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Snake River Hatchery Juveniles Same Early Marine Survival as Lower Columbia Fish by Staff Columbia Basin Bulletin, April 5, 2013

In a paper published this week in the peer-reviewed journal Proceedings of the U.S. National Academy of Sciences, researchers, using acoustic tagging and tracking technology, say they have learned that survival during the first month of life at-sea of juvenile Snake River spring chinook salmon was the same as that of a downstream population which did not first migrate through the Snake River dams.

The Canadian researchers say that result debunks a widely accepted theory that stress from Snake River dam passage causes reduced survival of Snake River chinook salmon once they enter the ocean, leading to fewer adults returning to the river two years later.

"Using a large-scale telemetry array, we tested whether survival of hatchery-reared juvenile Snake River spring chinook salmon is reduced in the estuary and coastal ocean relative to a downstream, hatchery-reared population from the Yakima River," according to the abstract for the paper, "Influence of multiple dam passage on survival of juvenile chinook salmon in the Columbia River estuary and coastal ocean."

"During the initial 750-km, 1-mo-long migration through the estuary and coastal ocean, we found no evidence of differential survival; therefore, poorer adult returns of Snake River chinook may develop far from the Columbia River," the abstract says. "Thus, hydrosystem mitigation efforts may be ineffective if differential mortality rates develop in the North Paci?c Ocean for reasons unrelated to dam passage."

The research paper can be found at: www.pnas.org/content/early/2013/03/27/1219910110.full.pdf+html

Authors are Erin L. Rechisky, David W. Welch, Aswea D. Porter, Melinda C. Jacobs-Scott, and Paul M. Winchell; Kintama Research Services Ltd., Nanaimo, BC, Canada V9S 3B3

Snake River salmon populations have persistently lower adult survival rates upon return from the ocean than do apparently similar populations from the mid-Columbia River such as the Yakima fish, which were used in the study, the study says. The downstream populations migrate through three or four dams as compared to eight for the Snake River fish used for the study.

"What was previously unclear was whether the large difference in adult return rates was actually attributable to delayed mortality due to the prior stress of exposure to the Snake River dams, or whether factors other than dams play the dominant role in determining salmon survival," according to a press release announcing the paper's publication.

The new study reports that early marine survival of the Snake River population was essentially identical to the mid-Columbia population, despite the smolts passing through the Snake River dams. The lower adult return rates of Snake River chinook therefore seems to be set later in the marine phase, sometime after the first month of life in the sea.

In their study, researchers used acoustic tags to follow the migrations of small juvenile Pacific salmon, called smolts, from their freshwater release sites far upriver to distant ocean destinations, which they say is a major step towards understanding the full life cycle of this species.

To do so the authors constructed a series of acoustic tracking receivers to monitor tagged smolts from two hatchery populations as they migrated downstream through the dams, into the ocean and along the Pacific coast. A census of the survivors was taken one final time as they passed over receivers situated off the northwest tip of Vancouver Island, some 1,400 kilometers (870 miles) from the release site and more than one month after release.

"Chinook salmon population collapses are not unique to the Columbia River basin. In recent years, similar collapses have been reported from California to Alaska, and marine survival is considered one of the leading causes of these declines," says lead author Erin Rechisky. "Managers may need to adopt a more pragmatic view of what level of technical 'fix' to compensate for poor ocean conditions is possible within the Columbia Basin, because fluctuations in ocean survival are much larger than those occurring in freshwater."

"Our results are important because mitigation efforts in the Columbia River basin, which are partially based on the assumption that the effects of the dams on ocean survival are large, may be ineffective if the higher mortality of Snake River spring chinook occurs in the North Pacific Ocean for reasons unrelated to dam passage."

The new results have important economic implications, the authors say. Breaching the four Snake River Dams was estimated nearly a decade ago to cost more than one billion dollars, and the net present value of the foregone power generation is worth billions more. Decommissioning the Snake River dams therefore has multi-billion dollar financial implications.

"Although our new study does not remove all uncertainty concerning whether delayed mortality exists, it provides relatively definitive experimental results that delayed mortality does not exist in large, hatchery smolts, despite their lower return rate," Rechisky said. "We do not, as yet, know whether delayed mortality occurs in smaller, wild smolts, but if it does, the delayed effect of Snake River dam passage on survival must be large, because it is not found in the larger smolts we studied."

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