DISEASE OF SALMON 



39 



trough showed signs of the disease. It appeared 

 that the disease was kept somewhat under control 

 after this time by sanitary measures and the fish 

 were put in outside ponds on July 18. The dis- 

 ease became less prevalent in midsummer but in- 

 creased in severity in the fall. It is possible that 

 the decreased mortality in summer was related to 

 the increase in water temperatures. Table 2 lists 

 the water temperatures i range, 44° to 59° F.) for 

 the period February 12 to October 9, and gives 

 the biweekly mortality data. The daily mortal- 

 ity rate showed a definite increase during the 

 latter part of the last 2-week period, although the 

 table shows only the biweekly mortality. The 

 characteristics of the diseased fish indicated that 

 the disease was definitely approaching an epi- 

 zootic stage when they were planted, having 

 undergone a 39.6-percenl mortality attributed to 

 this disease. 



Appearance of Diseased Fish 



The kokanees were approximately 3 months old 

 and about 1 inch long when the epidemic began 

 in May. As a group, the larger fish appeared to 

 sutler greater mortality than those of average or 

 small size. Signs id' the disease in the kokanees 

 were similar to those in the bluebacks. The fish 

 were apathetic: their gills were pale: some showed 

 a flush of red at the base of the fins or on the 

 operculum; and some exhibited popeye (exo- 

 phthalmos). The spleen was often pale; hemor- 

 rhagic areas on the air bladder or necrotic areas 

 in the musculature near the vent or dorsal area 

 were found in some: and the gastrointestinal 

 tract contained a watery fluid. The diseased fish 

 appeared black from above while normal finger- 

 lings were olive green. 



At the end of the season spinal curvatures were 

 not as frequent in the 2-inch kokanees as they 

 were in the 4-inch bluebacks previously ment ioned. 

 About 0.1 percent of the total population of ko- 

 kanees developed scoliosis, mainly during the 

 period July to October. In the fall, when the 

 mortality rate increased, fish with retracted heads 

 became common. In figure 3, the characteristic 

 posture is shown. The lateral view shows how 

 the head was retracted. The dorsoventral dis- 

 tance was increased remarkably just posterior to 

 the orbital area. 



Experimental Transmission 



The infectious nature of the disease among the 

 kokanees was demonstrated in the following ex- 

 periment. Twenty thousand kokanees from the 

 last trough to show signs of the disease were divid- 

 ed equally between two adjacent troughs. One 

 group served as the control. For a period of 2 

 weeks the other group was treated every other day 

 for 1 hour with 1 : 100,000 pyridyhnercuric ace- 

 tate. There was no diminution of the mortality 

 rate. 



Figure 3. — Normal (upper) and diseased (lower) 2-inch 

 living kokanee Qngerlings from the Leavenworth 

 station. The diseased Bsh shows the retracted head. 



In another experiment, two adjacent troughs 

 were each stocked with 10,000 healthy kokanee-. 

 One trough was cleaned with equipment also used 

 on the troughs containing diseased kokanees, the 

 other with separate equipment disinfected with 

 roccal. About 12 days after initiation of the 

 experiment an increased mortality was noted 

 among the fish in the trough cleaned with con- 

 taminated equipment. During the next 4^ 

 hours, an additional large number of fish became 

 sick, showing characteristics of the disease. The 

 disease did not appear in the second group, and 

 the experiment was terminated at this time. The 

 infectious nature of the disease affecting the ko- 

 kanees was thus shown to he similar to that affect- 

 ing the blueback-salmon fingerlings. 



