Table 12. — Returns of tagged fish at Rock Island Dam by 

 ladder, 1953-56 



1 Tagged members included Chinook, sockeye, and steelhead. 



Rock Island Dam. An illustration was the Bonneville 

 experiment (app. p. 366) where a distinctive copper 

 and black disk tag was used ; only 6 of these tags 

 were reported at Rock Island, but 14 were re- 

 covered upstream. In view of these examples, 

 it was undoubtedly true that the counters missed 

 tagged fish. 



Tagging operations have caused mortalities. 

 Schaefer (1951) concluded that there was a serious 

 differential mortality among the tagged and un- 

 tagged fish during the long migration between the 

 Harrison trap and the Birkenhead River. Nelson ' 

 found a differential mortality between tagged and 

 untagged sockeye migrating between Karluk 

 River weir and weirs on the tributary spawning 

 streams of Karluk Lake. 



The loss of tags from tagged fish apparently was 

 not a reason for the missing tags at Rock Island 

 Dam. No tag-scarred salmon were reported by 

 the counters at Rock Island Dam, and neither 

 were any caught in the traps, though many 

 tagged fish were captured there. 



Tagged salmon may have refused to reenter the 

 fishways after tagging. In 1956, five tagged 

 salmon were observed in the Oregon Fish Com- 

 mission's trap at McNary Dam, located approxi- 

 mately 160 river-miles downstream from Rock 

 Island Dam. Also, a tagged sockeye was observed 

 at the Redfish Lake weir in Idaho, about 700 

 river-miles from Rock Island Dam. Howard 

 (1948) reported that handling during tagging 

 operations at Cultus Lake, British Columbia, 

 apparently caused some of the tagged sockeye to 

 remain in the area immediately above the tagging 



location, rather than continue their migration. 

 Undoubtedly some of the missing tags at Rock 

 Island Dam could be attributed to straying. 



Finally, mortalities could be caused by the dam. 

 Mortalities were indicated for some experiments 

 but not for others when comparing upstream 

 recovery ratios from tagged fish released above and 

 below the dam. 



Returns by area oj release. — The results of tag 

 returns by ladder are shown in table 13. These 

 data show a consistent pattern of passage over the 

 ladders for the 3 years and for the two release 

 areas. Only 6 to 12 percent of the fish used the 

 right ladder. 



The observed differences in returns from the two 

 banks over the 3 years would indicate that right 

 bank tag releases were not as successful in passing 

 the dam as left bank releases. 



Table 13.- 



-Tag returns by area of release and ladder, 

 1954-56 



i Nelson, Philip R., unpublished data 1947 and 1948. U.S. Fish and Wild- 

 life Service, Seattle, Wash. 



i Unidentifiable tags were excluded. 



Since almost all of the fish were trapped in the 

 left ladder or in the forebay trap at the head of 

 this ladder, and since they were released at both 

 banks below the dam, it was important to know 

 whether these fish had learned the route for their 

 second passage of the dam. Accordingly, the tag 

 returns were listed by trapping site and by area 

 of release (table 14). Regardless of where the fish 

 were trapped or released, they returned in approxi- 

 mately the same ratio over the three ladders with 

 the least returns always at the right ladder. It is 

 evident that the salmon did not learn a route and 

 then repeat this route for a second passage over 

 the dam. Neither were they necessarily 

 frightened away from a ladder, since the majority 

 of fish returned to the left ladder, where most were 

 trapped previously. 



LOSS AND DELAY OF SALMON PASSING ROCK ISLAND DAM 



357 



