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NW Fishletter #381, May 7, 2018

[5] CSS Presentation Delves Into Study Of Dam Removal

Up to four times as many salmon and steelhead could return if four Snake River dams were breached and spill at four lower Columbia River dams was increased to 125 percent total dissolved gas, according to modeling results presented at the annual Fish Passage Center meeting April 17.

These conclusions, laid out in the center's annual Comparative Survival Study (CSS), are based on modeling used to predict survival of salmon and steelhead at different stages of life, and under different conditions. The study's findings have been used as the basis for court decisions, including U.S. District Judge Michael Simon's recent order to increase spring spill at eight Columbia and Snake river dams, which began early this month.

Bob Lessard, quantitative fisheries scientist with the Columbia River Inter-Tribal Fish Commission, made the final presentation, which provided conclusions of the study's evaluation of the relative survival and recovery benefits of different spill and breach scenarios.

These scenarios included spill levels starting at the 2014 BiOp levels, and increasing to the current spring spill at gas-cap levels--defined as spill that generates the maximum-allowed total-dissolved-gas level--and at 125 percent of total dissolved gas. Smolt-to-adult return ratios (SARs) are then calculated in different flow years, comparing the high flows of 2011 with average flows of 2009 and low flows of 2010.

"As you spill more, whether breach or non-breach, the abundance goes up by a factor of around two," Lessard told the group. "But immediately, under the breach scenario at the lower four [Snake River] dams, already everything is pushing twofold or greater. And if you combine breaching the lower four and spilling to 125 percent on the lower Columbia River dams, we're seeing in some cases a fourfold increase in abundance, and in others, a two-and-a-half [times increase] at least."

The simulations assume the harvest rate will increase to about 20 percent at and above 5,000 returns, and 20 percent of migrating juveniles will be transported past dams while 80 percent will remain in-river. If dams are removed, the model assumes no juveniles will be transported, and all will migrate in-river.

The conclusions, Lessard said, show more spill always predicts higher survival and abundance, regardless of flow, with a potential of four times the abundance, and an improvement in smolt-to-adult ratios that are two to three times what they are now. He said these SARs are in line with goals of a 2- to 6-percent rate set for salmon and steelhead recovery by the Northwest Power and Conservation Council.

The annual CSS attempts to assess salmon and steelhead survival rates across various life stages, and builds on itself each year as new data is collected and incorporated. The annual meeting with presentations comes after the final report is released and reviewed by the Independent Scientific Advisory Board, with comments and responses to them included in an appendix.

In its review of the 2017 study, which was finalized in December, the ISAB wrote that adding breach scenarios to the life-cycle model was a "nice compliment to the spill scenarios, producing interesting results. Further consideration of assumptions used in both sets of scenarios and recommendations for experiments (short of an actual breach) that could be done to test model results would be useful."

Michael Milstein, spokesman for NOAA Fisheries, said his agency is always interested in the study's findings, but "It's modeling, so it's educated guesswork." He added, "It's one prediction of what we're likely to see, but we won't know for sure unless it happens."

The study was initiated in 1996 by states, tribes and the U.S. Fish and Wildlife Service to assess the effects of hydro-system operations on hatcheries. It relies on data collected from PIT-tagged salmon and steelhead throughout the Columbia and Snake river basins.

In addition to an analysis of adult returns under different spill and dam-removal scenarios, this year's presentations reviewed many other chapters from the study and included many conclusions.

USFWS fish biologist Steve Haeseker presented information about how, increasingly, spring and summer Chinook salmon are returning to spawn at younger ages. He said Chinook mature and return to spawn generally between three and five years after migrating to the ocean. Older females, which come back at four or five years, lay more eggs and tend to be more productive, he said. On average, the majority--or about 67 percent--return at age 4, while 23 percent return at age 5 and 10 percent return at age 3, he said.

But since 2007, scientists have seen a dramatic increase in the proportion of Chinook returning to the Columbia River Basin at age 3, Haeseker said. The increase has held true for both wild and hatchery fish, and for juveniles that are transported and those remaining in-river on their way to the ocean. He said smolt-to-adult return rates and fishing below Bonneville Dam are also not associated with the younger returns. Specific causes have not yet been determined, he said, but both genetic and environmental factors appear to be drivers.

In an earlier presentation, Haeseker reported how juvenile mortality rates increase with time spent migrating downstream, noting that dams slow velocity of currents, and therefore migration time. He said that, with construction of dams, the migration time from Lewiston, Idaho, to Bonneville Dam has increased from about two days to about 20 days. The migration downstream can be even longer in low flow years, as in 2001, when it took about 32 days for fish to make the journey.

Scientists also explained other parts of the study, which looked at the impacts of temperature on survival; concluded that total dissolved gas up to at least 125 percent has no detrimental effects on fish; and validated the NWPCC's goal of between 2- to 6-percent smolt-to-adult return ratios for recovering salmon and steelhead. -K.C. Mehaffey

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NW Fishletter is produced by Energy NewsData.
Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: K.C. Mehaffey
Phone: (206) 285-4848 Fax: (206) 281-8035

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