NATURAL -REARED 

 SALMON 



In order to compare the incidence of 

 tuberculosis in wild fish with that in hatchery- 

 reared fish, a small number of fish of likely 

 natural origin were examined in 1956 and 1957. 

 Areas where adult fish of likely natural origin 

 could be obtained in Oregon proved to be some- 

 what limited. It cannot be safely assumed that 

 the adult fish in a particular stream are of 

 natural origin because no hatchery -reared fish 

 were released there, since hatchery-reared 

 fish are widely distributed and the homing in- 

 stinct of the salmon is not an infallible one. 



As can be seen in table 12, no evidence 

 of tuberculosis was found in these two groups of 

 wild fish. On the basis of these data it may be 

 anticipated that tuberculosis is a comparatively 

 rare event among stocks of salmon of natural 

 origin that are removed from areas where hatch- 

 ery-reared fish are liberated. In areas where 

 hatchery-reared fish are liberated the possibility 

 must be considered that the disease may become 

 established in natural populations as a result of 

 the upstream migration and spawning of tuber- 

 culous adult salmon originating in hatcheries . 

 It is obvious that further work must be done in 

 order to establish evidence on this particular 

 point . 



Two particular areas presented the best 

 apparent opportunity to obtain significant num- 

 bers of wild fish for examination. These were 

 Tenmile Lakes for silver salmon and the Snake 

 River in Idaho for fall chinook. Tenmile Lakes 

 (fig. 1) support a run of silver salmon generally 

 in excess of 50,000 yearly. Except for a small 

 number of marked fish planted in 1956, no lib- 

 erations of hatchery-reared salmon have been 

 made in Tenmile Lakes in recent years. Pos- 

 sible straying of hatchery-reared fish from 

 other streams into Tenmile Lakes are probably 

 "diluted" to a great extent by the large number 

 of silvers migrating naturally into Tenmile 

 Lakes. 



It is highly probable that fall chinook 

 salmon spawning in the main stem of the Snake 

 River in the area below Swan Falls Dam (see 

 fig. 1) are exclusively of natural origin. No 

 liberations of hatchery-reared fall chinook have 

 been made in the Snake River, in fact, the near- 

 est tributary of the Columbia where liberations 

 of hatchery-reared fall chinook are made is lo- 

 cated several hundred miles downstream from 

 Swan Falls. 



The results of the examination of salmon 

 from Tenmile Lakes and the Snake River are 

 shown in table 12 . Silver salmon from Tenmile 

 Lakes were obtained on fie spawning grounds of 

 the tributary streams in December 1956. Most 

 of the fall chinook from the Snake were obtained 

 on the spawning grounds in the vicinity of Swan 

 Falls Dam in November 1957; an additional small 

 number was obtained from anglers in the vicinity 

 of Brownlee Damsite in September and October 1957. 



LITERATURE CITED 



H. Spaulding, and H.W. 



Kolmer, J. A., E. 

 Robinson 

 1951. Approved Laboratory Technic. 

 Fifth edition. Appleton -Century- 

 Crofts, Inc., New York, xii 

 1, 180 pp. 



Wood, J. W. 



1958. Tuberculosis in Pacific salmon 



and steelhead trout. Master's 

 Thesis, University of Washington, 

 75 pp. 



Wood, J. W., and E. J. Ordal 



1958. Tuberculosis in Pacific salmon 



and steelhead trout . Fish Commis- 

 sion Oregon., Contr. No. 25 



Table 12. — Incidence of Tuberculosis in 



Natural-reared salmon examined 

 in 1956 and 1957 



(Swan Falls)104 

 (Brownlee 



Damsite) 13 



33 



