percent ascended the left ladder, while 16 and 

 9 percent ascended the middle and right 

 ladders, respectively (table 4). In 1964, per- 



centages of Chinook (jacks excluded) which 

 used the left, middle, and rights ladders were 

 about 51, 28, and 21, respectively. 



Table 3. — ^Maximum daily counts, salmon and steelhead, 

 Rock Island Dam, 1965 



Table 4-. — Semimonthly number and percent of chlnook salmon counted over each fish ladder, Ptock 



Island Dam, 1965 



Chinook Jacks 



Some chinook return from the ocean as 

 mature fish in the year following their sea- 

 ward migration and are referred to as "jacks." 

 These fish are small, and males predominate. 

 At Rock Islaind Dam in 1965, all chinook 22 in. 

 (inches) or less in length were counted as 

 jacks. Each counting board has a nnarked 

 section, 22 in. long, for use of the counter in 

 gaging the length of these small chinook. The 



maximum daily count of 557 jacks occurred 

 on September l4 (table 3), 2 days later than 

 in 1964. The 17,113 fish recorded as jacks 

 constituted about 47 percent of the total 

 chinook run as compared to about 36 percent 

 in 1964, In 1965, about 81, 10, and 9 percent 

 of the jacks used the left, middle, and right 

 ladders, respectively (table 5), as compared 

 to about 74, 14, and 12 percent in 1964. 

 Monthly totals are given in table 2. 



