Birge. — Plankton of the Lakes. 127 



up in the water and contribute to the organic material at the botom of 

 the lake, on which the insect larvae like the Mayfly larvse and the blood- 

 worms, may feed. 



Mr. Titcomb : Do you not think that most fishes like to live on the 

 bottom? In a deep water lake it is not customary to find any of the various 

 species feeding on the surface over the deep areas ; they are in comparatively 

 shallow water. This means that all this food, this great abundance of 

 food in the plankton, is wasted. 



Dr. Birge : A great amount of it must go to waste. Yet Green Lake has 

 many small fish that go out in the deepest water. On a calm day your boat 

 may be surrounded by hundreds of them. I refer particularly to one of 

 the shiners, Notropis atherinoidss. The young of this species is very abun- 

 dant in the open water. 



Mr. Titcomb : In connection with the Lake George survey, one of the 

 interesting things we found there was that the small form of whitefish 

 which inhabits that lake, and which is the main food of the lake trout 

 when they are young, apparently comes to the surface at night to feed. 

 They come to the surface just at dusk, when it is cool; you can see large 

 schools of them on or near the surface. I suppose they come up for this 

 food you speak of? 



Dr. Birge : Probably. A great many of the deep water fish come to 

 the surface at night. But the habits of both the fish and the water fleas 

 vary greatly. You must bear in mind that there are scores of species of 

 water fleas. For instance there is Daphnia pulex— a big, heavy-bodied 

 water flea. That is the type you find in ponds ; it is not found as abundantly 

 in the open water of the lakes, and if present there it is found ordinarily 

 in the deeper and colder water. In Lake Mendota it lives in the surface 

 water during the winter and early spring, and as the temperature warms up 

 it moves down from the surface ; as the summer goes on the water at the bot- 

 tom loses its oxygen and there are only a few feet of water that have a 

 sufficient supply of oxygen to meet its needs. There are other delicate-bodied 

 forms which are characteristic open water forms — they live in the open water 

 of the lakes. 



You will see that the study that I have reported relates chiefly to the 

 food derived from the open waters of lakes and from the deep water as 

 well. Its direct bearing on fish, therefore, is primarily with the fish of the 

 open water, like whitefish, with the j^oung of shore-living species, which 

 may come out into the open water, and with the fish, which, like perch 

 or white bass, are regularly shallow water forms, but which also come 

 out into deep water for food. We have not studied these relations between 

 fish and food; we have been determining the quantity of the food from the 

 plankton and its general food value. Some of the commercially valuable 

 fish of the Great Lakes, notably herring and whitefish, go back more directly 

 to the plankton for food, than do the fish which most of you are raising. 



If the Bureau of Fisheries could carry on such studies as ours in the 

 Great Lakes the results might be more directly useful to the commercial 

 fisheries than our studies on smaller lakes are to you. But we have been 



