.«**■£ 



Figure 2b. — Rock Island Dam after modification. 



Dam on the Wenatchee River; (2) Zosel Dam on 

 the Okanogan River; (3) Okanogan River spawn- 

 ing grounds above Zosel Dam; and (4) all other 

 areas combined. 



In addition to the recovered tagged fish, nu- 

 merous tagged sockeye were observed, but not 

 recovered, in the recovery areas. These could be 

 assigned to tagging date and release location, 

 because different color combinations of tags were 

 used for different release date-location combina- 

 tions. They could not be individually assigned 

 to age groups, however, hence, it is necessary to 

 estimate the age composition of the tagged fish 

 observed. 



Spawning ground surveys of the Columbia 

 River tributaries above Rock Island Dam indi- 

 cated that 3-year-old sockeye migrate only to the 

 Okanogan system, where they are found in vary- 

 ing numbers and proportions. On the basis of 

 these surveys, together with the absence of re- 

 coveries of tagged 3-year-old sockeye in recovery 

 areas 1 and 4, it is reasonable to assume that all the 



tagged fish observed in recovery areas 1 and 4 

 were 4-year-old sockeye. 



The age composition of the tagged fish observed 

 in recovery areas 2 and 3 can be estimated from 

 proportions of 3-year-old sockeye in the recoveries 

 from areas 2 and 3. Multiplying the proportions 

 of 3-year-old sockeye in the recovery samples by 

 the number of tagged fish observed provides 

 estimates of the numbers of 3-year-old sockeye 

 among the tagged fish observed. The numbers 

 of 4-year-old sockeye among the tagged fish 

 observed are obtained by subtraction. Table 3 

 shows the estimated numbers of 3- and 4-year-old 

 sockeye observed in recovery areas 2 and 3. 



In a number of instances, tagged sockeye 

 recovered at Zosel Dam (recovery area 2) and 

 released again after being checked for tag number 

 and tag color were recovered a second time on the 

 spawning grounds above Zosel Dam (recovery 

 area 3). It is likely, therefore, that other tagged 

 fish recovered or observed at Zosel Dam also were 

 observed or recovered in recovery area 3. It is 

 likely also that some tagged fish observed on the 



342 



U.S. FISH AND WILDLIFE SERVICE 



