The exceptions to the typical mortality pattern occurred at the 

 Winthrop hatchery in 1952 and 1953 (fig. 2), in the infected kokanee 

 population at the Leavenworth hatchery in 1951, and in some of the 

 troughs of sockeye fingerlings which became infected later in the summer 

 at the Leavenworth hatchery in 1953. 



The disease in the kokanee population at the Leavenworth station 

 was first recognized in one trough on May 18, 1951 (Rucker, et al ., 1953). 

 The losses increased rapidly, and the fish in this trough were destroyed. 

 On May 27, five troughs became infected, but the mortality rate did not 

 rise rapidly. The fish in these troughs, as well as the fish in 37 troughs 

 which showed no symptoms of the disease, were moved to three outside hold- 

 ing ponds on July 18. The mortality decreased, and by July 16 only 36 per- 

 cent of the kokanees had died. Just before disposal of these fish on 

 October 9, there was a significant rise in the mortality attributed to the 

 disease, and the total mortality at time of disposal was 39<>6 percent. 



The virus disease at the Winthrop hatchery caused a 145-percent 

 mortality in 1952 and a 59 oU-per cent mortality in 1953, in infected popula- 

 tions. When the disease became apparent at this hatchery in 1952, all the 

 sockeye -salmon fingerlings were confined to one pond. Shortly after the 

 onset of the disease, half of the fish were transferred to a clean rearing 

 pond and the other half to a canal. The volume of water flowing in the 

 canal allowed for a much greater flushing action than found in the rearing 

 pond. About U5 percent of the fish placed in the canal died within 30 days; 

 after that, there were no significant losses. In contrast, the mortality 

 of the fish in the clean rearing pond continued at a low level for the next 

 90 nays until the total mortality reached U5 percent; the death rate then 

 returned to normal. 



In 1953, the 90,000 fingerlings in the winthrop hatchery, reared in 

 one holding pond (pond No. 120), showed the first evidence of the disease 

 on June 20. After the disease was recognized, the fish were not moved 

 either to the clean rearing ponds or the canal as had been done the pre- 

 vious year; instead, they were retained in the original pond. Sixty-five 

 days after the recognition of the disease in the pond, 59«J4 percent of 

 the fish had died, (fig. 2). 



The majority of the 69 troughs and 8 ponds which became infected 

 at the Leavenworth hatchery in 1953 showed the initial symptoms of the 

 disease between June 15 and July 15. Late in August, however, infection 

 appeared in healthy fish adjacent to a trough in which there were diseased 

 fish, and in October 3,000 healthy fingerlings in another trough were ex- 

 perimentally infected to determine the virulence of the disease during 

 this period. The fish in the troughs in which the initial onset of the 

 disease occurred between June 15 and June 30 suffered a mortality of more 

 than 95 percent. The total mortality in the troughs infected between 



