487 



Importance of the Plankton 



The economic importance of the plankton is largely in the pre- 

 dominance of these minute animals and plants in the food of the 

 young of our most important fishes, — a predominance which may 

 be expressed, without serious exaggeration, in the aphorism : no 

 plankton, no fish. Furthermore, adults of many useful species, the 

 crappies and the sunfishes, for example, often cram their stomachs 

 with plankton organisms when these are especially abundant, as in 

 spring. The youngest fishes and adults differ, however, as a rule, 

 in their mode of obtaining this kind of food, the latter straining it 

 out of the water by means of their gill-rakers and the former cap- 

 turing the minute animals one by one, as full-grown predaceous fishes 

 capture their larger prey. There is, indeed, an important exception 

 to be made to the foregoing too sweeping statement. The young of 

 the sucker family seem to take their earliest food much as do the 

 adults — by sucking it up from the bottom — and while many plankton 

 organisms are caught by them in this way, these are largely forms 

 which commonly live on or near the bottom, the free-swimming spe- 

 cies (Daphnia, Cyclops, and the like) being commonly scarce in the 

 food of this family. The young of some other species also 'feed 

 habitually near the bottom, so that the bottom-loving plankton or- 

 ganisms are represented in their food in disproportionate numbers as 

 compared with the average product of the plankton net. Neverthe- 

 less, conditions which bring about an abundance of true-floating or 

 free-swimming plankton, have, generally speaking, a like effect on 

 the more significant part of the minute bottom-life of our waters 

 generally, and the yield of the collector's net is thus a fairly relia- 

 ble index to the food supply of young fishes, of whatever habit. It 

 is true that these, as a rule, make little use of the plants of the 

 plankton as compared with its animals, and that the numbers of the 

 two often do not vary together; but if we remember that the plank- 

 ton animals themselves feed largely on microscopic plants, we shall 

 see that the latter are indirectly useful to fishes, even when not di- 

 rectly so. 



Changes in River Levels 



It follows from the foregoing discussion that, one of the im- 

 portant points to be determined in our earlier operations of 1909 

 and 19 10 was the effect on the plankton content of the Illinois River 

 and its connected waters traceable to the opening of the Chicago 



