BLUEBACK SALMON, ONCOHYNCHUS NERKA 

 AGE AND LENGTH AT SEAWARD MIGRATION PAST 

 BONNEVILLE DAM 



INTRODUCTION 



The Columbia River blueback salmon 

 ( Onchorhynchus nerka) is a valuable commer- 

 cial species. One of the best blueback runs in 

 recent years was in 1947, when an estimated 

 338,000 fish returned to the river to spawn. 

 The commercial catch tasen from the river 

 that year was 219, 000 bluebacks, worth $664,000 

 to the canners. The 1946-53 average annual 

 value was $290,000 (Pacific Fisherman, Year- 

 book Number, 1954); the 1949 pack was omitted 

 from the average because it included bluebacks 

 that were not native to the Columbia River . 



Fluctuations in the number of adults 

 returning to spawn (in 1945 the total run num- 

 bered 10,600 fish) and virus infections in the 

 hatcheries (Rucker et al., 1953) in recent 

 years have focused added attention on the man- 

 agement of this resource The age and length 

 of bluebacks at the time of seaward migration 

 are of general interest and add to the knowledge 

 of the life history. Also, knowledge of the time 

 of migration and of lengths of downstream mi - 

 grants will be of value when young salmon are 

 guided past dams. The purpose of this report 

 is to provide this information for Columbia 

 River blueback salmon . 



Bluebacks spawn principally in streams 

 that flow into lanes. The fry descend to the 

 lakes soon after hatching, and remain there 

 until they migrate to the ocean. The age at 

 which Columbia River bluebacKs migrate sea- 

 ward varies, but most migrate as 2 -year-olds 

 or as yearlings. At the present time, Wenatch- 

 ee and Osoyoos are the principal populations of 

 this species (see fig. 1). 



Bonneville Dam, 140 miles above the 

 river's mouth, and below all blueback spawning 

 areas, is well located for obtaining general in- 

 formation on migrant blueback, and in the 

 future the dam may contribute valuable informa- 

 tion for conservation and management of the 

 fishery. The Bonneville catch data are not 



usable at the present time as an index of abun- 

 dance of returning adults, but should be usable 

 when it is determined how conditions at Bonne- 

 ville Dam affect the fingerling-trap catches. 

 No tests have been made to determine whether 

 the proportion of downstream migrants caught 

 by the fingerling traps fluctuates during the 

 year, but it is suspected that the efficiency of 

 the traps varies . 



The recovery at Bonneville of marked 

 bluebacks released by hatcheries gives a good 

 indication of the time and rate of migration of 

 hatchery releases, if it is assumed that marked 

 and unmarked fish are similar in these respects. 

 The survival of releases is difficult to deter- 

 mine from the Bonneville catches because the 

 fish released in the spring and those released 

 in the fall may be distributed differently with 

 respect to the rest of the population, and be- 

 cause conditions at Bonneville Dam are not 

 constant. These problems are discussed on 

 pages 17 and 33. No attempt is made in this 

 report to compute survival rates of blueback 

 releases, but the number of marked bluebacks 

 caught at Bonneville is listed later in the 

 report. 



The following took part in the collection 

 of data for this report: R.L. Bagwell, C.J. 

 Burner, D.R. Craddock, H.A. Hanson, C.C. 

 Jensen, B.A. Lehman, E.M. Maltzeff, L.G. 

 Schedin, L.C. Schlotterbeck, KG. Weber, 

 R.C. Wilson and P. D. Zimmer. K.H.Mosher 

 assisted in scale reading. 



METHODS 



Collection of samples 



Random samples were taken from the 

 fingerling traps on 5 working days each week, 

 except that in the spring when large numbers 

 of salmonids were migrating downstream it 

 was necessary to tend the traps on weekends. 



