BROODSTOCK, SPAWNING, AND EGG HANDLING 175 



further chance for fertiUzation. In salmonids, water- hardening generally 

 takes from 30 to 90 minutes, depending on water temperature. 



The sperm consists of a head, midpiece, and tail, and is inactive when it 

 first leaves the fish; on contact with water or ovarian fluid, it becomes very 

 active. Several changes take place when the sperm penetrates the egg. Nu- 

 clear material of the egg and sperm unite to form the zygote. This zygote, 

 within a few hours, divides repeatedly and differentiates to form the em- 

 bryo. 



Schematic drawings of trout and salmon egg development (Figure 6l) 

 can be applied in general to other species as well. 



SENSITIVE STAGE 



Trout and salmon eggs become progressively more fragile during a period 

 extending roughly from 48 hours after water- hardening until they are eyed. 

 An extremely critical period for salmonid eggs exists until the blastopore 

 stage is completed. The eggs must not be moved until this critical period 

 has passed. The eggs remain tender until the eyes are sufficiently pigment- 

 ed to be visible. 



EYED STAGE 



As the term implies, this is the stage between the time the eyes become 

 visible and hatching occurs. During the eyed stage, eggs usually are 

 shocked, cleaned, measured and counted, and shipped. 



At hatching, the weight of the sac fry increases rapidly. Water content of 

 the fry increases until approximately 10 weeks after hatching, when it is 

 approximately 80"(i of the body weight. Water content remains fairly uni- 

 form in a fish from this point on. 



As the embryo develops, there is a gradual decrease in the protein con- 

 tent of the egg. The fat content remains fairly uniform, but there is a gra- 

 dual decrease in relative weight of these materials as water content 

 increases. There is no significant difference in the chemistry of large and 

 small eggs. However, several studies have shown that larger eggs generally 

 produce larger fry and this size advantage continues throughout the growth 

 and development of the fish. 



Enumeration and Sorting of Eggs 



A number of systems for counting eggs are in general use. Enumeration 

 methods should be accurate, practical, and should not stress the eggs. 



