Table 14. — Tag returns by ladder of salmon trapped at both ladders and released at both banks below Rock Island Dam, 1954-55 



Year and trapping area 



Area released 



Tagged 



Observations by ladder ' 



Center 



Right 



196i 



Right ladder... 



Left ladder 



Left ladder 



1966 



Right ladder 



Left ladder 



Left ladder 



Right bank 

 Right bank 

 Left bank . . 



Right bank 

 Right bank 

 Left banlc.. 



Number 

 178 

 748 

 923 



363 

 430 

 691 



Number 



56 



331 



354 



169 

 142 

 302 



Percent 

 64.4 

 57.9 

 55.9 



72.5 

 65.7 

 63.4 



Number 

 20 

 172 

 187 



49 

 42 

 104 



Percent 

 23.0 

 30.1 

 29.5 



21.0 

 19.4 

 21.8 



Number 

 11 

 69 

 92 



Percent 

 12.6 

 12.1 

 14.5 



6.4 

 14.8 

 14.7 



1 The percentage by ladder is based on total observations. 



Tagged and untagged fish movements. — Table 15 

 shows the comparison of choice of ladders between 

 tagged and untagged salmon lor each season of 

 tagging. Chinooks and sockeye were combined 

 because of the error of confusing chinooks (jacks) 

 with the sockeye. The percentage returns at the 

 right ladder agreed closely with those of the un- 

 tagged run. A somewhat larger proportion of the 

 untagged run, compared with the tagged fish, 

 chose the left ladder for passage. The percentage 

 returns at the middle ladder were greater than the 

 proportion of untagged fish at this ladder. This 

 may be because right bank releases, failing to enter 

 the right ladder, were attracted by the center lad- 

 der before the left ladder. Generally speaking, 

 however, the tagged and untagged salmon were 

 similar in their patterns of passage. 



During the years of the tagging experiments, 

 the four spillway regulating gates nearest the 

 right ladder were kept closed so the high-velocity 

 flows would not interfere with fish approaching 

 along the right bank. In 1957, in order to test 

 their influence on fish passage, these gates (one 



Table 15. — Combined chinook and sockeye counts of the 

 tag returns and the untagged runs by ladder, 1954-56 



or more) were opened when most of the fish passed 

 the dam. The percentage of chinooks and sockeye 

 using this ladder in 1957 increased to 24 percent, 

 compared to an average of 15 percent for 1953-56. 

 This was the pattern of fish passage at the right 

 ladder by species — chinooks, 19.8 percent; sockeye, 

 26.8 percent; and steelhead, 16.6 percent. No 

 tagging was done in 1957, and a comparison of 

 tagged fish movements could not be made. 

 Apparently the salmons' choice of ladders can be 

 influenced by manipulating the regulating gates 

 and by the resulting changes in flow patterns 

 below the dam. 



DETERMINATION OF DELAY AT 

 ROCK ISLAND 



In this paper delay means the period of time 

 that Rock Island Dam checks the migration of 

 salmon under present conditions. 2 Delay may 

 cause salmon mortalities prior to spawning. Some 

 causes of delay may be flood waters in rivers, low 

 flows, high water temperatures, areas of difficult 

 passage (as in precipitous canyons), or dams in 

 rivers. Delay here is measured as days elapsing 

 between release of tagged fish and subsequent 

 observations at the counting boards. The delay 

 also is assessed by comparing, at upstream points, 

 the time of arrival of tagged fish released above 

 and below the dam. The difference in time of 

 arrival at an upriver point would be a measure of 

 delay at the dam, and this difference should 

 correspond to the delay observed at Rock Island 

 Dam. It will be shown that this was true. 



2 An argument advanced is that studies of delays at dams sriould consider 

 comparisons of travel rates before and after a dam is built. This has seldom 

 been accomplished; subsequent to this study, comparisons of travel rates 

 were made before and after Rocky Reach dam was built. Major, Richard 

 L. and James L. Mighell. A study to measure delay to upstream migrating 

 salmonids at Rocky Reach Dam. Manuscript. Bureau of Commercial 

 Fisheries Biological Laboratory, Seattle, Wash. 



358 



U.S. FISH AND WILDLIFE SERVICE 



