population may resolve the difficulties mention- 

 ed above, but the data are not available at this 

 time . Rather than withhold all the information 

 in this report until scale studies of the Colum- 

 bia River bluebacK salmon are complete, the 

 ages are presented as the best determinations 

 possible at this time. 



We have little knowledge how the age 

 composition of the downstream migrants com- 

 pares with that of the adult returns . Scale 

 samples from 280 adult bluebacks of the Okano- 

 gan River 1953 run showed that of 279 fish that 

 had migrated seaward in their second year, 271 

 returned in their third year and 8 in their fourth 

 year. None had migrated seaward as I's. A 

 single 5 -year -old fish had migrated to the ocean 

 as a 3— . Adult 3 -year -olds have been observed 

 in the Okanogan River in other years; however, 

 it is known that most Columbia River bluebacks 

 mature in 4 years. Apparently the 1953 Okano- 

 gan River run was anomalous. Observers at 

 Bonneville Dam and Celilo Falls on the Colum- 

 bia River noted that the bluebacks that returned 

 in 1953 appeared to be smaller than those in 

 previous years. The downstream -migrant age 

 composition in 1952 was similar to that in the 

 other years of this study, so age composition 

 of the downstream migrants would not explain 

 this abundance of adult 3's. 



River Conditions During Modal Weeks 



As previously stated, the Bonneville 

 catches indicate that the peak migration of 

 bluebacks past the dam occurs in the spring and 

 extends for a relatively short period. During 

 this period the water temperature, river flow, 

 and turbidity are increasing, but since these 

 factors at Bonneville may not be related to the 

 same factors at Lakes Osoyoos and Wenatchee 

 their effect on migration cannot be evaluated 

 accurately . 



River flow could affect the Bonneville 

 catch data by altering conditions at the dam, 

 for the flow into the auxiliary -water screen pits 

 is not directly proportional to river flow. As 

 the river rises in the spring, and the spillway 

 gates are opened, the fingerling traps sample 

 a lesser proportion of the total flow; if the pro- 

 portion of down.'itream migrants sampled is 



inversely proportional to flow, then figures 5-8 

 may not represent the exact percentage of mi- 

 grants passing the dam each week. However, 

 when corrected catches were computed for the 

 1949-53 data by multiplying the actual catch by 

 the ratio of river flow to auxiliary-water flow, 

 the modes in figures 5-8 were not changed. 

 The corrected catches do not necessarily rep- 

 resent the true abundance of downstream - 

 migrant bluebacks passing the dam, because 

 the number of fingerlings that pass through the 

 spillway and powerhouse channels as compared 

 to the number that enter the auxiliary -water 

 screen pits is not known. 



Foerster (1937) in his work at Cultus 

 LaKe found that the temperatures during January, 

 February, and March have an effect on the time 

 of migration, and that migration does not start 

 until the water temperature reaches 40° F. 

 This condition could also affect the time of 

 migration of Columbia River bluebacks but no 

 water -temperature records are available for 

 Lakes Osoyoos and Wenatchee. A few down- 

 stream -migrant bluebacks have been caught at 

 Bonneville when the water temperature was less 

 than 40° F. 



Age Groups and Time of Migration 



The earliest appearance of unmarked I's 

 in the catch was in week 19 in 1951; the I's 

 caught that year in weeks 17 and 18 were mark- 

 ed hatchery fish. Most of the I's were caught 

 after week 25. A few continued to migrate from 

 that date until the end of the year . 



The 2's dominated the catches through- 

 out the year, but were especially abundant in the 

 peak migration period each spring. 



The 3's have a migration pattern similar 

 to the 2's but are much less abundant. The 4's 

 and 5's migrate only during the spring months. 

 May 28 (week 22) was the latest date either of 

 these age groups appeared. 



2/ Age -analysis files of the Pacific Salmon 

 Investigations, Fish and Wildlife Service, 



2725 Montlake Boulevard, 



Seattle, Washington . 



17 



