5 15 25 5 15 25 



APR. MAY 



Figure 11. — Secchi disk visibility in the Columbia River at Bonneville Dam, 1950-55. 



maintenance operations have sometimes neces- 

 sitated shutting down portions of the fish-passage 

 facilities, but these have rarely affected passage 

 for long. 



Many improvements have been made in the 

 fish passage facilities at Bonneville Dam over the 

 years. Among them are alteration of the power- 

 house collection system, addition of auxiliary 

 attraction flows at the entrance to the ladders, 

 installation of flow baffles below the ladder en- 

 trances, and installation of barrier screens below 

 the spillway bays adjacent to the ladder en- 

 trances. 



In 1946 a submarine viewing chamber made of 

 a section of large steel pipe with the ends sealed 

 off and with two watertight windows, was used 

 by the U.S. Army Corps of Engineers and FWS. 

 A series of observations of migrating fish were 

 made by an observer inside the chamber, which 

 was lowered into a fish ladder. Fish within the 

 ladder experienced no difficulty in moving through 

 the ladder (Hanson et al., 1950). Other observa- 

 tions through the years have shown that once 

 fish have entered the ladders they generally pass 

 through with little or no injury or delay. 



Because there is no evidence that fish are in- 

 jured in the ladders, we eliminated this possible 

 source of mortality from further consideration. 



DISEASE 



Another possible cause of salmon mortality is 

 disease. In 1954 and 1955, to determine the causes 

 of death we collected all of the recently killed 

 fish we could find below the dam. 



None of the fish collected in 1954 were suffi- 

 ciently fresh to warrant a detailed examination, 

 although gross injuries of external origin were 

 obvious in some instances (table 8). 



In 1955, we recovered three chinook and six 

 sockeye salmon, four white sturgeon, one shad, 

 and two carp in fresh condition (table 8). Seven of 

 the fish (two chinook and five sockeye salmon) 

 were immediately frozen and later autopsied by 

 Edward M. Wood, fish pathologist at the FWS 

 Fish Nutrition Laboratory, Carson, Wash. In 

 summarizing the results of his examinations. 

 Wood said: 



"In some cases these fish had severe in- 

 juries which clearly caused death. In these 

 instances we have attempted to determine if 

 there were any contributing factors which 

 might have made the fish more susceptible 

 to injury such as disease. At other times, how- 

 ever, there was no gross evidence of injury 

 other than slight internal hemorrhage, con- 

 gestion of various organs with blood, or 



CHINOOK SALMON MORTALITY IN COLUMBIA RIVER NEAR BONNEVILLE DAM 



487 



