99] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 99 



males come into the shallows when the water first reaches a temperature of 

 4.3°C; that the females arrive when the temperature has dropped to 3.8°C 

 and that the spawning season is at its height with the water at from 3.6°C 

 to 3.3°C; and finally, that the season is abruptly over as the water drops 

 below this figure. Table 24 gives the conditions during other years. 



Table 24. 

 CONDITIONS UNDER WHICH CISCO SPAWNED FROM 1916 to 1921 



1916 1917 1918 1919 1921 



M M O O O 



First cf in shallows Dec. 3 Nov. 29 Nov. 30 Nov. 28 Nov. 26 



Temperature water: 4.3 4.4 4.4 4.3 4.3 



First 9 in shallows Dec. 5 Dec. 2 Dec. 2 Nov. 30 Nov. 29 



Temperature water: 3.8 3.9 3.8 3.7 3.65 



Maximum spawning: 3.3 3.5 3.5 3.4 3.3 



Spawning over: 3.0 3.1 3.0 3.0 3.1 



M = Lake Mendota O = Oconomowoc Lake 



From these data it must be concluded that the critical temperature at 

 which the male cisco enters the shallows is very close to4.3°C ;for the female 

 it is 3.8°C; and that the optimum temperature is between 3.3° and 3.5°C. 

 As to the ratio between the sexes, it appears that the males outnumber 

 the females to some extent, as shown by the following: 



No. ciscos examined Lake Males Females 



163 Mendota 101 62 



744 Oconomowoc 429 315 



907 530 377 



This is in the ratio of 100 males to 71.2 females. This difference can 

 be explained by the greater mortality among the females during the late 

 summer, as will be seen later. 



The number of eggs laid by the female varies considerably, depending 

 of course upon the size and age of the fish. By the water displacement 

 method the following count of the number of eggs laid by a single female 

 was obtained: 



Amount of water displaced by eggs 46.5 cc 



To displace 1 cc water (av. 5 counts) 329 eggs 



Total number of eggs 15,238 



Weight of female before stripping 465 grams 



Weight of female after stripping 405 grams 



Weight of 15,238 eggs . . . 60 grams 



