Fish Hatchery Management - fisheries & aquaculture

FISH HEALTH MANAGEMENT 287against infectious disease that are intrinsic to the species and individual.These defenses account for the innate resistance of various species andraces of fish to certain diseases. For example, IHN virus affects sockeyeand chinook salmon and rainbow trout fry, but coho salmon appear resistantto the disease. Rainbow trout are less susceptible to furunculosis thanbrook trout.Fish have immunological capabilities. Under favorable circumstances fishare able to produce gamma globulins and form circulating antibodies inresponse to antigenic stimuli. They also are capable of immunologicalmemory and proliferation of cells involved in the immune response. Theimmune response of cold-blooded animals, unlike that in warm-bloodedones, depends upon environmental temPerature. Lowering of the watertemperature below a fish's optimum usually reduces or delays the period ofimmune response. Other environmental factors that stress fish also canreduce the immune resPonse.Adaptive responses to disease occur in natural populations of fish. Significantheritabilities for resistance to disease exist, and selection to increasedisease resistance in controlled environments can be useful. Intentionallyor unintentionally, specific disease resistance has been increased atmany hatcheries by the continued use of survivors of epizootics asbroodstock. Increases in resistance to furunculosis in selected populationsof brook and brown trout have developed in this way. Potential exists forgenetic selection and breeding to increase disease tolerance in all propagatedfishes but certain risks must be anticipated in any major breeding program.Under controlled environmental conditions, resistance to a single diseaseagent through a breeding program can be expected. However, simultaneousselection for tolerance of several disease agents can be extremely difficult,except perhaps for closely related forms. In any natural population,individual fish may be found that are resistant to most of the commondiseases. Pathogenicity of disease agents varies from year to year and fromlocation to location, probably as a result of environmental changes as wellas strain differences of the disease agents. When environmental conditionsare favorable for a pathogen, the fish that can tolerate its effect have aselective advantage. However, when conditions favor another pathogen,other individual fish may have the advantage. Natural recombination ofthe breeding population assures that these variations are reestablished ineach new generation of the population. Any propagation program must ensurethat this variability is protected to retain stability of the stocks.Managers always run the risk of decreasing the fitness of their stock inselective breeding programs; changes in gene frequencies resulting fromselection for disease resistance may cause undesirable changes in the frequenciesof other genes that are unrelated to disease resistance'