fying most tags upon which the days-out data were 

 derived, the occurrence of tagged salmon, regard- 

 less of tag color or kind, was plotted by date and 

 compared with the total number of salmon tagged 

 (fig. 17). An examination of the graphs reveals 



Table 17. — Modal values in days out for each tag release 

 area and ladder of passage 



(Data from tips. 14, 15, and 10) 



Ladder 



Right 



3 



4-5 



that the peaks and troughs of the tag occurrence 

 curve occur 2 to 4 days after the start and finish of 

 a tagging period. This 2- to 4-day lag compares 

 with the days-out curve of figure 13 and indicates 

 the counters were approximately correct in identi- 

 fying most of the tags from which the days-out 

 data were derived. 



Undoubtedly the difference in time required for 

 salmon released above and below Rock Island 

 Dam to reach an upstream point reflects the 

 amount of time required for the below-dam releases 

 to pass the dam. Thus, it is reasonable to con- 

 clude that tagged sockeye were delayed 2 to 4 

 days below Rock Island Dam. Assuming that 

 tagged and untagged salmon migrate at similar 

 rates, Rock Island Dam delays sockeye approxi- 

 mately 2 to 4 days. 

 Chinook 



At Rock Island Dam spring chinook peaked 

 after 1 day-out and summer chinook on the third 



RIGHT BANK BELOW 



- Left ladder 

 Center ladder 

 Right ladder 



RIGHT BANK BELOW 

 — Left ladder 



Center ladder 

 -- Right ladder 



I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 2021 222324 25 

 DAYS— OUT 



9 10 II 12 13 14 15 16 17 18 19 2021 22 23 

 DAYS-OUT 



Figure 15. — Days-out of tagged sockeye at Rock Island 

 for each area of release, expressed^ in percentages of 

 total observations by ladder, 1955. 



Figure 16. — Days-out of tagged sockeye at Rock Island 

 for each area of release, expressed in percentages of total 

 observations by ladder, 1956. 



I.OSS AND DELAY OF SALMON PASSING ROCK ISLAND DAM 

 7T4-711 O— 66 5 



361 



