44: T^i'cnfy-si.rtli Aiiuual Meeting 



yields a sample of the plankton in a lake, and that such sample 

 may be weighed and measured and its constituents counted. 



This method was first introduced by Professor Hansen, who 

 used it in the study of the marine plankton and described it as 

 early as 1887. It was subsequently modified and used in some 

 of the fresh water lakes of North Germany by Hansen's pupil 

 Apstein in 1890 and later. Since then the method or some modi- 

 fication of it, has been widely used. When we remember that 

 aquatic plants arc dependent for their nourishment on the ma- 

 terials dissolved in the water, antl that aquatic animals are directly 

 or indirectly dependent on plants for nourishment, we realize that 

 a measurement of the plankton is a measurement of the relative 

 productive capacity of a body of water. We thus have for the 

 first time a method of determining how much organic matter a 

 given Ijody of water is capal)le of yielding, and the importance 

 of this method for fish culture has hardly yet been realized. 



Investigation by this method soon showed that the plankton 

 of a lake was uniformly distributed. The lake might be compared 

 to a field of wheat in which the plants were growing uniforndy 

 over the whole field. A scpiare yard anywhere in such a field 

 would yield approximately the same measure of wheat grains. 

 Sinnlarly it was found that the plankton net gave approximately 

 the saiue results, no matter in what part of the lake it was used. 

 Thus it became evident that in order to test the plankton pro- 

 duction of a lake it was necessary to make l)Ut a single haul of 

 the plankton net. 



Now let us see what the results have been of the comparison 

 of dift'erent lakes by this method. We are apt to think of a 

 lake area as we do of a land area and to imagine that if a lake 

 an acre in extent produces a certain weight of fish a lake one 

 thousand acres in extent should produce one thousand times 

 that weight of fish. When we turn to a very large lake, such 

 as Lake Michigan, wc are apt to think of it as we think of tlie 

 ocean, as being inexhaustible. In thus imagining that the pro- 

 ductive capacity of a lake is proportioned to its size, we fail to 

 take into account certain important facts. The whole source of 

 food supply for the inhabitants of a lake is contained in solution 

 in its waters. The plants live directly on the materials thus in 

 solution in the waters of the lake, and the animals m their turn 

 feed upon the plants, or upon one another. When we inquire 

 as to the source of the materials in solution in the water of a 

 lake, we find that they have all been introduced from 

 without. They are brought in by streams, they are 



