1955 SEASON 

 Number Togged - 1484 

 Number Observed - 987 



rTUKIflJk t^JW 



20 30 10 i 



JUNE JULY AUGUST 



DATE OF TAGGING AND TAG OBSERVATIONS 



SPRINGS 

  Left bank 

 Right bonk 



:S'-.W,\ 



SUMMERS 



Left bank 

 R.gtil bonk 



I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 2021 22 23 2425 

 DAYS - OUT 



Figure 17. — Numbers of salmon tagged below Rock Is- 

 land, and numbers of daily tag observations at the 

 counting stations. 



Figure IS. — Comparison of days-out at Rock Island of 

 tagged spring and summer run chinook, by area of re- 

 lease, 1956 season. 



day after tagging (fig. 18). The drawn-out time 

 period noted for many summer chinooks may have 

 resulted from errors in species identification as this 

 run coincides with sockeye, or it may be charac- 

 teristic of the summer chinook to take longer to 

 pass the dam. 



DISCUSSION 

 EXPERIMENTAL SITUATION 



In view of conflicting results for different years 

 and for different tagging areas, it is pertinent to 

 review briefly the experimental situation and 

 sources of error of the tagging experiments. 



We have assumed that the likelihood of recover- 

 ing or observing a tagged fish in a given recovery 

 area was the same regardless of the area of release 

 or time of tagging. This seems a reasonable as- 

 sumption, because recovery efforts on spawning 

 grounds were directed to all parts of the spawning 

 areas and throughout the period of spawning. 

 Tugged fish also were recorded as they passed 

 Zosel Dam on the way to the spawning grounds, 

 although sockeye could pass Zosel Dam without 



362 



passing through counting traps. At Tumwater 

 Dam, all fish, in order to pass the dam, had to pass 

 through a trap where tagged fish were easily ob- 

 served and counted. 



We have assumed also that proportions of 

 tagged fish migrating to different spawning areas 

 were the same for the different release lots. In 

 sampling fish for tagging, salmon were trapped 

 throughout the duration of the run and usually 

 during the height of the daily migration. In many 

 instances, individual truck loads of fish were 

 divided between above-dam and below-dam lag- 

 gings. Other times, successive loads of fish were 

 divided between left and right bank releases and 

 above and below-dam releases. On this basis we 

 feel that the above assumption is reasonable. 



Another factor relating to experimental control 

 was the kind of tag used for the various experi- 

 ments. All the fish released above the dam were 

 tagged with paired plastic disks. Different color | 

 combinations or single colors were used for dif- 

 ferent release groups. Tags used on fish released 

 below the dam consisted of disks of single colors 



U.S. FISH AND WILDLIFE SERVICE 



