RETURNS FROM UPSTREAM 

 Sockeye Salmon 



It was necessary that days-out of tagged fish at 

 upstream points be measured at a point where a 

 minimum of mixing of (1) chronological order of 

 movement and (2) delay might occur. Zosel Dam, 

 on the Okanogan River, approximately 145 river 

 miles above Rock Island Dam, appeared the best 

 place for measuring the period of migration of 

 tagged sockeye salmon. The sockeye passing 

 this dam were headed for Lake Osoyoos and the 

 spawning areas above the lake. They passed the 

 dam easily, either through fishways or through the 

 spillway. Unfortunately, not many tagged fish 

 were released in the forebay at Rock Island in 



1954, the year many sockeye and tags were 

 observed at Zosel Dam. In 1955 and 1956, larger 

 samples were released above the dam at Rock 

 Island but most fish passed Zosel Dam unobserved. 

 These data, at Zosel Dam, however, did allow us 

 to compare the migration period of different lots 

 of tagged fish. 



Tumwater Dam on the Wenatchee River, the 

 only other fish-passage observation point, pre- 

 sented a delay and mixing problem which pre- 

 cluded a reliable comparison of time periods for 

 different tagged lots. At this dam, all fish had to 

 find and ascend the ladder to pass the dam, and 

 this offered chances for mixing and delaying of 

 various parts of the salmon runs. The data ob- 

 tained at Tumwater Dam are given for comparative 

 purposes. 



Figure 12 shows the days-out period at Zosel 

 Dam for sockeye released at three areas at Rock 

 Island. Looking at the modal values, we note a 

 2-day difference between forebay and left bank 

 below the dam releases for both 1954 and 1955. 

 The right bank modes, however, show a 3-day 

 difference for 1954 and a 4-day difference for 



1955. This would indicate right bank releases 

 were delayed longer than left bank releases. The 

 mean values do not indicate this difference be- 

 tween banks. Since the means are influenced by 

 extreme values, however, the modal values should 

 be preferred when making comparisons, provided 

 the data are sufficient for forming smooth distri- 

 bution curves. The number of observations leave 

 something to be desired, but they indicate a delay 

 of from 2 to 4 days at Rock Island, and a greater 

 delay for right bank releases than left bank 

 releases below the dam. 



Figure 12. — Number of days out at Zosel Dam of tagged 

 sockeye released at three different areas at Rock 

 Island, 1954-55. 



At Tumwater Dam, the difference in days-out 

 between above- and below-dam experiments aver- 

 aged 2 to 3 days in 1954 and 1955 (table 16). We 

 observed that for these 2 years right bank releases 

 took longer to pass than left bank releases. The 

 1956 results may have been affected by changed 

 flow patterns at Tumwater Dam, the entire river 

 was spilled over the dam and no water was di- 

 verted for power production as in previous seasons. 

 The great range in individual days-out of tagged 

 sockeye is indicated by the large standard devia- 

 tions shown for the data. 



Chinook 



Our only upstream measure of the days-out 

 period of tagged chinook salmon was obtained 

 at Tumwater Dam. All other tag recoveries 

 were from the spawning grounds or hatcheries, 



Table 16. — Means and standard deviations of days-out at 

 Tumwater Dam of tagged sockeye released at different areas 

 at Rock Island Dam, 1954-56. 



[Figures in parentheses are standard deviations] 



LOSS AND DELAY OF SALMON PASSING ROCK ISLAND DAM 



359 



