Factors Associated with Travel Time and Survival of Migrant Yearling Chinook Salmon and Steelhead in the Lower Snake River

Abstract

Simple and multiple linear regressions were used to evaluate factors associated with travel time and survival of yearling chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss migrating in the lower Snake River. Factors were release date and environmental variables measuring river discharge (flow), water temperature, and the percentage of total flow passed over spillways at dams. Data were collected from migrant salmonids tagged with passive integrated transponder (PIT) tags from 1995 through 1999. The greatest distance over which survival could be estimated during all 5 years was from the Lower Granite Dam tailrace to the McNary Dam tailrace (225 river km encompassing four dams and reservoirs). Release groups consisted of PIT-tagged fish leaving Lower Granite Dam daily. Data from more than 451,000 PIT-tagged yearling chinook salmon and 204,000 PIT-tagged steelhead were analyzed. For each daily group, indices of exposure to environmental factors were calculated as the average value for the factor during an index period of the group's downstream passage. For both species, flow volume and travel time were strongly correlated within single years, and the regression equation was consistent from year to year. Survival estimates changed very little within any given migration season, despite sometimes great fluctuations in environmental factors. Correlations between river discharge and survival between Lower Granite Dam and McNary Dam and between travel time and survival were neither strong (within or between years) nor consistent from year to year. Thus, survival benefits to the stocks from increased flow in this stretch of the river were at best minimal; any measurable benefits occurred downstream from the Snake River.