The Columbia River Basin Fish and Wildlife Program calls for a regular system of independent and timely science reviews of the (FPC) analytical products. These reviews include evaluations of the draft annual reports for the Comparative Survival Study (CSS). This ISAB review of the is the ISAB’s fifteenth review of CSS annual reports.
The Fish Passage Center has developed a valuable long-term database on the hydrological performance of the hydrosystem and its effects on salmon and steelhead survival during their seaward migration as juveniles, at sea, and during their upstream migration as returning adults based on detections of salmon tagged as smolts (e.g., smolt-to-adult return: SAR). The CSS reports since 1998 summarize the trends and provide analyses of the effects of the hydrosystem on salmon, steelhead, and other species in the Columbia River Basin. ISAB reviews from 2010 to the present have critically evaluated the analyses in the CSS reports and made suggestions for improved methods, interpretations, and presentation of results.
The annual CSS report is a mature product, typically including updates of analyses using the latest year of data and expansion of analyses when data are sufficient. As more data are acquired, some new patterns may emerge. The passing years may also bring scientific advances and perspectives, leading to new conclusions, and these are now the primary focus of the ISAB’s reviews.
Major findings and regional issues identified in this year’s ISAB review that warrant attention and potential decisions and actions include:
- Heterogeneity and Factors that Influence Smolt-to-Adult Return Proportions
Chapter 6 is an omnibus analysis of the relationship between SARs and covariates. An empirical logistic regression analysis was conducted of SARs versus water transit time (WTT), and an index of powerhouse encounters (PITPH) for Chinook and steelhead was combined with random effects for group and year. Several models were fit and ranked using information criteria. However, all of the models assumed simple additivity between the factors (e.g., the incremental effect of water transit time was the same for both species). Additional models should be fit to investigate if the incremental effects of WTT and PITHP are the same for both species. Additionally, the effects of other possible explanatory variables (e.g., marine phase-related, freshwater habitat-related, migration dimensions), in addition to WTT and PITPH, are known and their effects should also be considered in the model building.
- Influence of Flow Augmentation from the Snake River above Brownlee Reservoir on Salmon and Steelhead Survival
Chapter 7 examines the efficacy of flow augmentation. Only small effects on the hydrosystem were detected, so it is not surprising that there was little evidence of an overall effect on fish survival. However, flow augmentation may be beneficial under certain conditions (e.g., low flow years), so further analyses are needed to investigate its effects under different hydrological conditions and when salmon are scarce (i.e., if even relatively small benefits could benefit the population’s status).
- Estimating Powerhouse Passage Proportions at Hydropower Dams
Chapter 8 is a useful addition to the previous chapter of the 2022 CSS Report that describes the history of the hydrosystem, development of management of spill to benefit salmon and steelhead, and the formation and evolution of the CSS study to inform state, federal, and tribal fisheries managers. The ISAB notes some concerns about the use of some water gaging stations. This new chapter, and the chapter from the 2023 report on the history of the hydrosystem development, are valuable foundations for readers of the CSS reports, especially those without extensive prior knowledge of the system. Perhaps these types of chapters could be combined into a single document that is available as “background” on the CSS for future readers.
Suggested Topics for Further Review
Since 2011, the ISAB has suggested topics that warrant further CSS or regional review. The ISAB offers the following suggestions based on this year’s review:
- Coho salmon is notably missing from the list of stocks and numbers of years of data. Although largely absent in the formative years of the CSS, hatchery and rewilded coho salmon are becoming an important part of the mix of stocks and species being studied. The CSS should consider including coho salmon (e.g., in the PIT tagging effort and reporting) to help understand the dynamics of success of the large reintroduction effort in the mid and upper Columbia River, and the potential for inter-specific effects on other salmonid populations as coho salmon become more abundant.
- Last year, the ISAB recommended that the CSS consider how to incorporate the influence of climate-related and density-dependent factors on the marine survival of Columbia River salmon in future reports. In 2023, we emphasized that:
“In addition to concerns about SARs, from 2008 to 2022, an average of 74% of adult Snake River Chinook salmon migrating upstream past Bonneville Dam survived to Lower Granite Dam, but in the warm year of 2015, only 52% of the adults survived from Bonneville to Lower Granite. The frequency of warm years is likely to increase in the future under climate change, and the survival of adults may decrease more than recent averages illustrate. The collective ongoing poor survival of Columbia River salmon and steelhead warrants a comprehensive assessment of the long-term consequences of these trends and consideration of likely scenarios of climate change and warming.”
