147 



22 



For most wild and natural stocks, this recommendation is secondary to 

 those regarding handling of hatchery stocks that may interact with them. 

 This recommendation complements advice regarding effective population 

 size and artificial selection of hatchery stocks. As in hatchery stocks, 

 maintenance of genetic resources and life history patterns in the naturally 

 reproducing component of donor stocks is influenced by effective 

 population size. If agencies establish a lower limit on numbers of 

 individuals remaining to spawn naturally, they must recognize that 

 different lower limits may be necessary for different stocks. Attributes of 

 the natural stock to consider include its population dynamics, viability in 

 the face of fishing mortality and degraded natural habitats, and historically 

 natural levels of genetic variation. Also as in hatchery stocks, prevention 

 of intense artificial selection on the naturally reproducing component of 

 the donor stock guards against loss of genetic variation needed for 

 evolution and perpetuation in natural environments. 



Threatened and endangered stocks, which have experienced severe 

 declines in abundance due to cumulative human impacts, present 

 particularly difficult cases. When stock rehabilitation relies primarily on 

 hatchery production as a means of rapidly increasing abundance, natural 

 reproduction is further reduced by removal of gametes from many or all 

 adults. When rehabilitation relies primarily on improving conditions for 

 successful natural reproduction, use of hatchery production may be 

 precluded. Genetic conservation implications of these alternatives and 

 ways to strike a balance between them must be carefully considered. 

 Agencies must rely on best available scientific information, but recognize 

 that decision making will inevitably involve educated value judgements. 



4. INDIRECT GENETIC IMPACTS OF HATCHERY FISH 



Frequent releases of large numbers of hatchery fish may have indirect 

 genetic effects on wild and natural stocks. They may swamp mixed-stock 

 fisheries, leading to reproductive overfishing of less productive wild and 

 natural stocks. Genetic consequences of reproductive overfishing are 

 reviewed in Kapuscinski and Jacobson (1987) and are beyond the scope of 

 this report. Ecological interactions, such as competition for spawning sites 

 and other limiting resources in natural habitats, also may indirectly change 

 genetic resources and life history patterns of wild and natural stocks 

 (Vincent 1987. Skaala et al. 1990). 



