Table 7. — Estimated mortality rates due to Rock Island Dam based on total 4-year-old sockeye recoveries 



[95 percent confidence limits] 



years. In 1956, returns from tagging at the right 

 bank were considerably lower than those from the 

 left bank; however, the reverse was true for 1955. 

 No reasonable explanation can be offered for these 

 differences. Perhaps conditions which may have 

 caused low recoveries from the right bank one 

 season were not present the other season, or it 

 may have been chance that upstream recoveries 

 varied as they did in the 2 years. 



The data for 1956 indicated a loss of fish re- 

 leased below the right hank fishway when recovery 

 proportions were compared to recovery propor- 

 tions of fish released above the dam (19.5 percent 

 compared to 24.4 percent). In terms of mortality 



rates, in the same manner as was done for the 

 sockeye, this would indicate a 20 percent mortality 

 for fish tagged below the right bank. No mortali- 

 ties were indicated when comparing total returns 

 from below the dam with returns from above the 

 dam. 



While summer chinook tag returns are com- 

 paratively few, the recovery ratios are consistent 

 over the years and for the different areas of release. 

 Only in 1954 was there an indication of mortalities 

 for right bank tag releases (7.1 percent recovered 

 compared with 7.8 percent recovered from above 

 the dam). In the other 2 years there was no 

 indication of mortalities. There was no indication 



352 



U.S. FISH AND WILDLIFE SERVICE 



