Airborne and aquatic organisms <, — No positive evidence was obtained 

 to incriminate either airborne or aquatic organisms as possible vectors 

 of this disease. However, no comprehensive survey was made during the 

 epizootic to determine if ther_e was a correlation between the incidence 

 of the disease and the aquatic and airborne populations. The water supplies 

 other than those originating from springs or wells (table 2) were obtained 

 from cold, fast-flowing mountain streams where the plankton population was 

 comparatively low. It was demonstrated that in at least one instance no 

 aquatic vertebrate was acting as a vector for the virus. At the Cook 

 hatchery in 1953, the water supply was obtained from a spring which origin- 

 ated approximately 100 yards from the hatchery. No vertebrates lived in 

 this water supply prior to its entrance into the hatchery. 



Eggs. - — It is possible that the disease is transmitted from the adult 

 to the finger ling sockeye salmon through the eggs. Table 3 lists the 

 rivers in which adult sockeye return to spawn and the hatcheries in which 

 their progeny were reared in 1951-53. 



Epizootics occurred at all five hatcheries although the eggs came 

 from adults from four different rivers. Three of these four rivers are 

 widely separated geographically. As shown in table 3, fingerlings became 

 infected at one hatchery, but not at another, although the original 

 source of eggs was the same for both hatcheries. 



Environmental Factors 



Water temperatures. — Seven of the eleven epizootics occurred when 

 the water temperature was between U5° and J48 F., but four epizootics 

 occurred when the temperatures were between 50° and 53° F. (table 1) . 

 Four of the epizootics occurred when the water temperatures were rising, 

 five when the water temperatures were relatively constant, and two when 

 the water temperatures were declining. 



Time of year. — Of the eleven epizootics investigated, one started 

 in March, three in April, two in May, three in June, one in July, and one 

 in August (table 1) . 



Shock. — During June and July the fish were subjected to more handling 

 and movement than earlier or later in the year. During this period the 

 fingerlings were subjected to the shock of being moved into other troughs 

 or ponds. In addition, during this period, and in some instances in early 

 May, weekly prophylactic treatments with pyridylmer curie acetate were given 

 to prevent gill disease. Possibly, the shock suffered by the fish from 

 either the moving or the prophylactic treatments was sufficient to make 

 them more susceptible to the virus disease . 



Host Factors 



Age. — Epizootics were most prevalent in 5- to 6-month-old sockeye 

 fingerlings. Six of the eleven epizootics occurred in populations of 

 this age, three in populations 3 to k months old, and two in populations 



1U 



