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 2026 Draft CSS Annual Report: Comparative Survival Study of PIT-tagged Spring/Summer/Fall Chinook, Summer Steelhead, and Sockeye is the ISAB’s seventeenth 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.
CSS annual reports are mature products, containing analytical updates that use the most recent data available. Many of the draft reports’ chapters include methods that were reviewed in previous ISAB reports. The ISAB reviews those methods for any changes and, as more data are acquired, whether new patterns in the results are emerging. The passing years may also bring scientific advances and new analytical approaches, leading to new conclusions, and these new analyses and chapters are now the primary focus of ISAB reviews. In addition to ongoing analyses, the CSS Draft 2026 Annual Report includes several new analyses and two new chapters, which are highlighted in the ISAB report’s summary.
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. For example, in response to the ISAB’s suggestions in 2025, the CSS made gains in understanding the impact of early marine factors on salmon survival and returns. The ISAB does not expect the CSS to respond immediately to new requests for further analyses. The ISAB understands that some of the requests might be beyond the existing mainstem passage-focused scope and/or budget of the CSS and could be the subject of broader efforts coordinated among the numerous entities conducting fish passage research in the basin.
- CSS analyses have started to include information on an index of non-native fish predator abundance (Smolt Monitoring Program walleye counts). Smolt and adult salmon predation by avian, marine mammal, and fish predators is a significant management concern in the Columbia River Basin (ISAB 2019-1). Improvements to the existing monitoring programs to ensure standardized sampling frameworks that inform the status and trends of these potential predators of concern should be developed (see ISAB 2016-1). The ISAB recognizes that the CSS is not responsible for this index but encourages them to coordinate with the relevant agencies to improve the sampling design to ensure that useful information can be extracted.
- Best scientific practices have evolved regarding methods used to help interpret results of analyses using multiple models. The ISAB recommends that the CSS compare the results from their current methods with alternative methods to better understand the limitations and advantages of the different approaches.
Importance of Information in CSS Reports
As stated in previous CSS reviews, the ISAB strongly emphasizes the importance of the CSS reports for effectively monitoring and evaluating salmon co-management and hydrosystem operation. With more than 28 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 sets are particularly critical when assessing years with extreme conditions, including low flows, warm temperatures, and other atypical seasonal patterns. Such cases, at the edges of the historical distributions, are expected to occur with greater frequency with climate change, and long-time series are needed to capture enough of these uncommon conditions to make reliable assessments. In other words, what seems like extreme conditions from the perspective of the historical record must be increasingly viewed as part of a new but poorly understood normal. Improving that understanding requires long-term datasets. The ISAB continues to emphasize the need to maintain the CSS’s long-term monitoring and analyses and to increase sample sizes and enhance detection probabilities at the dams and in the lower Columbia River.
