Ill] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 111 



7. After hatching the young fish seek the deepest parts of the lake where 

 they remain until their first breeding season, at the age of three years, 

 except when driven out by the thermocline. 



8. The rate of growth and weight increase is given for fish up to ten 

 years of age. 



9. The cisco has a seasonal migration in a vertical plane. This is due 

 to the formation of the thermocline and the accompanying zone of water 

 deficient in oxygen. The result is that the species is slowly driven from the 

 deeper water into water of lesser depth and increasing warmth. 



10. Since the cisco is a deep water fish, adapted to cold water, the 

 result of this condition is death to thousands of fish. Death is due to 

 suffocation as the fish require more oxygen in warm water than in cold, 

 and the supply is insufficient to their needs. 



11. The result of 941 stomach examinations is given, showing the food 

 habits the year around. 



12. Entomostraca form a very important item of the food. Since 

 these organisms are negatively phototropic, being down during the day 

 and up at night, a second migration, or daily rhythm on the part of the 

 fish, also in a vertical plane, coincides with the movement of their food. 



13. The fish are active day and night the year around. 



14. This activity in summer assumes a definite co-ordinated movement, 

 extending around Oconomowoc lake in a clockwise direction. 



15. This movement is connected with the feeding habits as suggested 

 by the fact that the migrating schools can be stopped and held by the 

 presence of an abundant food supply. 



16. The decrease in abundance and size of the cisco in Oconomowoc 

 lake call for protective and conservative measures to prevent the destruc- 

 tion of a valuable food fish. 



