Biological Survey — Genesee River 



13 



dozen days during; July and August when such are likely to pre- 

 vail. Nevertheless it is on these very days that the water tempera- 

 tures may reach the critical point. Where so many streams are 

 concerned, it is not possible to actually cover the ground on the 

 few hot days at one's disposal. Consequently it is frequently 

 necessary to estimate what the maxima will be. 



Estimations of this kind are not difficult to make in the case of 

 shaded streams frequently fed by large springs, for here water 

 temperatures are not markedly influenced by the sun's heat. Nor 

 is it so important to have the maximum temperatures of a stream 

 in which trout are observed in abundance. Here one may assume 

 that all conditions are favorable. However, there are many 

 streams which are not abundantly spring fed and whose tempera- 

 tures must range above what may be considered the optimum for 

 trout but not necessarily up to the critical point. Such of these 

 streams as are well provided with food produce some of our best 

 trout fishing. It is in such cases that the maximum temperatures 

 are so important and at the same time so difficult to estimate. 



The estimates in connection with the Genesee survey have re- 

 sulted from a comparison of the water and corresponding air 

 temperatures obtained in the field on moderately warm days with 

 those recorded in the writer's past experience. 



The following table has been the guide in separating brook- 

 trout streams from others in so far as the temperature factor has 

 been concerned. It is based upon experimental evidence obtained 

 in the hatching station of Cornell University and upon observa- 

 tions made in the streams about Ithaca, N. Y. It should apply 1o 

 the Genesee valley section during the months of June, July and 

 August but not necessarily to others without some modification : 



Table 1. — ■ Comparisox of Air and Water Temperatire.s ix a Brook 



Trout STREA:\r 



Max. air temp. deg. Fahr. . . 

 Max. water temp. deg. Fahr. 



94.0 

 75.0 



The first line gives a series of air temperatures and the second 

 the corresponding water temperatures, which should not be 

 exceeded, if the stream is to be considered suitable for brook trout. 



In order to use the table, one must have a record of air and water 

 temperatures taken at the same time and preferably between noon 

 and 5 p. m. Referring to the first and last column, if a stream 

 temperature below 65° happens to have been taken when the air 

 tem]:>erature was 80°, one would predict the stream temperature 

 to be about 75°, if the air were 94°. Likewise, the water should 

 not exceed 66.6° on days when the air shows 82° and so on 

 throughout the given range. 



One must remember that the temperature of flowing water often 

 lags behind a sudden rise in air temperature, sometimes as much 

 as 24 hours, and that three or four successive days of the same 



