7 to 8 months old. No information is available concerning epizootics in 

 fingerlings reared under natural conditions, or in sockeye salmon from 

 the time they are released from the hatcheries until they return to spawn. 



Size. — -The onset of the disease occurred in populations in which 

 the usual length of the fish was between 1 and h inches. Five of the 

 epizootics occurred in fish 2 to 3 inches long, three in fish 1 to 2 

 inches long, and one in fish 3 to k inches long. At the Leavenworth 

 hatchery in 1953, the fish were segregated into two size groups. In one 

 group the fish were 1 to 2 inches long, and in the other group 2 to 3 

 inches long. When the mortality rates of infected populations, each with 

 a different average size, were compared no significant differences could 

 be found. However, in any infected population, in general, the majority 

 of the survivors were the smaller, weaker fish. 



Heredity . —This disease has not been limited to one race of sockeye, 

 but has occurred in the progeny of adults which returned to spawn in four 

 different rivers. Moreover, the disease has occurred in fish of this 

 species which spend their entire life cycle in fresh water as well as in 

 other fish of this species which spend only the first 12 to 2u months of 

 their life cycle in fresh water and most of the next 2 years in salt water. 



Although the disease was manifest in four different races, the 

 mortality was significantly less in one race in 1952 and 1953- Although 

 most of the fish from the native stock at Winthrop, reared at that hatchery, 

 died in the epizootic of 1951, only U5 to 55 percent of the fish from the 

 native stock died during the epizootics at Winthrop in 1952 and 1953. 



EXPERIMENTAL WORK 



Objectives 



To support the epizootiological findings we initiated an experi- 

 mental program, but with limited personnel and time we realized we had 

 our choice of concentrating our efforts on one small facet of the problem 

 or conducting a broad exploratory program, the results of which would 

 guide in directing future research in the most profitable direction. We 

 accepted the broad general approach for the present set of experiments, 

 hoping it would reveal facts which would allow us to concentrate upon more 

 specific facets in future investigations. 



The objectives of our program were to determine the effects of the 

 chemical, physical, and environmental factors on the disease, and also, 

 to investigate the basic nature of the etiological agent and to determine 

 the virulence and specificity of the agent and the immunities of blueback 

 salmon and other species of fish. 



16 



