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 2023 Draft CSS Annual Report: Comparative Survival Study of PIT-tagged Spring/Summer/Fall Chinook, Summer Steelhead, and Sockeye is the ISAB’s fourteenth 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 based on detections of tagged smolts and returning adults (e.g., smolt-to-adult-return: SAR), and during their upstream migration as returning adults. 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 and interpretations.
The annual CSS report is a mature product and now typically includes mostly updates with the latest year of data, continuation of the analysis of long-term trends, and addition of new analytical approaches. As more data are acquired, new patterns may emerge. The passing years also bring scientific advances and perspectives, leading to new conclusions, and these are now the primary focus of the ISAB’s reviews.
Summary of Key Findings
Major findings and regional issues identified in this year’s ISAB review that warrant attention and potential decisions and actions include:
- Effects of Bypass Actions and Spill Management
Analysis of juvenile bypass systems and spill management strategies to reduce mortality of juvenile salmon and steelhead and to increase the return of adults has been a major focus of CSS studies in recent years (CSS Chapters 2, 7, and 8). The CSS has concluded that bypass systems result in hydrosystem-related delayed mortality (i.e., below Bonneville Dam, in the estuary, and early marine period). The CSS now has enough raw data available to refine their analyses and create summaries indicating the scale and longitudinal pattern of impacts of bypass events. The ISAB provides specific suggestions to strengthen the analyses and interpretation, for example, regarding fish guidance efficiency estimates.
The 2022 and draft 2023 CSS Reports (CSS Chapter 2) evaluated the initial responses of juvenile fish travel times and survival to the Flex Spill Agreement and the Preferred Alternative for the 2019 BiOp. The CSS will assess smolt-to-adult return (SAR) and the ratio of returns of fish transported to in-river migrants. The analyses indicate that increased survival likely is related to increased spill. While these trends are consistent with the intended benefits to juvenile salmonids, models indicate that they are not sufficient to even meet the lower targets for SARs of the Columbia River Basin Partnership. Based on initial results, additional actions or other changes in conditions would be necessary to achieve SARs of 2% or greater. This ongoing study will provide critical information about flow management strategies in the Columbia River Basin.
The study of spill management in Chapter 2 recommends that future analyses should evaluate the impact of daily load following on juvenile fish survival and travel time. The ISAB concurs and recommends that the CSS should include this new recommendation in the Conclusions section. The CSS report makes very clear and compelling recommendations regarding the need for more spill, which they have recommended in past years. In its present form, the analyses show promise but could be improved in the “learning” part of Adaptive Management. The ISAB encourages the CSS to continue and expand its efforts to highlight the most recent body of evidence related to these issues for regional decision makers and researchers.
- Smolt-to-Adult Returns (SARs) and Survival of Upstream Migrants
The SAR analysis provides important long-term data for the Columbia River Basin and management of the hydrosystem. Trends in SARs in the Columbia River Basin are a major concern regionally. SARs for all hatchery and wild stocks of spring Chinook, fall Chinook, steelhead, and sockeye in the upper Columbia and Snake rivers are below the Northwest Power and Conservation Council’s 2% minimum SAR objectives (CSS Chapter 4). Only wild spring Chinook salmon and wild steelhead from the mid-Columbia (i.e., farther downriver) meet the minimum objectives in most years. The ISAB provides several recommendations for the CSS to provide additional explanations for major assumptions and provide tables of key input values in the analyses.
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 (CSS Chapter 5). The frequency of warm years is likely to increase in the future, 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 warming.
- Uncertainties and Risk Associated with Breaching of the Lower Snake River Dams
One of the major issues explored in the Columbia River System Operations Environmental Impact Statement(CRSO-EIS) and Biological Opinion in recent years has been the potential breaching of the four lower Snake River dams to increase the survival of juvenile and adult salmon and steelhead. The causes of the declines in abundance of many Chinook salmon populations along the coast continue to be a topic of scientific debate, as are the actions to reverse those declines. While scientists are weighing the effect of breaching on survival, major policy discussions are ongoing and influenced by these studies, so the importance of the CSS studies cannot be overemphasized.
