36 Tivciity-si.vfh Annual Meeting 



and August, and the season was not a particularly abnormal one 

 any way. 



Chairman Whitaker: May I ask a question of you? There 

 are some things in connection with the paper that seem to me 

 might iiave a bearing on the general movement of fish from one 

 depth to another, in relation to fish food. If I understand you 

 right, as -the season progresses, up to Septeml^er and October, 

 the conditions of temperature are reversed, the top growing 

 cooler as it nears the fall months, and the temperature gradually 

 rising at the bottom. 



Professor Birge: That is not quite correct. The story is 

 rather a long one. In one lake which has been investi- 

 gated, that is 60 feet deep, the bottom remains of a constant 

 temperature until November. The lake being so small the bot- 

 tom temperature practically rises only a fraction of a degree 

 until the water begins to be mixed by the influence of the winds. 

 Of course, the area of the lake makes a very great difference in 

 regard to the effect of winds. 



Chairman Whitaker: When that change takes place in the 

 water is it bv reason of violent winds, or by changes of season, or 

 by transmission of caloric from the top to the l^ottom, and wh.cn 

 it has changed, are the bottom waters richer in plankton than 

 the surface? 



Professor r>irge: \'ery much richer than they were, but never 

 actually richer than the surface strata. 



Chairman Whitaker: It seems to me this is a very interest- 

 ing question. May that question not govern somewhat the 

 movement of fishes? May they not find a richer field at the bot- 

 tom in certain months in which to live? If they do not hibernate, 

 but if they actually do go to the bottom and feed, may not that 

 result from the changes nature sets up in this way? 



Professor Birge: I cannot speak with knowledge of that, 

 except with reference to the perch in Lake Mendota. They go 

 to the bottom in wnitei ; are caught in immense quantities in the 

 lake in anywhere from 40 to 60 feet in depth. But the stomachs 

 of the perch during winter are pretty nearly free from food. 



Mr. Clark: Do they not go to that great depth to get a 

 warmer temperature? 



Professor Birge: They don't get a much warmer tempera- 

 ture at the bottom during the winter; the temperature near the 



