A new analysis of PIT-tag data from upper Columbia spring Chinook found the adults experienced lower survival rates from Bonneville Dam to their tributaries than scientists expected.
Dan Rawding, Columbia River salmon recovery coordinator for the Washington Department of Fish and Wildlife, said between 50 and 70 percent of the adult spring Chinook that pass Bonneville Dam make it to their upper Columbia tributaries to spawn. "We thought it would be closer to 70 to 90 percent," he said.
Rawding presented findings of the analysis to the Northwest Power and Conservation Council's Fish and Wildlife Committee on Oct. 13. "These are among the most at-risk populations we have in the Columbia Basin," he said.
The analysis is part of the Fish Passage Center's Cumulative Survival Study, and included improvements recommended by the Independent Scientific Advisory Board in 2018.
Rawding studied 93,000 wild fish from three stocks tagged from 2009 through 2013. He said all three populations—from the Methow, Entiat and Wenatchee rivers—had relatively high abundance in the 1980s, low abundance in the 1990s, and some increases in the 2000s.
The analysis found that all three populations had smolt-to-adult return rates close to 1 percent, below the Council's 2 to 6 percent SAR goal necessary for recovery, he said.
Adult survival from Bonneville Dam to the tributary averaged 50 percent to the Methow River, 60 percent to the Entiat River, and 70 percent to the Wenatchee River. "Part of the take-home here is that the Methow is nine dams up from the mouth, and the Wenatchee's only seven dams up," he said. The difference in adult survival rates is likely due to how many dams they have to pass, and the difference in distances they have to travel, he said.
Rawding said compared with the Wenatchee and Methow fish, spring Chinook from the Entiat River had much lower survival rates during their first year in the river, before migrating to the ocean as smolts. The Entiat fish tend to spend their first winter in the Columbia River, whereas the Wenatchee and Methow fish stay in the tributaries, he said.
About 2 percent of all three populations of smolts that passed Bonneville Dam survived the ocean to return as adults to the dam, he said.
"We're pretty far from delisting for all three of these populations," Rawding said.