 



