564 



perch and minnows. In its youngest stages the black bass was 

 found to be a plankton {Entomostraca) feeder, and later chang- 

 ing to a fish diet. It is interesting to note that its principal 

 fish food, young Dorosoma, is itself in its younger stages a 

 plankton feeder, secondarily adopting a limophagous habit. 

 The bottom slime thus eaten by the growing and adult Doro- 

 soma is a food largely because it contains so many organisms 

 normal or adventitious to the plankton. Thus at all seasons the 

 plankton forms an important link in the chain of food relations 

 leading to the black bass. 



The buffalo-fishes and the German carp are likewise to a 

 large degree dependent for food upon the plankton in early 

 stages of growth, and, like Dorosoma, subsequently adopt the 

 limophagous feeding habit. The organisms of the plankton 

 thus at all times enter largely into the sources of their food 

 supply. The contents of the digestive tracts of these impor- 

 tant food fishes examined by me at Meredosia during the 

 spring months of 1899 were found uniformly to contain com- 

 minuted vegetable debris, which constitutes the greater part of 

 the unstable ooze or slime which abounds in the backwaters of 

 the river, and, associated with this, many Entomostraca, rotifers, 

 rhizopods, and unicellular or colonial algse, belonging to species 

 common in the plankton at that season of the year. The mori- 

 bund, the spore-forming, the egg-laden organisms of the plank- 

 ton sink to the deeper strata, and together with the normal 

 denizens of the bottom slime, which are everywhere adventi- 

 tious in our plankton, form the food of these fishes of greatest 

 commercial importance. 



A very striking instance of the adaptation of the breeding 

 seasons of fishes to food conditions is found in their nice ad- 

 justment, for the most part, to the seasonal course of plankton 

 production in our waters. Most of our minnows, the dogfish 

 and gar, the Catostomidce, the carp, Dorosoma, and the Etheosto- 

 midce spawn in central Illinois during April and the first of May, 

 while the bass, the sunfishes, and many of the Siluridce follow 

 in May. This brings the maximum number of young fish, re- 



