EPIZOOTIOLOGY 

 Record of Epizootics 



In 195l> epizootics occurred in the sockeye-salmon populations 

 at both the Leavenworth and iinthrop hatcheries, and in the kokanee- 

 trout population at the Leavenworth hatchery (table l) . At Leavenworth 

 the disease in the sockeye populations was restricted to 6 of 102 troughs, 

 but at the Winthrop hatchery it was present in all sockeye troughs; at 

 Leavenworth the disease was manifest also in 15 of the 37 kokanee troughs. 

 Almost 100 percent of the infected sockeye populations died, but only 

 39»6 percent of the infected kokanees. 



The seriousness of the disease became increasingly apparent in 1952 

 when three of the five hatcheries (Leavenworth, Winthrop, Cook) reported 

 epizootics. The entire sockeye population became infected at each of 

 these hatcheries, and the mortality at both Leavenworth and Cook exceeded 

 90 percent. Altogether, the number of sockeye fingerlings lost at these 

 three stations during 1952 totaled 2,368,000, which represents a loss of 

 91.5 percent of all the sockeye fingerlings reared at these hatcheries 

 between June and September, and more than 35 percent of all sockeye finger- 

 lings reared in the State of Washington. 



The epizootics were considerably more widespread in 1953 } when the 

 disease occurred at all five of the hatcheries that raise sockeye salmon 

 in the State of Washington (see table 1). A total of 2,053,000 fish died 

 during the epizootics, representing a loss of 65 percent of all the sock- 

 eye fingerlings reared in the State that year; the losses probably would 

 have been more serious had not UUl,000 fingerlings been released before 

 the onset of the disease. 



Mor tality P att ern During an Epizootic 



Under normal conditions, the average daily mortality of sockeye salmon 

 fingerlings reared in a hatchery is about 0.01 percent of the total popula- 

 tion. During epizootics the daily mortalities increased rapidly and in 

 some populations reached 32 percent within 20 days after the initial in- 

 crease in mortality. The cumulative mortality pattern was similar, in the 

 majority of the epizootics, to that described by Watson (I95u) for infected 

 ponds at the Leavenworth hatchery in 1952. At this hatchery the daily 

 mortality rose to 1 percent a day within 6 to 9 days after the onset of 

 the disease. The cumulative mortality reached 50 percent within 16 to 22 

 days after the disease was first seen. Within 25 to UO days after the on- 

 set of the disease, 90 percent of the fish were dead. 



