American Fisheries Society 185 



In this general field we now need merely to finish our 

 studies along special lines, and to extend somewhat the 

 geographical range of our detailed survey. We shall then 

 be both ready and free to take up special problems of im- 

 mediate economic interest. Indeed, much has already been 

 done by us on such practical problems, as may be seen from 

 a few illustrations. 



We learned a good many years ago — and this fact was 

 first established in Illinois — that virtually all our young 

 fishes, whatever their adult habits may be, live at first on 

 the same kind of food. All which hatch in like situations 

 and at approximately the same time, consequently, compete 

 with each other when they first begin to feed. We have 

 also learned that this first food — the minute plant and 

 animal life of the water, called its plankton — is produced 

 almost wholly in the backwaters. Although flowing streams 

 often carry an enormous quantity of it, this mainly perishes 

 presently in our great silt-laden rivers. When, as in very 

 low water in midsummer, the contributions from the back- 

 waters are reduced to a minimum, or perhaps wholly cut off, 

 the plankton of the stream also falls off to little or nothing. 

 Left to itself, indeed, even so slow a river as the Illinois 

 would virtually empty itself of plankton in a little while. 

 The fish-producing capacity of the stream is thus propor- 

 tionate, other things being equal, to the extent and fertility 

 of the backwaters accessible from it, and contributing to it 

 at the hatching time of fishes. The plankton content of a 

 stream at that time is, in fact, an excellent index to the 

 productive capacity of the waters as a whole. 



These facts have some interesting consequences, one of 

 which is that every useless fish is an injurious one, since 

 it competes for food, at least when young, with the useful 

 kinds. By a useless fish, however, I must be understood to 

 mean one which is both valueless to us and which does not 

 contribute in any important way to the maintenance of 

 valuable kinds. 



There is a notable harmony between the time of highest 



