Figure 2a. — Rock Island Dam before modification. 



placed in the counting rooms for reference but 

 counters were not told of the tag applied each day. 



Upstream tag recoveries were made from spawn- 

 ing ground surveys, at fish hatcheries, and from 

 fish counters' observations at Tumwater and 

 Zosel Dams (fig. 1). As different colors and com- 

 binations of tags were used for each experiment, a 

 tag observed on a live fish provided the same infor- 

 mation as an actual recovery, except for the 

 identity of the particular fish. Therefore, up- 

 stream tag recoveries include both visual observa- 

 tions of tags and actual recoveries. 



To determine mortalities at the dam, we ana- 

 lyzed and compared the percentage tag returns at 

 Rock Island from releases below the left and right 

 bank ladders; for upstream tag returns we com- 

 pared recovery ratios from the different release 

 areas above and below the dam. In addition, 

 percentage tag returns from the two release areas 

 below the dam were compared by date of tagging 

 and by volume of water flow at Rock Island. To 

 determine delay we computed the elapsed day-out 

 (number of days between tagging and subsequent 



tag observations) periods of tag returns at Rock 

 Island; for upstream returns we computed the 

 difference in day-out periods for releases above and 

 below the dam. 



ESTIMATION OF TOTAL UPSTREAM 

 RECOVERIES OF TAGGED SOCKEYE 



Upstream tag recoveries and observations for 

 the 1954-56 individual sockeye experiments at 

 Rock Island are given in table 2. The numbers of 

 tagged fish released are given by date, location, and 

 age group. Releases at the four locations in the 

 vicinity of the dam were designated as follows: 

 below dam, left bank; below dam, right bank; 

 above dam, left bank; above dam, right bank. 

 The two age groups represented in the releases of 

 tagged fish were 3-year-old sockeye, recognized by 

 their small size, and sockeye over 3 years old. 

 Most of the fish in the latter group were 4 years 

 old; hence, this group is designated as 4-year-old 

 sockeye. 



The recoveries from each release lot are given by 

 area of recovery above the dam: (1) Tumwater 



LOSS AND DELAY OF SALMON PASSING ROCK ISLAND DAM 



341 



