Table 7. — Comparalive mortality ofchinook salmon at Bonneville Dam in spring, summer, and fall periods between 191*3 and 1956 



C.m.8. 



Spring: 



May 1-31, 1952 8,700 



April 20 to May 10, 1965 1,050 



Summer: 



July 1-31, 1954 7,900 



June 20 to July 22, 1955 9,020 



June 20-29, 1956 10,500 



Fall: 



Sept. 1-15, 1943 620 



Sept. 1-30, 1946 140 



Sept. 1-20, 1950 480 



Sept. 1-15, 1954 1,160 



' A commercial fisherman reported "thousands' 



of dead Chinook salmon below the dam. 



The "high" and "low" mortality ratings are 

 subjective classifications based on the number of 

 chinook salmon floaters relative to the average 

 daily chinook salmon counts at Bonneville Dam. 

 Major differences in search techniques and in the 

 lengths of search periods make detailed compari- 

 son between most of the periods of dubious value. 

 Only for our study in 1955, and for a brief followup 

 study in 1956, can we be certain that search 

 techniques were comparable. We, therefore, gave 

 the greatest weight to data for these years in 

 reaching our conclusions. Data for earlier years 

 were useful, however, in lending support to the 

 conclusions. 



In spring 1955, when spillway flows and water 

 temperatures were low and numbers of salmon 

 were high, only 0.009 chinook salmon floater was 

 found per hour of search. Later the same year 

 (midsummer), when spillway flows averaged 9,000 

 c.m.s., water temperatures were higher, and num- 

 bers of salmon were low, 0.322 chinook salmon 

 floater was found per hour of search. 



A pattern of characteristic circumstances ac- 

 companying high mortality periods was evident: 

 floating dead salmon were likely to be especially 

 numerous below the dam when either high spill- 

 way flows or exceptionally large numbers of salmon 

 occurred. If high flows and large numbers of 

 salmon occurred simultaneously, even greater 

 numbers of floating dead salmon could be pre- 

 dicted downstream from the dam. The data in 

 table 7 generally substantiate this conclusion. 



After completing our study in 1955, we wished 

 to test the hypothesis that large numbers of 

 chinook salmon would die and float near Bonne- 

 ville Dam whenever large numbers of fish and 

 high spillway flows coincided. Late June 1956 had 

 such a coincidence. 



From June 20 to 29, spillway flow at the dam 

 averaged 10,500 c.m.s. and coincided with chinook 

 salmon counts over the dam averaging over 4,300 

 per day. We predicted that at the prevailing 

 water temperatures of 13.3° to 15.6° C. floaters 

 from a given day's mortality would appear down- 

 river about 7 days later and a large number of 

 floaters would be evident below the dam by about 

 July 3. On July 3, during 1 hour and 50 minutes 

 of search at the Cape Horn station, five chinook 

 salmon carcasses were observed; on July 5, 14 

 carcasses were seen during 4 hours and 40 minutes 

 of observation. In terms of numbers of chinook 

 salmon floaters recovered per hour these numbers 

 were greater than for any previous observation 

 period (except September 1943), indicating, as 

 predicted, that serious mortality had occurred. 

 The cause of the high mortality of September 

 1943 is unknown. 



FLOW 



We were unable to determine which specific 

 conditions at Bonneville Dam contribute to salm- 

 on deaths, but evidence is strong from our study 

 and from observations in earlier years that high 

 mortality in spring and summer occurs during 

 high flows. The annual peak flow of the Columbia 

 River at Bonneville Dam is in May or June, 

 depending on the time of maximum snowmelt in 

 the upper watershed; the peak flow dates from 

 1938 to 1955 fell between May 11 and June 28. 

 Flows of more than 8,500 c.m.s. may occur for 

 up to 3 months. 



The maximum combined flow capacity of the 

 10 power turbines at Bonneville Dam is about 

 4,100 c.m.s. when the Columbia River is at flood 

 stage. At low riverflows, maximum turbine capac- 

 ity is about 3,400 c.m.s. This means that flows 



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