the fact that the two groups of fish became diseased at different 

 times; the second group had fewer infected troughs than did the first 

 group of fingerlings. Possibly, this differential was due to the fact 

 that each group was being fed viscera from different lots. Also, it 

 might be that diseased viscera was fed in Hay to the first group, while 

 the second group was not fed infected viscera until late in June or early 

 July. Moreover, since the fish in more of the troughs in the first group 

 became infected than in the second group it is entirely possible that more 

 diseased viscera was fed to the first group than to the second. 



Since it was impossible in this investigation to recover the virus 

 directly from the viscera, and because all troughs of sockeye fingerlings 

 in any one hatchery did not become infected, it may be assumed that all 

 batches of the viscera were not contaminated with the virus. Moreover, 

 it may be assumed that the food was not completely homogenized before 

 feeding and that the infectious agent was irregularly distributed in the 

 food. 



No evidence has been obtained to suggest that the virus is trans- 

 mitted from the adults to the fingerling salmon through the egg. Records 

 were not available to allow the following of eggs from one female or 

 several females to determine whether there was any indication that the 

 virus was transmitted in this manner. 



Likewise, no strong evidence has been obtained to indicate that the 

 virus was introduced into a hatchery population through aquatic or airborne 

 organisms. It is believed, however, that aquatic vertebrates do not act as 

 vectors in this disease. This conclusion is based on the evidence that no 

 vertebrates were present in the water source at one hatchery during the 

 epizootic and the failure to demonstrate experimentally that species of fish 

 other than 0^_ nerka are susceptible or can act as vectors. 



Although it is quite possible that an aquatic vertebrate was not act- 

 ing as a vector in this disease, it is conceivable that either aquatic or 

 airborne insects may have been serving as vectors. However, since the 

 rivers from which the waters were obtained were widely separated geograph- 

 ically, it seems highly questionable whether a common insect would be found 

 in all of these water supplies. 



It would be expected that if an aquatic or airborne population were 

 acting as a vector in this disease, and if all troughs in a hatchery did 

 not become infected, the infected troughs would be distributed at random 

 throughout the hatchery. This was not the case. 



Why the majority of the epizootics started during the period from 

 May through July is not known. If the virus was introduced into the 

 hatchery populations through the eggs or the food, it would be expected 

 that the incidence of disease during these months would be no higher than 



32 



