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Paper, Memo Discuss Ongoing Issue Of Delayed Mortality For Salmon Migrants Negotiating Hydro Project
Posted on Friday, February 10, 2012 (PST)

Scientific discussion continues regarding the existence, extent and/or causes of delayed or latent mortality in salmon and steelhead that must negotiate, particularly, the Columbia-Snake river hydro system.


Over the past month thoughts have come from a couple of sources. A research paper published online Jan. 30 in the journal “Transactions of the American Fisheries Society” says that hydro operations such as spill can reduce freshwater passage stressors that result in increased mortality in the estuary and ocean and mitigate to some degree for saltwater mortality.


Lead author for “Assessing Freshwater and Marine Environmental Influences on Life-Stage-Specific Survival Rates of Snake River Spring–Summer Chinook Salmon and Steelhead” is Steven L. Haeseker of the U.S. Fish and Wildlife Service. Co-authors are Jerry A. McCann, Jack Tuomikoski and Brandon Chockley of the Fish Passage Center.


The article can be found at:


Meanwhile, The Independent Scientific Advisory Board in a memo issued Jan. 3 says that, while delayed mortalities theories may indeed have some validity, the jury is still out as regards the source.


The ISAB review of three FPC memos as well as its Cumulative Survival Study annual reports regarding latent mortality of in-river migrants due to route of dam passage was done at the request of the science panel’s oversight board, which is made of the Northwest Power and Conservation Council chair, the executive director the Columbia River Inter-Tribal Fish Commission and the science director for NOAA Fisheries Northwest Fisheries Science Center.


The memo can be found at:


The published article says that “researchers have found that juvenile out-migration conditions can influence subsequent survival during estuarine and marine residence, a concept known as the hydrosystem-related, delayed-mortality hypothesis.”


“In this analysis, we calculated seasonal, life-stage-specific survival rate estimates for Snake River spring–summer Chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss and conducted multiple-regression analyses to identify the freshwater and marine environmental factors associated with survival at each life stage,” the article says. “We also conducted correlation analyses to test the hydrosystem-related, delayed-mortality hypothesis. We found that the freshwater variables we examined (the percentage of river flow spilled over out-migration dams and water transit time) were important for characterizing the variation in survival rates not only during freshwater out-migration but also during estuarine and marine residence.


“Of the marine factors examined, we found that the Pacific Decadal Oscillation index was the most important variable for characterizing the variation in the marine and cumulative smolt-to-adult survival rates of both species. In support of the hydrosystem-related, delayed-mortality hypothesis, we found that freshwater and marine survival rates were correlated, indicating that a portion of the mortality expressed after leaving the hydrosystem is related to processes affected by downstream migration conditions.


“Our results indicate that improvements in lifestage-specific and smolt-to-adult survival may be achievable across a range of marine conditions through increasing spill percentages and reducing water transit times during juvenile salmon out-migration,” the article abstract says.


The ISAB memorandum “re-examines analyses conducted by the Fish Passage Center (FPC) and Comparative Survival Study (CSS) to evaluate whether the route of dam passage affects subsequent survival ‘latent mortality’ of in-river migrants.”


The ISAB finds that collectively “these analyses demonstrate that fish bypass systems are associated with some latent mortality, but the factors responsible for latent mortality remain poorly understood and inadequately evaluated,” the memo says. “The significant association between fish bypass and latent mortality might only reflect a non-random sampling of smolts at the bypass collectors (the selection hypothesis) rather than injury or stress caused by the bypass event (the damage hypothesis).


“Because these hypotheses have very different implications for hydrosystem operations, FPC and CSS conclusions should be re-examined to consider alternative explanations discussed in this review. Further research will be needed to resolve this issue,” the memo says.


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