36 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



These healthy blueback salmon were placed in 

 outside ponds for rearing during the early part of 

 June. They continued in excellent condition 

 throughout the summer and had a very low 

 mortality. 



The water temperatures increased during the 

 summer as shown in table 2: but when they began 

 to drop in the fall, the mortality rate again in- 

 creased slightly among the blueback salmon and 

 the disease reappeared. This was indicated by 

 the mortality of 0.4 percent for the 11-day 

 period ended October 5. This represented a mor- 

 tality rate more than twice that of the preceding 

 2 weeks and more than four times the rate during 

 August and the first part of September. There 

 was a progressive increase in daily mortality 

 during the last 9 days prior to planting the fish 

 on October 5. 



Table 2. — Biweekly mortality in salmon flngerlings with 

 average water temperature, Leavenworth station, 1951 



[Original number: 1,400,000 blueback salmon and 788,000 kokanees] 



Period ended 



Feb. 12. 



26. 

 Mar. 12. 



20. 



9.. 



23. 



7.. 



21. 

 June 4.. 



18. 



2.. 



16. 



30. 



13. 



27. 

 Sept. 10. 



Apr. 

 May- 



July 



Aug. 



Oct. 



24. 



5. 



9. 



Total .. 



Blue- 

 back 

 mortal- 

 ity 



Percent 



0.8 



.7 



.6 



1. 1 



.8 



1.2 



2.4 



2.5 



.fi 



.2 



.1 



.2 



.2 



.1 



1 



.1 



.2 



.4 



Kokanee 

 mortal- 

 ity 



Percent 



3.6 

 3.4 

 2.4 

 5.4 

 1.2 

 10.6 

 14.4 

 2 7 

 1.6 

 1.5 



.5 



To 



Average 



water 

 tempera- 

 ture 



' F. 



46 

 46 

 46 

 45 

 45 

 45 

 45 

 44 

 44 

 46 

 49 

 56 

 59 

 59 

 59 

 54 

 52 

 49 

 49 



Note.— Mortality is based on the percentage of fish alive at the beginning 

 of each biweekly period. Total mortality is therefore not equal to the sum of 

 the biweekly percentage mortalities. 



Oct. 5 and Oct. indicate planting dates, not biweekly periods. 



The disease among the blueback-salmon finger- 

 lings at the Leavenworth station in 1951 was dem- 

 onstrated by a number of techniques to be 

 contagious. The 1,400,000 young bluebacks were 

 held in 102 troughs arranged in pairs separated 

 by aisles. The troughs were tended by two 

 workers using alternate aisles. Thus, each worker 

 cared for the fish in the troughs on both sides 

 of alternate aisles as shown in table 3. The pat- 

 tern in which the disease spread among six 



troughs cleaned by one worker suggested an infec- 

 tious disease, probably transmitted by equipment 

 used to clean the troughs. As soon as it was sus- 

 pected that an infectious agent was present, all 

 cleaning equipment was sterilized between use in 

 individual troughs by dipping it in a disinfectant. 

 A 1 : 2.000 solution of roccal was used. It is re- 

 markable that the infection was limited to the 

 six troughs, presumably as a result of this pre- 

 caution. 



Table 3. — Course of the disease through <i troughs of bliic- 

 liiick flngerlings, Leavenworth station, 1951 



[Total of 102 troughs and 1,400,000 flngerlings) 



Appearance of Diseased Fish 



A definite diagnosis could not be made either 

 from the appearance of the sick fish or from the 

 findings at autopsy. The findings conformed to 

 a genera] pattern that might also occur in fish 

 with other diseases. 



Diseased fish appeared lethargic and did not re- 

 act to stimuli, such as moving an object over the 

 trough or jarring the trough. Normal fish would 

 dart about, but sick fish usually did not move. 

 After a few days, the abdomen was slightly 

 bulged. A few fish showed hemorrhagic areas in 

 some of the fins, at the base of paired fins, or in 

 the isthmus; and the gills were pale. 



In autopsied fish, the spleen was often light in 

 color; the stomach was distended witli a milky 

 fluid, giving the impression of pyloric constric- 

 tion: the gall bladder was filled with green bile, 

 which in fish is typical of anorexia: the intestine 

 was full of a watery, straw-colored fluid : and the 

 large intestine often was red and apparently in- 

 flamed. Fluid expressed from the anus was yel- 



