Sididae and Daphniidae 217 



82 ° F. All of these factors have been mentioned before by various 

 investigators, especially by Dr. A. M. Banta and his associates. All of 

 the above mentioned factors except predatory enemies favor the produc- 

 tion of so-called winter eggs, which is usually an indication that the 

 culture is on the wane. 



In order to have strong cultures at all seasons it becomes necessary to 

 keep the individual Daphnia in the active condition of producing asexual 

 or so-called summer eggs only, which demands the elimination of the 

 inhibitive factors just mentioned. 



A too dense population, of course, is easily reduced by using the 

 Daphnia. The excessive accumulation of waste matter is corrected by a 

 change of water in the pond. Predatory enemies are controlled partly by 

 sterilizing the pond bottom and sides immediately before starting a cul- 

 ture and later by spraying the surface with some non-toxic animal oil, 

 such as herring oil, salmon oil, or cod-liver oil. The water temperature 

 cannot be controlled and, consequently, where it ranges above 82 ° F. for 

 any length of time it may be difficult or impossible to produce cultures 

 of Daphnia magna. 



A population density reaches the maximum under the experimental 

 conditions maintained in our ponds in from 16 to 26 days with the water 

 temperature varying between 70 and 8o° F. Consequently cultures are 

 permitted to develop for 2 1 days, when they are fed to the fish and an 

 entirely new culture with fresh water is started. The schedule of opera- 

 tions is as follows: 



First day — Pond is drained, bottom and sides thoroughly disinfected 

 with a strong solution of chlorinated lime, allowed to stand 6 hours; 

 then refilled, fertilized, and stocked with large Daphnia from an active 

 culture. 



Fifth to seventh day — Second fertilization. 



Tenth to fourteenth day — Third fertilization. 



Twenty-first day — Drawing the pond and using the Daphnia. 



In general this routine was continued through several summers from 

 May to October and when a proper amount and kind of fertilizer was 

 administered, the results were consistently good. 



Although we have tried to maintain pure cultures of Daphnia, other 

 organisms have naturally appeared and multiplied. Some of these are 

 desirable food animals but others are predators. 



Several little hard-shelled ostracods have appeared in considerable 

 numbers, especially late in the culture period. We believe there is some 

 connection between their abundance and a decline in the production of 

 Daphnia, without as yet having direct evidence that they actually prey 

 upon living Daphnia. Disinfection of the pond does not entirely elimi- 

 nate them but nevertheless helps to keep them under control. 



