BROODSTOCK, SPAWNING, AND EGG HANDLING 145 



program is chosen, an elaborate recordkeeping system is necessary in order 

 to evaluate progress of the program. 



Inbreeding occurs whenever mates selected from a population of 

 hatchery broodfish are more closely related than they would be if they had 

 been chosen at random from the population. The extent to which a partic- 

 ular fish has been inbred is determined by the proportion of genes that its 

 parents had in common. Inbreeding leads to an increased incidence of 

 phenotypes (visible characteristics) that are recessive and that seldom occur 

 in wild stocks. An albino fish is an example of a fish with a recessive 

 phenotype. Such fish typically are less fit to survive in nature. Animals 

 with recessive phenotypes occur less frequently in populations where mat- 

 ing is random. 



Problems that can arise after only one generation of brother-sister mat- 

 ing include reduced growth rate, lower survival, poor feed conversion, and 

 increased numbers of deformed fry. Broodstock managers must be aware of 

 the problems that can result from inbreeding and employ techniques that 

 will minimize potential breeding problems. To avoid inbreeding, managers 

 should select their broodstocks from large, randomly mated populations. 



Significant differences have been found in rainbow trout between females 

 of different ages in egg volume, egg size, and egg numbers per female. 

 Three-year-old females provide a higher percentage of eyed eggs and 

 larger, more rapidly growing fingerlings than two-year-old females. Growth 

 of the fingerlings is influenced by the age of the female broodfish and is 

 directly related to the size of the egg. The egg size is dependent on the age 

 and size of the female broodfish. Generally, the egg size increases in fe- 

 males until the fifth or sixth year of life and then subsequently decreases. 



If inbreeding is avoided, selective breeding is an effective way to im- 

 prove a strain of fish. A selective breeding program for rainbow trout at 

 the Manchester, Iowa National Fish Hatchery resulted in fish 22% heavier 

 than fish hatched from unselected individuals. Selective breeding in trout 

 has increased growth rate, altered the age of maturation, and changed the 

 spawning date. 



A system has been developed for maintaining trout broodstocks for long 

 periods with lower levels of inbreeding than might be experienced in ran- 

 dom mating. It requires the maintenance of three or more distinct breeding 

 lines in a rotational line-crossing system. The lines can be formed by: (a) 

 an existing broodstock arbitrarily subdivided into three groups; (b) eggs 

 taken on three different spawning dates and the fry reared separately to 

 adulthood; or (c) three different strains or strain hybrids. Rotational line- 

 crossing does nothing to reduce the level of inbreeding in the base 

 broodstock, but serves only to reduce the rate at which further inbreeding 

 occurs. Consequently, it is essential that a relatively high level of genetic 

 diversity be present in the starting broodstock. The use of three different 



