12 
We have also learned that this first food—the minute plant and ani¬ 
mal life of the water, called its plankton—is produced almost wholly 
in the backwaters. Although flowing streams often carry an enor¬ 
mous quantity of it, this mainly perishes presently in our great silt¬ 
laden rivers. When, as in very low water in midsummer, the con¬ 
tributions from the backwaters are reduced to a minimum, or per¬ 
haps 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 proportionate, other 
things being equal, to the extent and fertility of the backwaters ac¬ 
cessible 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 val¬ 
ueless 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 flood 
in our great rivers, the spawning time of the bulk of our fishes, and 
the climax period in the development of the plankton. All coming 
together or following one another in quick succession, as they nor¬ 
mally do, conditions are as favorable as possible for a large stock 
of young fishes. The longer the period and the larger the scale 
of the spring overflow, the better is the prospect for a heavy annual 
contribution to the population of the stream. To this no doubt is 
due the fact, clearly indicated by our recent river work, that the 
plankton product of the Illinois system has been greatly increased 
by the opening of the drainage canal from Lake Michigan and the 
consequent raising of the average level of the river by about three 
feet, this rise of river level of course resulting in a more wide-spread 
and longer-continued overflow. 
On the other hand, nothing can be more dangerous to the con¬ 
tinued productiveness of these waters than a shutting of the river 
into its main channel and the drainage of bottom-land lakes for agri¬ 
cultural purposes. It is fortunate for our fisheries when one of these 
lakes comes into the possession, or under the control, of a hunting 
or fishing club, for this insures its maintenance. The time has come, 
