(mean 154 mm.). The bypass-trap sample aver- 

 aged 23 mm . longer than the hatchery sample . 

 The 193 -mm. blueback from the above release l. 



had scale markings that made it appear a 3 . 

 The loss of the combination of fins listed above 

 could have been accidental, but the evidence 

 seemed conclusive enough to include this fish 

 with the 2's, in spite of the age indicated by 2 . 



scale study. 



In every case the average lengths of the 

 marked bluebacks caught at Bonneville were 

 greater than the samples sent by the hatcheries, 

 even though the hatchery samples extended over 

 a wide range of lengths and two age groups . Un- 

 fortunately, no data are available that can be 4. 

 compared with the Bonneville catches to test the 

 traps for selectivity. However, the fingerling 

 traps catch all sizes of migrants, and there is 

 no evidence that they are size selective . Possib- 

 ly the samples selected by the hatcheries were 5. 

 biased, and were of a smaller size than the mean 

 size of the releases . The fish released in the 

 spring may have added the growth necessary to 

 compensate for the differences in length between 

 the time of release and the time of recapture at 

 Bonneville. Another possibility is that the larg- 

 er fish of the releases had a better survival 6. 

 rate than the smaller fish. 



The separation of the hatchery and wild 

 fish by inspection of length -frequency graphs of 

 the trap catches is not possible at this time. 

 More data on lengths and abundance of the popu- 

 lations in LaKes Osoyoos and Wenatchee and of 

 the hatchery fish would be necessary. Few 

 hatchery fish were released in the fall of 1952. 

 The catches in 1953 showed that those fish 

 measuring from 115 to 130 mm. were much 

 scarcer than in other years. This length group 

 may be composed of fall-released hatchery fish. 7. 



More fall releases of marked bluebacks into 

 Lakes Osoyoos and Wenatchee would be helpful 

 in separating hatchery fish from the wild fish 

 caught at Bonneville Dam . 



SUMMARY AND CONCLUSIONS 



Ages of blueback downstream migrants 

 caught at Bonneville Dam ranged from 1 

 to 5 years; 93 percent were 2 -year -olds 

 or in their second year. 



The Bonneville catches indicate that the 

 bulk of downstream -migrant bluebacks 

 pass the dam in April and May. 



3 . The lengths of migrants of each age 

 overlap; thus estimation of age from 

 length is not possible for all fish. 



Length -frequency graphs show no con- 

 sistent separation of the races of 

 Columbia River bluebacks at the time 

 they pass Bonneville. 



More marked bluebacks were caught at 

 Bonneville from spring than from fall 

 releases from Leavenworth and Winthrop 

 hatcheries. The reverse was true of a 

 spring and fall release from Little White 

 hatchery . 



The rate of travel of marked bluebacks 

 varied. The fastest migrated 400 miles 

 in 8 days. The slowest, from the same 

 hatchery release, took 877 days to reach 

 Bonneville . Limited data indicate that 

 fish released in the spring migrate 

 immediately, whereas those released in 

 the fall do not migrate until the following 

 spring. There is some evidence that fall 

 releases from Little White reach Bonne- 

 ville sooner than fall releases from 

 Leavenworth and Winthrop. 



The Bonneville catches are not usable to 

 forecast abundance of returning adults 

 without additional data . 



35 