The CSS reports since 2017 have modeled and assessed the projected responses of salmon and steelhead under various management scenarios. Chapter 6 in the 2023 CSS Draft Report examined potential uncertainties and risks associated with dam breaching for salmon populations. The analysis focused on some isolated effects of dam breaching and not on broader changes to the ecosystem or hydrosystem operations that would be expected to occur with dam breaching. Thus, the analyses of dam removal in Chapter 6 substantially underestimate the complexities (and resulting uncertainties) associated with dam breaching. A core assumption is that the system will immediately return to a state simply represented by changed model parameter values for water transit time and expected number of powerhouses encountered by migrating juveniles (PITPH) and that other factors affecting SARs will remain the same. The ecological processes that influence salmon and steelhead after dam breaching cannot be reduced to water travel time and exposure of smolts to structures. Removal of large dams can have uncertain effects, particularly for ecosystem processes in the years immediately following dam breaching. Further, variables representing the ecosystem condition may not be representative of a likely future state that involves greater climatic and ecological variability independent of the dam breaching. Consequently, quantitative predictions of the environmental conditions and effects on salmon will be uncertain.
The current structure of the models and the assumed relationships may not accurately predict SARs and abundances after breaching. Predictions of SARs after breaching using models configured with and completely informed by data collected when dams are present must be viewed cautiously. While it is not possible to exactly represent the physical and ecological conditions that will occur after dam breaching, the inherent uncertainty and implications for risk assessment should be clearly stated.
Within a narrow range of hydrological and climate conditions comparable to those in recent years, the model results may be more predictive, but outside the range of recent records, the model results have much greater (and unquantified) uncertainty. The CSS should provide a discussion of the consequences of climate change and its effect on model realism and how historical conditions can be used to represent future conditions. We encourage the CSS to explore approaches to incorporate the influence of climate-related and density dependent factors on the total life-cycle survival of Columbia salmon in future reports.
Given the exceptional importance and interest around the potential for breaching of the lower Snake River dams, the representation of breaching in the models requires further evaluation and the limits of what is represented need to be clearly stated alongside the results. The CSS analysis alone does not constitute a comprehensive analysis of breaching, but it is an important component of a broader analysis that would also have to incorporate the other important geomorphological, ecological, and management changes that would occur with breaching. The ISAB supports and encourages these types of analyses by the CSS and others to examine critical and challenging topics such as the uncertainties and risks associated with the ecological responses to breaching the lower Snake River Dams. Despite some of the concerns raised through this review, the use of existing modeling results to explore this new application is an excellent example of the CSS looking forward and exploring important and challenging questions with their analyses.
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:
- Building upon the 2019 model comparison, Basin Partnership 2022, and Chapters 2 and 6 in the 2023 Report, continued analysis of the benefits, uncertainties, and risks of breaching the lower Snake River dams is warranted. The ISAB views this as a critical effort going forward, as the issues to be addressed likely involve changes to models, adding sources of uncertainty not previously considered, and using modified models to perform new simulations.
- With the long-term data available and changes in some of the dams, additional dam-specific information is available to include in the analyses. What are the most important differences among dams that affect passage and survival? Did the modification of dams increase survival? For example, the estimates of fish guidance efficiency are now 20 years old and they might be revisited in light of the new data and changes in the dam structures.
- The CSS could consider how to incorporate the influence of climate-related and density dependent factors on the marine survival of Columbia River salmon in future reports.
- Given the value of the time series for comparative analyses, a useful addition would be a recurring chapter that synthesizes similarities and differences between hatchery and wild fish in SARs, fish travel time, and other response variables. With the long-term data, the CSS could explore the interplay between origin, size, and timing on survival of salmon and steelhead.
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. With more than 25 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. The physical (e.g., PIT tag detection arrays) and human capacity and expertise added over the 25 years are extremely valuable and the maximum benefits of these efforts should be obtained.
Long-term records of fish abundance and environmental conditions are extremely difficult and expensive to develop but are essential for the Fish and Wildlife Program. Maintaining the long-term data set is especially critical to support assessments of years with extreme conditions, such as low flows, high temperatures, or other atypical seasonal patterns. These “edge of the distribution” cases may occur more often under climate change, and a long-time series is needed to capture enough of these uncommon conditions to make reliable assessments.
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 and editorial comments on each chapter of the draft 2023 CSS Annual Report.