The Columbia River Basin Fish and Wildlife Program calls for a regular system of independent and timely science reviews of the Fish Passage Center’s analytical products. These reviews include evaluations of the draft annual reports for the Comparative Survival Study (CSS). This ISAB review of the 2025 Draft CSS Annual Report: Comparative Survival Study of PIT-tagged Spring/Summer/Fall Chinook, Summer Steelhead, and Sockeye is the ISAB’s sixteenth consecutive 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. 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 evaluated the analyses in the CSS reports, noted the usefulness and applicability of the CSS’s analyses, and made suggestions for improved methods, interpretations, and presentation of results.
The annual CSS report is a mature product, including updates of analyses using the latest year of data and expansion of analyses when data are sufficient. Many of the methods have been reviewed in previous ISAB reports and now only receive a confirmatory examination. However, 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 our reviews. In addition to ongoing analyses, the CSS Draft 2025 Annual Report includes several new analyses and chapters, and two are highlighted in this summary.
First, the draft includes a new chapter on steelhead overshoot and fallback rates that examines the migration routes of wild adult steelhead in the Columbia River Basin and movement of those that migrate past (overshoot) their natal tributary, those that eventually fall back to their natal stream, and upstream strays that were never detected in their natal basin. Straying of natural-origin and hatchery salmon and steelhead and not returning to their natal rivers have been ongoing concerns for Columbia River hydrosystem management, especially in the Snake River for the four lower Snake River dams (see ISRP 2025-3). The ISAB commends the CSS for investigating this extremely important management issue and encourages them to refine and continue this analysis.
Second, the draft includes a new chapter titled “Have We Followed the PATH?” thatexamines the context and milestones of the historical development of the CSS analyses. In the early 1990s, the listing of anadromous salmonids under the Endangered Species Act changed management in the Columbia River Basin, and an analytical process named Plan to Analyze and Test Hypotheses (PATH) was initiated to resolve differences and reduce uncertainty about the effects of hydrosystem management on listed fish species. The chapter describes the origin of the current scientific debates and how the CSS has followed the PATH process. The ISAB found this chapter to be well-written and informative to all readers, from the interested public to informed river managers.
Suggested Topics for Further Review
Since 2011, the ISAB has suggested topics that warrant further CSS or regional review. The CSS has incorporated many of the ISAB’s past suggestions, and the ISAB greatly appreciates the CSS’s effort to respond to these past suggestions. The ISAB does not expect the CSS to necessarily respond immediately to new requests for further analyses, and the ISAB understands that some of the requests might be beyond the existing mainstem passage focused scope and/or budget of the CSS.
- The ISAB continues to call for the CSS and/or others to develop a focused assessment of the influence of climate-related and density-dependent factors on the marine survival of Columbia River salmon. Climate and density-dependence will increasingly play a role in influencing population responses to hydrosystem operation and management actions, and further assessment will improve understanding of causal factors in trends in smolt-to-adult returns (SARs).
- Current monitoring of the hydrosystem above Bonneville Dam is now well developed and mature. While studying fish in the estuary is somewhat beyond the current scope of the CSS, the ISAB recommends that the CSS conduct a brief review on the methods currently and previously used by others and potential new methods that could be used to better understand smolt and adult survival in the estuary below Bonneville Dam, in the North Pacific, and throughout the Basin.
- The ISAB appreciates the CSS analysis of overshoot, fallback, and straying, which is an extremely important management concern in the Columbia River Basin. The ISAB encourages the CSS to refine and continue these analyses and include other stocks and species, such as hatchery steelhead and spring Chinook salmon. Future analysis by the CSS or others could also examine overshoot and straying at a finer resolution within basins or around specific dams or river confluences.
- The CSS increasingly uses multiple models to bound the uncertainty in predicted responses. Using multiple models can be a very powerful approach but is also a complex endeavor and can easily lead to mis-interpretations of results. The ISAB suggests that the CSS should develop a “multi-model guide” to help standardize the approach and documentation, allowing readers to better understand analyses that use multiple models and to properly interpret differences and similarities in predictions.
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 27 years of data, the conclusions reached are extremely valuable because the uncertainties in the results can be well estimated and outlier years identified. In addition, the physical (e.g., PIT tag detection arrays) and human capacity and expertise added over the decades are invaluable, 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. Long-term data are particularly critical when assessing years with extreme conditions, such as low flows, warm temperatures, or other atypical seasonal patterns. Though such cases are relatively infrequent now, they are expected to occur with greater frequency under climate change, and a long-time series is needed to capture enough of these uncommon conditions to make reliable assessments. However, collection of important long-term data from some sections of the Columbia River has been limited, interrupted, or eliminated in recent years. The ISAB continues to emphasize the need to extend PIT-tag trawl sampling in the lower river through August, increase the number of PIT-tagged fish, and enhance detection probabilities at the dams.
