FinnRE 4. — Lowering tagged fish off the deck of the dam. 



In table 7 are listed the estimates of mortalities 

 based on all 4-year-old sockeye recoveries. 



Compared with the proportions of tagged fish 

 recovered from the area AL (left bank above the 

 dam) releases, tagged fish released on the left bank 

 below the dam suffered mortalities of about 10, 6, 

 and 16 percent in 1954, 1955, and 1956, respec- 

 tively. Tagged fish released on the right bank 

 below the dam suffered mortalities of about 22, 

 12, and 18 percent in the corresponding years. 

 We would conclude from these point estimates 

 that tagged fish released below the dam suffered 

 a mortality due to the dam, and that the mortality 

 rate was greater for fish released on the right bank 

 below the dam than for fish released on the left 

 Imnk below the dam. 



Compared with the proportions of tagged fish 

 recovered from the AK (right bank above the 

 dam) releases, tagged fish released on the left bank 



below the dam suffered no mortality due to the 

 dam in either 1955 or 1956. Neither did tagged 

 fish released on the right bank below the dam. 

 The point estimates for 1956, it should be recog- 

 nized, are based on a release of only 16 fish in area 

 AR. 



In combining the data for the 3 years, we could 

 conclude, on the basis of recoveries from area AL 

 releases, that tagged fish released on the left bank 

 below the dam suffered a mortality of about 16 

 percent because of the dam, and that tagged fish 

 released on the right bank below the dam suffered 

 a mortality of about 21 percent because of the 

 dam. Little or no mortality was indicated on the 

 basis of releases in area AR (0 and 2 percent). 



When comparing proportions of tagged fish 

 recovered from total releases below the dam with 

 all releases above the dam, we obtained point 

 estimates of mortalities due to the dam of about 

 15 percent in 1954, 4 percent in 1955, 15 percent 

 in 1956, and 15 percent for all years. 



Figure 5. — Kinds of tags used during the Rock Island 

 tagging experiments. 



■MS 



U.S. FISH AND WILDLIFE SERVICE 



