suspected, therefore, that there might be a dif- 

 ferential take of tuberculous fish since diseased 

 fish, especially those heavily infected, are pos- 

 sibly not as able to free themselves from the 

 nets as non-diseased fish. If such a differential 

 existed in 1957, however, its importance is 

 minimized when it is considered that an estimated 

 45 percent of the April and May spring-run Chin- 

 ook, 35 percent of the summer -run chinook, 45 

 percent of the fall -run chinook, 45 percent of the 

 blueback, and 40 percent of the summer-run 

 steelhead entering the Columbia were caught by 

 the gill -net fishery. Since the bulk of the es- 

 capement through the fishery occurs during the 

 closed seasons, the actual proportions of fish 

 caught that were present in the river during the 

 open seasons are probably much higher than 

 those indicated above . 



An influencing factor of possibly greater 

 importance than a differential catch of tubercu- 

 lous fish in obtaining an accurate estimate of 

 the incidence of tuberculosis in fish entering the 

 river is that fish for the survey could only be ob- 

 tained during the open fishing seasons. Another 

 factor of possible influence is that tuberculosis 

 apparently influences the time of migration of 

 chinook salmon, in particular, into the Columbia 

 (Wood and Ordal, 1958). Tuberculous fall chin- 

 ook have been found to enter the Columbia dur- 

 ing almost any month of the year. The disposition 

 of abnormally early migrants after they enter the 

 river is generally unknown and it is quite possible 

 that they are exposed to the fishery for a longer 

 than normal period of time previous to entering 

 the tributary streams. If so, these particular 

 fish are probably sampled out of proportion to 

 the normal migrants entering the river . More 

 data are necessary before the degree to which 

 these abnormally early fall chinook migrants af- 

 fect the observed incidence of infection in chinook 

 caught during the spring and summer seasons . 



Due to the sampling of fish in the river 

 only during the open fishing seasons, the results 

 of the survey in 1957 are listed in this report as 

 the incidence of tuberculosis in the landings of 

 the gill -net fishery. In view of the above, this 

 is the most accurate estimate available of the 

 incidence of the disease in fish entering the Col- 

 umbia in 1957. 



Chinook salmon 



The sampling of chinook salmon com- 

 menced during the week ending May 18, thus 

 that portion of the spring run entering the river 

 during the open fishing season in February and 

 during the first two weeks of the season opening 

 April 30 was not sampled. The data on tuber- 

 culosis in chinook are shown in table 1 and are 

 divided to cover the three races. Weekly samples 

 of varying size were obtained in each full week of 

 open season. Livers were usually obtained from 

 fish caught in all statistical zones (fig. 1) with 

 the bulk of the sampling effort being concentrated 

 on Zones I and II near the mouth of the river, the 

 area where most of the chinook are usually taken. 

 Since the number of fish caught and the size of 

 the sample varied from week to week during the 

 open seasons, the sampling data in table 1 are 

 weighted by the number of fish caught during the 

 week to give the estimated tuberculous number 

 caught by week and by race . 



It may be seen in table 1 that an estimated 

 4.9 percent of the spring- run chinook, 2.9 percent 

 of the summer-run chinook, and 5.6 percent of the 

 fall -run chinook taken by the fishery in 1957 were 

 tuberculous. A direct comparison of (he percent- 

 ages is not thought to be entirely valid due to the 

 previously mentioned migration of tuberculous 

 fall chinook into the river during the spring and 

 summer seasons . It may also be noted in table 1 

 that the comparatively small landings during the 

 weeks ending August 3 and 10 are accompanied 

 by higher than average rates of infection. Again 

 in late October and November small landings are 

 associated with comparatively high rates of in- 

 fection. This would further indicate that the time 

 of migration of many tuberculous chinook is some- 

 how different than that of non-diseased chinook. 



In table 8 are Listed the number of chinook 

 examined, by race, according to the relative 

 abundance of acid-fast bacilli in the smears. 



Liver samples were taken from a large 

 portion of the marked fall chinook (fish marked 

 as juveniles by excision of fins and/or maxillary 

 bones) that were observed in the gill-net fishery 

 landings during August to November by biologists 

 of the Oregon Fish Commission and the Washing- 

 ton State Department of Fisheries. Data on marked 



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