Williams and Matthews: Flow and survival relationships for Oncorhynchus tshawytscha 



739 



<s 3 April through 26 June > 



Figure 4 



Comparison of 1973 and 1992 Snake River flows during the spring juve 

 nile salmonid seaward migration. 



Table 2 



Changes in average runoff (in thousands of cubic feet per 

 second) in the Columbia River at The Dalles Dam. 



May 



June 



July 



1926-58 

 1959-75 

 1976-91 



283 

 250 

 206 



353 

 344 

 199 



224 

 211 

 132 



hydropower corridor, but past research definitely 

 showed that low flows increased travel time (Ray- 

 mond, 1979). Certainly adult returns of spring and 

 summer chinook salmon have been greatly reduced 

 in most years since the mid-1970's (Matthews and 

 Waples, 1991). This poor return has coincided with a 

 substantial decrease in lower Columbia River flows 

 during the late spring and early summer smolt mi- 

 grations as a result of completion of storage reser- 

 voirs in the upper Columbia River Basin (Table 2). 

 The decreased volume of freshwater entering the 

 estuary and ocean may have delayed entry offish to 

 the ocean or may have changed the ecology of the 

 system sufficiently to affect predator-prey interac- 

 tions above and below the trophic level of the juve- 

 nile migrant fish and, thus, their survival. 



The 1970's juvenile survival estimates made by 

 NMFS in average-to-high flow years also reflected 

 the effects of debris for the proportion of the popula- 

 tion that passed through the juvenile collection and 

 bypass systems. These survival estimates are prob- 

 ably lower than those for present passage conditions, 

 even when one accounts for the installation of new 

 bypass systems at dams where they did not exist in 



the 1970's. Thus, they do not apply to 

 present-day migrants in the Snake and 

 Columbia River hydropower system, 

 and we recommend they not be used by 

 modelers, unless substantial modifica- 

 tions are made to adjust for the errors 

 that we have discussed. We also recom- 

 mend continued emphasis on research 

 to provide up-to-date survival estimates 

 under present system conditions, be- 

 cause the data gathered to date do not 

 cover a wide range of flows. 



Acknowledgments 



We thank the staff of the Northwest 

 Fisheries Science Center for many help- 

 ful editorial suggestions. We particu- 

 larly thank Steve Mathews from the University of 

 Washington whose helpful suggestions led to sub- 

 stantial revisions to the manuscript. 



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