This ISAB report reviews the Welch et al. (2020) paper "A Synthesis of the Coast-wide Decline in Survival of West Coast Chinook Salmon” that examined time series of annual smolt to adult return (SAR) values for many West Coast Chinook salmon populations. The analysis of Welch et al. (2020) highlights the generally low SAR values of Chinook salmon along the coast that have occurred recently, and the paper calls into question the view that Columbia River SARs are anomalously low. Their publication generated debate about the general efficacy of hydrosystem passage and freshwater habitat actions because of some of their broadly stated conclusions. Most importantly, they concluded that changes in freshwater habitat would have little impact on SARs and therefore have only small effects on Chinook salmon populations, a finding which could have major implications for how salmonids are managed in the Columbia River Basin. The ISAB also considered a review of the Welch et al. (2020) paper by the Fish Passage Center (FPC 2020) and Welch et al.’s (2021) response to the FPC review.
Welch et al. (2020) conducted simple analyses of the assembled SAR time series data and reach a series of conclusions – some supported by the analysis and some not supported. At a coarse resolution, the descriptive observations of Welch et al. (2020) that SARs for Chinook populations are low in the region and the values for the Columbia system are not dissimilar from those of other systems, including those with no dams, are supported by the analysis presented in the paper. We also agree with their findings that low SAR values from marine survival affect the realization of long-term population-level benefits of freshwater management actions. These findings are useful contributions to the wider literature on Chinook survival patterns and for informing management.
However, Welch et al.’s conclusions involving causal inferences, for example about the effectiveness of freshwater habitat initiatives (management actions), are not adequately supported by the evidence. Such conclusions therefore should be considered speculation, especially when interpreted for individual populations. Moreover, inferring management implications based on their results alone would be premature. In particular, the degree to which freshwater mortality (i.e., during migration from the point of smolt tagging to ocean entry) influences SAR values varies across populations has not been determined by the Welch et al. (2020) analysis. One cannot directly compare SAR values to infer how freshwater survival differs among populations without making major unsupported and untested assumptions (e.g., assuming marine survival is constant across populations).
The Welch et al. (2020) paper adds to other evidence for the need to further investigate SAR values across populations and to continue investigating oceanic and freshwater contributions to low SARs as a critical uncertainty in the basin. The ISAB offers recommendations for further analyses of coastwise SARs to increase our understanding of their general temporal trends and broad spatial patterns throughout the region.
The reader is referred to the full review for further information and details.
