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ISAB Commends Newest Chapters in Comparative Survival Study by K.C. Mehaffey NW Fishletter, November 18, 2025

Overshooting is a common behavior for steelhead, with between 30 and 55%
of each of the 13 populations examined swimming past their home tributaries

A new chapter on wild adult steelhead that don't initially return to the Columbia Basin tributaries where they were born is an important addition to the Fish Passage Center's draft 2025 comparative survival study (CSS), according to the Independent Scientific Advisory Board.

The chapter, "Steelhead Overshoot and Fallback Rates," examines how 13 major populations of Columbia Basin steelhead migrate through the Columbia Basin, and the percentages that bypass their natal rivers, along with the percentages of fish that ultimately return or fall back before spawning.

Completed in August, the 2025 draft CSS involves long-term analyses of the hydro system and its impact on juvenile salmon and steelhead migration and adult return rates.

It is funded by the Bonneville Power Administration with analyses coordinated by the Fish Passage Center and conducted by state, federal and Native American fish and wildlife scientists.

In the CSS chapter on straying steelhead, scientists note that summer steelhead enter the Columbia River almost a year before spawning and spend many months in the main-stem Snake and Columbia rivers.

They analyze the rates of overshooting -- when steelhead move upstream beyond their natal tributary; and fall back -- when they migrate back downstream to their natal stream to spawn.

"The combination of overshooting and failure to fallback represents a loss to the natal population. 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," the ISAB wrote in its review.

The CSS analysis found that overshooting is a common behavior for steelhead, with between 30 and 55 percent of each of the 13 populations examined swimming past their home tributaries.

Some populations have higher overshoot rates. Close to 55 percent of the John Day and Tucannon steelhead overshoot their natal streams, while between 35 and 49 percent of Umatilla, Walla Walla, Upper Columbia, Entiat and Fifteenmile Creek steelhead migrate past their home waters.

The CSS also found that for some populations, fewer than 50 percent of steelhead eventually return to their natal streams, and frequently, fewer than 70 percent fall back. Additionally, fish that overshoot further upriver tend to have lower fallback rates.

Overall, the ISAB commended the CSS for investigating steelhead overshoot and fallback.

The independent scientists suggested that the chapter could also look at how many of the fish that strayed past their natal streams also failed to return home. "The ISAB recommends that this is worth reporting the net loss to the natal basin, which is the most critical factor for population demographics," the review says.

The ISAB also recommended including populations that were not analyzed even though sufficient numbers of steelhead were tagged with passive integrated transponders, because their overshoot rates were low. "The Klickitat River had 139 recovered tags and 8 that overshot the river, and the Deschutes River had 804 recovered tags and 4 that overshot the river. These two rivers would demonstrate low overshoot rates," and should also be analyzed, the ISAB said.

The new chapter is a small part of the CSS report, and the ISAB offered many comments about other chapters and the report in general.

"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," the review states.

The scientists said annual data collection and analysis allows salmon managers to detect important changes. Long-term data can be especially critical when the region experiences extreme conditions like low flows, warm temperatures or other seasonal patterns, the ISAB said. With these conditions expected to occur more frequently, a long-term data series will be needed to make reliable assessments, it added.

The ISAB lamented that some data collection in the CSS has been limited, interrupted or eliminated in recent years, and emphasized the need to extend PIT-tag sampling in the lower river through August, increase the number of PIT-tagged fish, and improve detection probabilities at hydroelectric projects.