The CSS did not respond to this recommendation, which the ISAB repeats, emphasizing that juvenile Fish Travel Time (FTT) and Ordinal Day are strongly related to temperature and flow and are likely to be affected by changing climate. With lower flow (or increased WTT), the time spent feeding in or migrating through warmer water may increase and affect growth rates, thermal stress, and forage community composition. Similarly, Ordinal Day may reflect seasonal temperature and hydrological profiles which can also affect growth, stress, and diet (among other effects). Such indirect effects on survival and productivity warrant greater consideration.
- In 2023, the ISAB concluded that continued analysis of the benefits, uncertainties, and risks of breaching the lower Snake River dams is warranted and suggested that a more comprehensive effort to predict responses to simulate the complex ecological responses expected after breaching is needed. We suggested that the analyses could include scenarios for the geomorphic and environmental conditions after dam removal and evaluate additional sources of uncertainty (e.g., implementation uncertainty, realism of existing models for no-dam conditions) under present-day and plausible future (climate change) environmental conditions. We encourage the CSS to revisit our suggestion from the review of the 2023 CSS report on breaching by examining lessons learned from Klamath dam removals, which were removed this year, and other dam removals in the region. Information on the geomorphic, hydrologic, water quality, and ecological responses will soon be available for the CSS to expand their modeling analysis of breaching presented in Chapter 6 of the 2023 CSS Report.
- If analyses are preformed that involve a priori selection of specific and limited explanatory variables, the description of the methods should clearly describe why those variables were selected and the implications of their use for the results and interpretation. For example, in Chapter 6 only WTT and PITPH were included in statistical models of SARs even though other variables are important and available for inclusion. The limited variable models did not predict SAR values well yet were extensively interpreted for the importance of WTT and PITPH. The rationale for such analyses needs to be clearly stated and, in some cases, additional models that include more explanatory variables should be considered.
- Consider expanding certain analyses (e.g., SARs in Chapter 6 and benefits of flow augmentation in Chapter 7) to sockeye salmon. This is understandably a long-term objective. With the recent increases in sockeye salmon abundance, it would be wise to get prepared to do such analyses.
- Some analyses would benefit from further resolution of the ocean phase (e.g., Chapter 6 on SARs), analyses of flows to include the lower estuary (Chapter 8 on WTTs), and decomposing results into year-types to more fully understand the robustness of general patterns (Chapter 7 on flow augmentation).
- There is an agreement in principle between Canada and the United States for the Columbia River Treaty, but details are not yet available on the effects on the hydrosystem. The ISAB recommends that, when information is available on the details of the Treaty, the CSS should analyze the likely effects of the revised treaty on passage and survival metrics based on observed responses in the system to date.
Importance of Information in CSS Reports
The ISAB strongly emphasizes the importance of the CSS reports for effectively monitoring and evaluating salmon co-management and hydrosystem operation. There may be a tendency to consider the annual CSS reports to be just “more of the same” each year. With more than 26 years of data, the conclusions reached are now extremely valuable because the uncertainties in the results can be well estimated and outlier years can be identified. Moreover, annual data collection and analysis updating can play a “sentinel” role by permitting a standardized detection of changes within a contemporary period. The physical (e.g., PIT tag detection arrays) and human capacity and expertise added over the 26 years are extremely valuable, and the CSS annual reports provide an effective and useful resource to many involved with the Columbia Basin.
Long-term records of fish abundance and environmental conditions are extremely difficult and expensive to develop. The survival of salmon and steelhead during parts of their life cycle is affected by the hydrosystem, and these data are essential for the Fish and Wildlife Program. This is particularly critical when assessing years with extreme conditions, such as low flows, warm temperatures, or other atypical seasonal patterns. Such cases, at the edges of the distributions, are likely to occur more often under climate change, and a long-time series is needed to capture enough of these uncommon conditions to make reliable assessments. Collection of important data from some sections of the Columbia River has been limited, interrupted, or eliminated in recent years. Critical needs include extending PIT-tag trawl sampling in the lower river through August, increasing the number of PIT-tagged fish, and enhancing detection probabilities at the dams.
The ISAB appreciates the CSS’s detailed responses to suggestions provided in previous reviews, and we do not expect the CSS to necessarily respond immediately to new requests for further analyses. The ISAB’s full report includes an overview of the latest report’s findings, suggested topics for further CSS review, and general comments and editorial comments on each chapter of the draft 2024 CSS Annual Report.
