Early investigations on seaward migrating Snake River spring Chinook using PIT tag technology indicated that direct dam-related mortality could not explain the magnitude of the very poor adult returns, and this led to the conclusion that delayed mortality might be expressed somewhere in the lower river, the estuary, or coastal ocean. However, because survival beyond the hydrosystem was impractical to measure at that time, this hypothesis remained untestable until the development of large-scale acoustic telemetry arrays.
Location of Kintama acoustic arrays, 2009. Bathymetric and topographic data courtesy of the Government of Canada and NOAA.
In order to test the delayed mortality hypothesis two smolt populations were chosen: the Snake River population and the Yakima River population. The Snake River Chinook salmon population migrates through a total of 8 hydro dams while the Yakima River population enters the Columbia River 17 km upstream of the confluence of the Columbia and Snake Rivers, and only migrates through the four lower Columbia River dams. The Yakima River population was chosen for comparison because the smolt to adult return rate from 1999 to 2006 was on average four times greater than Snake River spring Chinook from the Dworshak Hatchery. Our study allows us to compare the survival of two populations that have a common migration path through the lower Columbia River, but where a significant difference between the two populations is in the additional passage of four dams and in-river migration.
Our results over several years have found no evidence that passage through additional dams caused delayed mortality in Snake River smolts relative to Yakima River smolts after migration past Bonneville Dam, but rather indicated that Snake River smolts had higher survival in the ocean than Yakima River smolts.
Overall, the segment-specific survival estimates are not depressed for Snake River smolts relative to Yakima River smolts and Kintama has found no support for the delayed mortality hypothesis. Our 2009 results, shown here, are similar to previous years. The survival disparity observed in adult return rates to the Snake River may thus develop later in the marine life history phase.
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2006-2009 BPA Multiyear report
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