176 FISH HATCHERY MANAGEMENT 



.GERMINAL DISC 



A. ONE DAY AFTER FERTILIZATION, 55.9°F AVERAGE TEMPERATURE 

 (23.9 T.U.). 



BLASTODISC 



B. TWO DAYS AFTER FERTILIZATION, 53.9°F AVERAGE TEMPERATURE 

 (43.9 T.U.). 



EDGE OF BLASTODISC 



EMBRYO 



C. FIVE DAYS AFTER FERTILIZATION, 51 .7°F AVERAGE TEMPERATURE 

 (98.4 T.U.). 



Figure 61. Schematic development of trout and salmon eggs. One temperature 

 unit (TU) equals 1°F above 32°F for a 24-hour period. See Glossary: Daily Tem- 

 perature Unit. (Source: Leitritz and Lewis 1976.) 



When small numbers of eggs are involved, counting can be done by 

 hand or by the use of a counting board that will hold a known number of 

 eggs. A paddle- type egg counter is constructed of plexiglass by drilling and 

 countersinking a desired number of holes spaced in rows. The diameter of 

 the hole will depend on the size of eggs being counted. The paddle is 



