136 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



After six weeks spent in determining the composition of the lake fauna and flora, 

 the attention of the laboratory staff was turned toward the question of the distri- 

 bution of these forms and toward measurements of the total volume of living forms 

 found in a given volume of water and of the numbers of each species in a given vol- 

 ume of water. For this purpose twenty-one stations were selected. Fifteen of these 

 were in Lake St. Clair. Since the time was limited, these fifteen stations were 

 located on the American half of the lake and were more numerous in the neigh- 

 borhood of the fishing-grounds. Their distribution is shown on the accompanying- 

 chart. It will be noticed that while they are not scattered over the entire lake, they 

 represent all the conditions of bottom, shore, and depth that are to be found in the 

 lake. Any conclusions concerning them probably hold for the entire lake. Three 

 stations were located near the head of the Detroit River, while for the purpose of 

 comparison three others were located in the western end of Lake Erie. 



At each station the various forms of nets described were used and the collections 

 made were preserved. Their study will determine the distribution of the animals 

 and plants. 



An attempt was also made to measure at each station the quantity of animals 

 and plants floating free in the water under each square meter of surface. No methods 

 have yet been devised for determining the quantity of the animals or plants that are 

 attached to the bottom or that live upon it. Measurements are, therefore, to be 

 made only upon those forms that are found floating free in the water. There is 

 reason to believe that whitefish fry (and probably the fry of many other young fish) 

 feed for a time upon minute animals (Crustacea) floating free in the water. If this 

 is true, it becomes at once a question of great practical importance to know where 

 these Crustacea are to be found in greatest numbers, so that the whitefish fry may 

 be planted in such localities. Such measurements have also a very high scientific 

 interest. 



Quantitative determinations of the living forms in the water have not been pre- 

 viously undertaken in this country and its methods are almost unknown among us. 

 Tbey have been conducted on a large scale by Prof. Victor Hensen, of the Uni- 

 versity of Kiel, in (Jermany. Hensen has examined the waters of the North Sea and 

 more recently those of the Atlantic Ocean, and has perfected very ingenious apparatus 

 for the purpose. Apstein,* a pupil of Heusen, has adapted this apparatus for use in 

 fresh water, and has made a careful study of several fresh-water lakes in the neigh- 

 borhood of Kiel. 



As it was found necessary for work in the Great Lakes to modify the apparatus 

 and methods used by Hensen and Apstein, I give here a somewhat detailed account 

 of our procedure. 



The name plankton has been given by Hensen to all those animals and plants 

 which are found floating free in the water and subjected to the action of the waves, 

 currents, or tides. Thus adult fish do not belong to the plankton, since they are able 

 to move about independently of waves or currents. Fish eggs and young fish fry are 



I >:is Plankton <lcs Susswassers und seine quantitative Bestimuiung. Schrifteu d. Naturw. Ver. f. 

 Scnleswig-Holstein. Bd. ix. Hit, 2. 1892. 



Quantitative Plankton-Studieu in Siisswasser, Hiologisches Centralblatt. Bd. XII. 1892. 



