AN ESTIMATE OF MORTALITY OE CHINOOK SALMON IN THE 

 COLUMBIA RIVER NEAR BONNEVILLE DAM DURING THE 

 SUMMER RUN OF 1955 



BY THEODORE R. MERRELL. JR.,> MELVIN D. COLLINS,' AND JOSEPH W. GREENOUGH' 



BUREAU OF COMMERCIAL FISHERIES BIOLOGICAL LABORATORY 



AUKE BAY, ALASKA 99821 



ABSTRACT 



In 1955 the Oregon Fish Commission estimated the 

 numbers of dead chinook salmon, Oncorhynchus 

 tshawytscha, near Bonneville Dam and studied the 

 probable causes of death. 



The estimates of numbers of dead fish were made 

 from ratios of tagged to untagged floating carcasses 

 below the dam. Tagged salmon carcasses were released 

 at the dam, and the river below the dam was searched 

 systematically to recover tagged and untagged car- 

 casses. The introduced tagged carcasses and the un- 

 tagged carcasses of fish that died in the river were 

 assumed to have equal chances of recovery, provided 

 they were not too severely mutilated to be recoverable. 

 This assumption was verified experimentally. 



On June 30 and July 1, 1955, when riverflows were 

 relatively high, 1,169 tagged chinook salmon carcasses 

 were released at Bonneville Dam. Thirty-one tagged and 

 117 untagged carcasses were recovered in searches down- 

 stream from the release point. On the basis of these 

 recoveries, an estimated 4,412 summer-run chinook 

 salmon died near the dam between June 21 and July 

 10. On the basis of this estimate, 15.8 percent of the 

 total chinook salmon run died at Bonneville Dam in 

 this period. 



The numbers of floating carcasses in 1954 and 1955 

 were directly related to spillway discharge; greatest 



numbers of floating dead fish coincided with Columbia 

 River flows in excess of 7,100 c.m.s. At Bonneville Dam 

 fall chinook salmon runs have never been subjected 

 to flows above 7,100 c.m.s. (killing flows); spring runs 

 are exposed to such flows in some years; and summer 

 runs nearly always encounter such flows. Water tem- 

 perature, turbidity, disease, and injuries from gill nets 

 did not affect the number of carcasses. Although the 

 specific causes of death and the precise areas at Bonne- 

 ville Dam where death occurred were not determined 

 in our study, the major source of chinook salmon 

 mortality was associated with the spillway during high 

 flows. Other investigators subsequently demonstrated 

 that during high flows the Columbia River that has 

 plunged over dam spillways is supersaturated with 

 atmospheric nitrogen. This supersaturation may be 

 one of the principal causes of death of fish at main- 

 stem dams. 



Bonneville Dam is only about 18.3 m. high, and hun- 

 dreds of thousands of salmon successfully negotiate 

 the fishways each year; yet many salmon are killed 

 during periods of high flow. Complacency about the 

 efficiency of salmon passage over large dams is, there- 

 fore, unwarranted, even when elaborate well-designed 

 passage facilities are present and few dead or injured 

 fish are noticed. 



The problem of facilitating passage of anad- 

 romous fish over dams and evaluating the effects 

 of dams on the fish has become a matter of in- 

 creasing importance and concern because of the 

 steady increase in the number of dams in the past 

 2 decades. Each dam is an impediment to migra- 

 tory fish. 



The Columbia River, one of the greatest rivers 

 in the world for chinook salmon, Oncorhynchus 

 tshawytscha, now has 11 dams across the main 

 stem and numerous dams across tributaries. 



Published February 1971. 



Bonneville Dam, the first dam on the lower 

 Columbia River, was completed in 1938; it is 227 

 km. from the river mouth. Before its construction, 

 anadromous fish migrated with little difficulty 

 into the upper Columbia and Snake Rivers and 

 their tributaries to spawn. The lower river had 

 two natural barriers — Cascade Rapids and Celilo 



' Theodore R. Merrell, Jr., Fishery Biologist, Bureau of Commercial 

 Fisheries Biological Laboratory, Auke Bay, Alaska 99821. He was Aquatic 

 Biologist with the Oregon Fish Commission at the time of the study. 



2 Melvin D. Collins, Aquatic Biologist, Fish Commission of Oregon 

 Research Laboratory, Clackamas, Oreg. 97015. 



3 Joseph W. Greenough, Biometrician, Bureau of Commercial Fisheries 

 Biological Laboratory, Auke Bay, Alaska 99821. 



FISHERY BULLETIN: VOL. 68, NO. 3 



461 



