Fig. 31« Living brook trout brood (vitellin sac brood) immediately 

 after hatching. Also two empty egg shells. 2 x natural size. 



5. Artificial incubation, shipping of eggs, counting of eggs and brood . 

 In artificial incubation we distinguish between two stages: 



(1) The pre-incubation period, that is, up to the time of the development 

 of the two black eye spots (Figs. 30 and 3L). One of these spots — 

 in trout — lies somewhat deeper and is not always clearly perceived. 



(2) The incubation period, that is, from the eyed stage to the moment of 

 hatching (Fig. 31). 



The period of incubatican of trout eggs is dependent upon many factors, not fully 

 understood as yet and differing in practically every case. The hatching period of e^Z,' 

 from one and the same mother fish will at times stretch over a period of more than a 

 week, 



TTiller and his pupils have ascertained that en;,ry of light, and also a low oxj'gen 

 content of the water will delay hatching for almost a week. Haempel and Lechler found 

 the reverse to be true when exposing eggs to ultra-violet rays. Sk lower notes a short- 

 ened hatching period in eggs from older fish. I have observed that insufficient 

 maturity increases the incubation period. But the most influential factor is the 

 temperature, of course. It is imperative for the fishbreeder — ^r.'ho has to know the 

 hatching time in advance — to know the approximate influence strength of temperature. 



As a rule, the incubation period is figured by "day degrees", that is, the total 

 of the daily averages. For example: 5 degrees at one day and 6 degrees at the next 

 are equal to 11 day degrees and so forth. 



Observations made at the incubator house at Eberswalde — the te.Tiperatures were 

 measured to l/lO of degrees — gave the following results: 



Eggs from 3 and /* years old rainbow trout, about 330 day-degrees; at an 

 average temperature of 9 degrees centigrade. 



Eggs from 3 and U years old brook trout, about ^60 day-degrees; at an 

 average temperature of U degrees centigrade. 



115 



