148 FISH HATCHERY MANAGEMENT 



(1) Avoid inbreeding, which includes father- daughter, mother-son and 

 brother-sister mating. Current practice is to keep the same broodstock 4 to 

 10 years, with replacement broodstock coming from progeny produced on 

 the farm. Furthermore, a beginning producer may have unknowingly 

 started a broodstock with full brothers and sisters having a narrow genetic 

 base. Catfish should be marked in some manner to identify broodstock for 

 pen mating to avoid inbreeding. The stocks can be clearly identified by 

 heat branding, applied when water temperature is 72°F or above, so that 

 healing proceeds rapidly. 



(2) Enrich bloodlines through the addition of unrelated stock. This can 

 be effective in correcting deterioration in quality of broodstock common to 

 inbreeding. The need to enrich bloodlines might be suspected if a high 

 percentage of deformed progeny, low hatchability of eggs, low survival of 

 fry, or poor growth becomes evident. 



(3) Crossbreed unrelated stocks. Stocks orginating from different river 

 systems and commercial sources are usually quite diverse, and may com- 

 bine with resulting hybrid vigor, especially in growth and disease resist- 

 ance. 



(4) Select broodstock carefully; as males grow faster than females in 

 channel catfish, blue catfish, and white catfish, rigorous selection by grad- 

 ing in ponds probably will result in practically all males. More properly, a 

 random sample should be taken at the first selection at 6 months of age, 

 with selection for growth and broodstock occurring at 18-24 months of 

 age. Select equal numbers of males and females. 



HYBRIDIZATION AND CROSSBREEDING 



Hybridization between species of fish and crossbreeding between strains of 

 the same species have resulted in growth increases as great as 100%, im- 

 proved food conversions, increased disease resistance, and tolerance to en- 

 vironmental stresses. These improvements are the result of hybrid vigor — 

 the ability of hybrids or strain crosses to exceed the parents in perform- 

 ance. 



Most interspecific hybrids are sterile. Those that are fertile often produce 

 highly variable offspring and are not useful as broodstock themselves. Hy- 

 brids can be released from the hatchery if they cause no ecological prob- 

 lems in the wild. 



Several species of trout have been successfully crossed, the more notable 

 being the splake, a cross between brook and lake trout. 



Hybridization of the chain pickerel and northern pike in a study in Ohio 

 did not produce hybrid vigor and the resulting offspring grew at an inter- 

 mediate rate to the parents. A cross between northern pike males and 

 muskellunge females has yielded the very successful tiger muskie. 



