Biological Survey — Erie-Niagara Wati^shed 67 



5. — The Macroplankfon of Lake Erie 

 By Charles B. Wilson 



Apparatus and Methods. — In the present survey the macro- 

 plankton was collected by two meter nets, one drawn along the 

 surface and the other just above the bottom, and by a Helgoland 

 trawl drawn along the bottom. Foot-nets were used for the 

 microplankton, and their contents were examined also for Crus- 

 tacea to make sure that none of the smaller species escaped. The 

 only thing they really added to the contents of the large nets was 

 a greater abundance of developmental stages, nauplii and meta- 

 nauplii. 



Importance of the Macroplankton. — The number of fish any 

 lake will produce depends almost entirely upon the amount of suit- 

 able food which the water of the lake is capable of furnishing. 

 Artificial feeding, especially in a body of water the size of Lake 

 Erie, is absolutely impossible. The food must be a natural pro- 

 duction of the lake itself, and must exist in sufficient abundance 

 not only to carry the fish through the earlier and more critical 

 period of their existence, just after they are hatched from the egg, 

 but also to keep them well fed as long as they live. It must be, 

 therefore, of such a nature that it can reproduce itself and thus 

 furnish a new supply as fast as the old is used up. The plankton 

 Crustacea fully meet these requirements by reproducing in great 

 numbers throughout the entire year, but especially at the time 

 when the newly hatched fish fry most need them. These fry almost 

 without exception feed practically exclusively upon the Crustacea, 

 and the latter also serve as food for those other organisms, which 

 make up the diet of larger fishes, such as insect larvae, smelts, 

 minnows and the like. Some fish, like the ciscoes, smelts, minnows 

 and darters, continue to feed more or less largely upon these small 

 Crustacea as long as they live. Hence the plankton Crustacea 

 occupy a critical and most important position in the economy of 

 fish propagation. 



Its Relation to the Microplankton. — Obviously if the Crus- 

 tacea are to multiply in sufficient numbers to feed the fish they 

 themselves must have an abundance of nourishing food. This 

 they obtain from the microscopic plants in the plankton, especially 

 the diatoms, and their mouth parts as well as their habits of life 

 are admirably suited for just this kind of food. By eating and 

 digesting these tiny plants the Crustacea convert vegetable sub- 

 stances into animal tissues which are not only more palatable to 

 the great majority of fishes, but also are vastly more nourishing. 

 Fortunately the diatoms and other microscopic plants in their turn 

 are able to manufacture their own food synthetically out of the 

 soluble inorganic substances and gases in solution in the lake 

 water. We thus find a complete cycle of interdependence from 

 the dissolved substances in the water up to the large adult fishes, 

 and failure on the part of any single factor of this cycle will result 



