The high-density rearing of trout and salmon and their large-scale transplantation in the egg, fry or smolt stages give rise to conditions suited for the transmission and spread of various diseases and environmental hazards, like the toxic algal blooms of Chrysochromulina polylepis, that developed on the Norwegian coast in 1988. Many of the diseases described are those diagnosed from trouts and salmons. The most common virus diseases are infectious pancreatic necrosis (IPN), infectious haematopoietic necrosis (IHN) and viral haemorrhagic septicaemia (VHS). Among the bacterial diseases are furunculosis, bacterial gill disease, enteric red mouth disease and vibriosis. The whirling disease caused by the protozoan parasite Myxosoma cerbralis infects almost all species of salmonids, particularly the young. Costiasis and icthyophthiriasis (ich or white spot disease) are also quite common. Infections by the copepod Argulus and the trematode Gyrodactylus do not cause direct mortality,but adversely affect the growth rate and marketability of the fish.
The diagnosis of these diseases and preventive and curative measures are discussed. The absence of any feasible curative measures for some of these diseases, particularly the viral infections, emphasizes the need for scrupulous observance of high levels of sanitation in salmonid culture establishments. Many of the diseases cause large-scale mortalities in fry and fingerlings and this clearly indicates the need for special care during hatchery and nursery operations. The high value of salmonids makes it possible to adopt immuno-logical techniques, especially in the case of viral diseases, for which there are no known cures. Regulatory measures that are implemented in some countries to control the spread of infectious diseases of salmonids should serve as a major means of protecting the very important salmonid aquaculture industries that are developing in many parts of the world, especially in the northern hemisphere.