FISH HEALTH MANAGEMENT 287 



against infectious disease that are intrinsic to the species and individual. 

 These defenses account for the innate resistance of various species and 

 races of fish to certain diseases. For example, IHN virus affects sockeye 

 and chinook salmon and rainbow trout fry, but coho salmon appear resis- 

 tant to the disease. Rainbow trout are less susceptible to furunculosis than 

 brook trout. 



Fish have immunological capabilities. Under favorable circumstances fish 

 are able to produce gamma globulins and form circulating antibodies in 

 response to antigenic stimuli. They also are capable of immunological 

 memory and proliferation of cells involved in the immune response. The 

 immune response of cold-blooded animals, unlike that in warm-blooded 

 ones, depends upon environmental temperature. Lowering of the water 

 temperature below a fish's optimum usually reduces or delays the period of 

 immune response. Other environmental factors that stress fish also can 

 reduce the immune response. 



Adaptive responses to disease occur in natural populations of fish. Sig- 

 nificant heritabilities for resistance to disease exist, and selection to in- 

 crease disease resistance in controlled environments can be useful. Inten- 

 tionally or unintentionally, specific disease resistance has been increased at 

 many hatcheries by the continued use of survivors of epizootics as 

 broodstock. Increases in resistance to furunculosis in selected populations 

 of brook and brown trout have developed in this way. Potential exists for 

 genetic selection and breeding to increase disease tolerance in all propagat- 

 ed fishes but certain risks must be anticipated in any major breeding pro- 

 gram. 



Under controlled environmental conditions, resistance to a single disease 

 agent through a breeding program can be expected. However, simultane- 

 ous selection for tolerance of several disease agents can be extremely diffi- 

 cult, except perhaps for closely related forms. In any natural population, 

 individual fish may be found that are resistant to most of the common 

 diseases. Pathogenicity of disease agents varies from year to year and from 

 location to location, probably as a result of environmental changes as well 

 as strain differences of the disease agents. When environmental conditions 

 are favorable for a pathogen, the fish that can tolerate its effect have a 

 selective advantage. However, when conditions favor another pathogen, 

 other individual fish may have the advantage. Natural recombination of 

 the breeding population assures that these variations are reestablished in 

 each new generation of the population. Any propagation program must en- 

 sure that this variability is protected to retain stability of the stocks. 

 Managers always run the risk of decreasing the fitness of their stock in 

 selective breeding programs; changes in gene frequencies resulting from 

 selection for disease resistance may cause undesirable changes in the fre- 

 quencies of other genes that are unrelated to disease resistance. 



