The artificial propagation of salmon starts with the capture of adults as they ascend rivers to spawn. After capture, they are retained in specially constructed holding areas until sexually mature. Though holding periods vary with species, the spring chinook (Oncorhynchus tshawytsca) may be held as long as 4 months before they are spawned. During this interval the fish are often closely confined at unfavorable water temperatures and fatal diseases sometimes reach epizootic proportions. It is important that methods be devised to control or prevent infectious diseases in adult salmon while they are in the holding ponds.
Mass treatment is the most convenient way to combat fish diseases. For example, drugs can be administered per os in diets, or chemicals can be added to the water. These methods are mostly ineffective in treating systemic infections of adult salmon because mature salmon do not feed, and many fish diseases cannot be controlled by chemical baths. Thus, effective treatment would require administering drugs to each individual.
