ISAB Follow-up Review: FPC and CSS analyses of latent mortality of in-river migrants due to route of dam passage

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In the attached memo, the ISAB 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 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. Details are provided in the memo.

Rich Alldredge, ISAB Chair, will be presenting this review at the Fish Passage Center Oversight Board meeting on January 9 in Portland, Oregon.

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