Table 2. — Time observers spent searching in boats for floating 

 salmon carcasses at eight stations on the Columbia River 

 near Bonneville Dam, April i to July 22, 1955 



(See text for definition of search areas) 



Station and month 

 (station number 

 In parentheses) 



River 



site 



River 

 vicinity 



Shore 



Total 



Hr. Min. Hr. Min. Hr. Min. Hr. Min. 



72 10 1 40 36 74 25 



65 15 1 45 67 



- 88 15 88 15 



Total 215 40 1 40 2 20 219 40 



The Dalies (8) 



May 



June 



July 



MolTett Creek (7) 



April... 



June 



12 35 

 11 10 



12 35 

 11 10 



Total. 



23 45 



23 45 



McGowan (6) 



April 



May 



June 



July. 



17 40 12 25 30 5 



63 50 25 5 1 10 80 5 



44 20 26 50 1 60 73 



69 30 23 50 24 15 107 35 



Total 175 20 88 10 27 15 290 45 



Cape Horn (5) 



June 



July.... 



1 40 10 ., 2 40 



79 60 28 50 1 20 110 



Total. 



Reed Island (4) 



April 



May.. 



June 



July 



81 30 29 60 1 20 112 40 



68 15 13 6 81 20 



78 6 12 10 25 90 40 



96 40 28 15 7 5 132 



47 25 33 15 8 25 89 6 



Total 290 26 86 45 15 55 393 6 



Ellsworth (3) 

 July... 



Total. 



Willamette (2) 



April.. 



May.. 



June 



July.. 



66 45 15 15 4 40 76 40 



56 46 15 15 4 40 76 40 



85 30 85 30 



101 45 101 45 



94 65 1 30 45 97 10 



92 35 2 35 7 30 102 40 



Total 374 46 4 6 8 15 387 6 



St. Helens (1) 



May 



June 



75 40 9 



67 40 10 60 



84 40 

 78 30 



Total 143 20 19 60 



Grand total. 1,361 30 245 36 59 45 1,666 60 



The greatest numbers of floaters were found 

 below the dam in July: between the dam and 

 station 2, 182 chinook and 53 sockeye salmon 

 floaters were recovered (0.40 chinook and 0.12 

 sockeye salmon per hour of search). Between 

 July 1 and 15, 23,034 chinook and 199,095 sockeye 

 salmon were counted over the dam. Thus, al- 

 though counts of chinook salmon were declining, 

 the rate of recovery of floaters in early July was 

 much greater than in earlier periods — nearly 

 double the next highest rate in June. Counts of 

 salmon at the dam and floaters in the river were 

 both declining by July 22 when the study ended. 



Searches above the dam were limited to station 

 8 from May 3 to July 22. Chinook salmon floaters 



found per hour of search were: May, 0.0; June, 

 0.04; and July, 0.17. 



Our experience in 1954 and 1955 demonstrated 

 that floaters could be effectively counted by 

 observers in boats. 



Table 3. — Chinook and sockeye salmon counted over Bonne- 

 ville Dam, April 1 to July 15, 1955, and number found as 

 floaters per search hour at stations below dam, April 1, to 

 July 22, 1955 



Salmon counted Floaters found ' Floaters per hour 



Time period 



Chinook Sockeye Chinook Sockeye Chinook Sockeye 

 salmon salmon salmon salmon salmon salmon 



Number Number Number Number Number Number 



April 1-30 84,436 4 0.02 



May 1-31 85,769 77 8 .02 



June 1-15 11,551 68 13 .07 



June 16-30 33,951 21,855 45 .23 



July 1-15... 23,034 199,095 182 53 .40 6.12 



* Floaters found and search time spent on shore search area (table 2) not 

 included. 



SEARCHES FROM AIRCRAFT IN 1954 AND 1955 



Synoptic observations of floaters in the Colum- 

 bia River were needed to determine if at any given 

 time the numbers of floaters were greater below 

 dams on the river than above. The aerial survey 

 method was chosen because Merrell had seen 

 carcasses of chinook salmon on low-altitude 

 spawning survey flights on the Columbia River 

 and the upper Snake River in Idaho — a distance 

 too great to cover with boat searches. At the 

 time of our study the Columbia River had only 

 two dams, Bonneville and McNary (237 km. up 

 the Columbia River from Bonneville Dam). 



A chartered two-place single-engine, high-wing 

 monoplane with a cruising speed of about 120 km. 

 per hour was used for all flights. The pilot sat in 

 the rear seat, and a biologist in the front seat, from 

 which point the visibility was excellent. A blind 

 spot directly below the aircraft when it was in 

 level flight did not significantly hamper observa- 

 tions. Merrell and Collins made all observations. 

 Altitude varied between 15 and 100 m. The river 

 was searched on both the upstream and down- 

 stream flights. Over sections where the river was 

 wide, observations were confined to one side of 

 the river on the first trip and to the opposite side 

 on the return trip; less than half the total river 

 surface could be observed in these sections. In 

 narrower sections, particularly between The 

 Dalles and McNary Dams, a greater portion of 

 the river surface, and at times the entire width of 



CHINOOK SALMON MORTALITY IN COLUMBIA RIVER NEAR BONNEVILLE DAM 



467 



