July 1 and July 15 was 90 percent, and the total mortality in the troughs 

 in which fish became infected between July 15 and July 30 was 70 to 80 

 per cento The trough in which fish became infected in August had a 37- 

 percent mortality, while the trough in which fish became infected in 

 October had only a 22-percent mortality (fig. 3). 



Distribution of Infected Fish in Hatcheries 



The incidence of epizootics was greater in ponds than in troughs 

 (see table l). When an epizootic occurred in a hatchery where finger- 

 lings were being reared in ponds and troughs, the infection was manifest 

 in all ponds, but usually not in all troughs. The only hatchery where 

 fish were reared in several ponds was at the Leavenworth hatchery in 

 1952 and 1953> where all ponds became infected. No sequence was observed 

 in the order in which they became infected to indicate that the virus was 

 being disseminated from diseased to healthy ponds. However, the virus 

 could have been spread easily between ponds by scavenging birds which 

 were observed dropping moribund fish. 



Rarely did all the fish in all of the troughs become infected, 

 Rucker, et al ., (1953) reported that when the disease was first observed 

 at Leavenworth in 1951 only the fish in six troughs became infected, al- 

 though sockeye fingerlings were being reared in 102 troughs. In 1952, 

 when the epizootic occurred all of the fingerlings were being reared in 

 rearing-ponds, and the fish in all ponds became infected. However, at 

 Leavenworth in 1953 a large number of fingerlings were being reared in 

 132 hatchery troughs and 8 ponds when the epizootic occured in fish in 

 69 of the troughs and all of the ponds* In the same year at the Cook 

 hatchery, the six troughs which became infected were in the center of the 

 hatchery. At this hatchery it appeared that the virus was being dissemin- 

 ated from diseased to healthy troughs. At the Entiat hatchery in 1953? 

 the majority of the diseased troughs were at one end of the hatchery (see 

 figo h) s while at the Leavenworth hatchery, in that same year, most of the 

 infected troughs were in two of the six sections of the hatchery where 

 sockeye fingerlings were being reared (see fig. 5). The order in which 

 the troughs became infected at these two hatcheries did not indicate that 

 the disease was spread from diseased to healthy troughs. 



Symptomatology 



During an epizootic most of the fish appeared normal until just before 

 death. Two to ten hours before they died the fish became sluggish and were 

 easily caught — some of the fish became dark in color. Approximately 10 

 percent of the fish became side-swimmers, apparently the result of a partial 

 paralysis. The left-handed swimmers swam in circles to the left and the 

 right-handed in circles to the right. The fish in this condition became 

 very excitable on occasion and dashed around wildly in an erratic manner, 

 often jumping out of the water. A spasm of this nature usually lasted h$ 

 seconds; then, as if exhausted from the sudden burst of activity, the fish 



