American Fisheries Society. 33 



bringing them in, and when you saw those fish they came from 

 the surface,out in the open lake, not from near the shore. 



Mr. Titcomb: I would like to inquire how the lake is sup- 

 plied with water. Is it by springs? 



Professor Birge: There is a small creek, but a good deal 

 more water comes in from large springs at the part of the lake 

 furthest removed from the city. 



Mr. Titcomb: In getting at the temperatures, of course the 

 waters nearest those springs would remain coolest the year round, 

 have an even temperature, would it not? 



Professor Birge: I have done very little work at that end of 

 the lake. The temperature of the creek during winter and the 

 bottom temperature of the lake, falls below the temperature of 

 springs. It falls to 35 or 36 degrees at the bottom and it does 

 not rise anywhere until after the ice goes out in the spring, so 

 that this inflow of water is not sufficient to raise the bottom tem- 

 perature, through, say, three and a lialf months of the winter. 



Mr. Titcomb: I was making incjuiries, because I have been 

 taking observations of temperatures in the trout lakes of Vermont, 

 and we have lakes fed there largely by springs, and the tempera- 

 ture remains very even, within 20 feet below the surface. You 

 go 20 feet below the surface and you will get a temperature of 

 40 to 46 the year around. The lake is about 1,500 fedt across 

 a" id two miles long. 



Professor Birge: The bottom temperature differs in differ- 

 ent inland lakes more with reference to the area than in respect 

 to the direct depth. In Oconomowoc Lake, which is perhaps a 

 mile or a mile and a half long, the bottom temperature is about 

 as you get it in Vermont, about 43 to 44 degrees, at 60 feet in 

 depth, while in Lake Mendota, which has a greater area, the bot- 

 tom temperature is 60 degrees. 



This peculiarity of the foulness of the liottom water is true 

 only of lakes where there is a rich plankton. The other 

 lakes of which I sjieak, Oconomowoc Lake and Pine Lake, are 

 tvpicallv plankton jjoor lakes, where there is not one-twentieth 

 as much of vegetal)le life as in Lake Mendota. In both those 

 lakes Crustacea go nearly or (juite to the bottom. The foulness 

 of the water is from the quantity of material dropped down there 

 from the surface. 



