102 Tzvciity-sez'cnth Annual Meeting 



Lake, the smallest, the temperature at ;^/ feet is as low as in 

 Mouse Lake at a depth of 60 feet, and in Okauchee at a depth 

 of more than 90 feet. All three of the smaller lakes have a bot- 

 tom temperature 5 or 6 degrees lower than that of Mendota at a 

 depth of nearly 80 feet. This feature of the temperature also de- 

 pends on the action of the wind. The water at the bottom of a 

 lake acquires most of its warmth between the middle of April and 

 the middle of May, and the amount to which the bottom water 

 would be warmed is largely dependent on the action of the wind 

 during that month. It follows, of course, that the larger lake 

 will acquire more warmth than the smaller lake. As the season 

 advances the gain of heat at the surface is so rapid that the sur- 

 face water becomes warm to such an extent that the wind is un- 

 able to distribute it through the deeper water. This condition 

 is reached earlier in the smaller lake, and the time during which 

 the bottom water can gain heat is consequently shorter, and the 

 effect of the wind is smaller during this time. The bottom tem- 

 perature is therefore lower. 



You see, therefore, that the water in lakes of different sizes 

 may possess a very dififerent temperature at the same depth, and 

 that the bottom temperature of a small lake is likely to be lower 

 than one would expect from its depth only, and that of a large 

 lake is likely to be higher than its depth alone would indicate. In 

 Garvin Lake, indeed, at a depth of less than 40 feet, the bottom 

 temperature is about as low as in Lake Geneva at a depth of 

 nearly 150 feet, or in Green Lake at a depth of nearly 200 feet. 

 This is because Green Lake and Geneva Lake are seven or eight 

 miles in length, and are therefore exposed to a much greater 

 action of the wind. ' 



I don't know that I ought to sa}- that these considerations have 

 any immediate practical bearing on fish culture, but I think that 

 any one must see that the small lake, with its shallow water and 

 cold bottom temperature, must form a different kind of home 

 for the fish from that afforded by a lake of equal depth but differ- 

 ent area, and consequently different temperature. 



Mr. Whitaker: How about tlie sliallower lakes? Is the 

 source of supply the same as that of the others? Are they spring 

 fed? 



Prof. Birge: Yes, I believe they are spring fed: the tempera- 

 ture of the "spring water is very close to 50 degrees, so that the 

 temperature here at the bottom of this lake is now four or five 

 degrees cooler than the temperature of the spring water. There 



