FISHERY BULLETIN: VOL. 70, NO. 1 



Table 10. — Tag release and observation data for spring chinook salmon seen passing 

 over fish ladders at Rock Island Dam, 1954 and 1955.' 



Dote 



Tagging 

 location 



Number of 

 fisli fagged 

 and released 



Number of fagged fish observed 



Left 

 ladder 



Center 

 ladder 



Right 

 ladder 



Total 



1954 



June 23-25 

 Juno 29-July 2 

 July 7-9 

 July 13 

 July 16-22 

 July 29-30 



1955 



June 7-9 



June 14-15 

 June 17-21 



June 28-29 

 June 30-July 1 

 July 5-6 

 July 7-8 



Right bonk 

 Right bonk 

 Right bank 

 Right bank 

 Left bank 

 Right bank 



Left bank 

 Right bank 

 Right bank 

 Left bank 

 Right bank 

 Right bank 

 Right bank 

 Left bank 

 Right bank 



26 

 36 

 57 

 26 

 3 

 7 



8 



14 



13 

 9 



19 



15 

 19 

 17 

 43 



6 

 5 

 7 

 15 

 3 

 1 



3 



7 

 7 



10 

 6 



10 

 3 

 8 



16 



3 

 2 



1 

 2 

 

 





 1 



1 







1 





 2 

 3 

 1 



7 

 2 

 5 

 

 

 1 



1 

 1 

 2 

 2 

 2 





 2 

 1 



4 



ily 7-8 Right bank 43 16 1 4 



' The total number observed may exceed the number tagged. See text (p. 132) for explanation. 



16 

 9 

 13 

 17 

 3 

 2 



4 

 9 



10 

 12 

 9 



10 



7 

 12 

 21 



Table 11. — Travel time of tagged spring chinook salmon 

 from tagging areas below Rock Island Dam to the Rock 

 Island fish ladders. 1954 and 1955. 



Date 



1954 



Tagging 

 location 



Mean travel time in half-days, 

 all ladders combined 



marked and repeatable difference in distribution 

 by ladder between the postencroachment years, 

 1964 and 1965, and the preencroachment years, 

 1954 and 1955. The center ladder took a dis- 

 proportionate share of the fish in 1964 (at the 

 expense of the left ladder), but this was less 

 pronounced in 1965. 



Travel Time from Release to Observation in 

 Fish Ladders 



Apparently no large-scale mortalities and no 

 great losses of tags were caused by encroachment 

 (these assumptions are supported by the high 

 percentages of tagged fish subsequently seen 

 passing Rock Island Dam). The overall eflfect 

 of encroachment is then best measured by com- 

 paring the travel times between the pre- and 

 postencroachment tagging studies. Although 

 we have already shown that the 1965 travel time 

 (5.8 half-days) was significantly less than the 

 1964 travel time (9.0 half-days), this difference 

 does not overshadow the fact that both values 

 are well below the comparable figures (12.4 and 

 16.0 half-days) for 1954 and 1955, respectively. 

 We can only conclude that travel time of spring 

 chinook has decreased markedly since encroach- 

 ment. 



SOCKEYE SALMON 



Results of the 1954 and 1955 tagging studies 

 with sockeye salmon are given in Table 12. Com- 

 parable data for 1964 and 1965 are presented in 

 Table 5. 



138 



