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, 
ag b : Total plankt 
Wiear Lae, st Average plankton content in cm.’ per m.° Beeneee ea 
fish marketed In channel | In backwaters | Total cubic meters 
1894 8,276,227 2.53 4.40 6.93 45,757 
1895 8,588,000 5-91 10.76 16.67 83,340 
1896 7,252,811 1.05 7.18 8.23 28,629 
1897 9,703,298 3-51 5.85 9.36 94,605 
1898 10,647,466 2.03 12.42 14.45 63,596 
Aver’ge 8,933,560 2Epy 8.42 Lets 67,750 
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 
