BROODSTOCK, SPAWNING, AND EGG HANDLING 147 



One potential problem with the system is the amount of separate holding 

 facilities required for maintaining up to 15 groups if each line and year 

 class are held separately. This problem sometimes can be overcome by us- 

 ing marks such as fin clips, brands, or tags to identify the three lines and 

 then combining all broodfish of each year class in a single rearing unit. 

 The total number of broodfish to be retained in each year class would be 

 determined by the production goals of the particular station, but equal 

 numbers of fish should come from each line. This method will not only 

 slow down inbreeding, but will also make a selection program more effec- 

 tive. 



Studies have been conducted on the growth and survival of progeny 

 from mating of hatchery and wild steelheads to determine if hatchery fish 

 differ from wild fish in traits that affect the survival of wild populations. 

 They indicated that wild fish x wild fish had the highest survival, and wild 

 fish X hatchery fish had the highest growth rates. In the hatchery, how- 

 ever, fish from a hatchery x hatchery cross had the highest survival and 

 growth rates. 



With salmon, where the adult returns to the hatchery exceed the number 

 of fish required to maintain the run, it has been possible to select that por- 

 tion of the population having the most desirable characteristics. Through 

 selective breeding, it has been possible to develop stocks of salmon that are 

 better adapted to the needs of both fisheries management and commercial 

 aquaculture. Changes in timing of spawning runs through selection have 

 resulted in delayed or advanced fish spawning when water in the spawning 

 streams has cooled or warmed to more desirable temperatures. In some in- 

 stances, fish that are much larger than most have been selectively bred to 

 produce many more eggs than the ancestoral stock. Greater temperature 

 tolerance and disease resistance of selectively bred fish can also increase 

 survival. Rapid growth of selectively bred fish shortens the rearing period 

 so that facilities may be used more efficiently, and earlier maturity de- 

 creases the rearing period for broodfish. 



Information on selective breeding of cool- and warmwater fish is limited. 

 Some work has been done toward improving the commercial value of these 

 fish, increasing their resistance to low dissolved oxygen concentrations, im- 

 proving feed conversion, and developing hybrid strains. 



Selective breeding of catfishes is relatively new. Some goals to be 

 achieved by selective breeding include resistance to low dissolved oxygen 

 levels, more efficient food conversion, and development of fish with smaller 

 heads in proportion to body size. Albino channel catfish have been report- 

 ed to possess the smaller head characteristic. However, albino channel cat- 

 fish fry have a significantly lower survival rate than normal fish. 



The following guidelines should be followed when catfish are managed 

 and selected: 



