Snake River Steelhead Marine Survival: Influential Periods and Conditions
Faculty Mentor Information
Dr. Matt Falcy, University of Idaho
Presentation Date
7-15-2026
Abstract
Returns of adult natural-origin Snake River steelhead have declined substantially over the past few decades, driven by low smolt-to-adult survival. Previous studies have identified the first year of ocean residence as the most limiting, influenced by intrinsic smolt characteristics, coastal conditions during outmigration, and environmental conditions during later ocean residence. We examined the relative importance of these factors by analyzing survival rates of ~28,000 emigrating natural-origin Snake River steelhead between the years 2006 and 2021. We estimated smolt-to-adult survival from Bonneville Dam to Bonneville Dam. We used logistic regression models to evaluate the influence of intrinsic characteristics and ocean conditions across three temporal scales: early-ocean entry, first summer/fall, and first winter. Model results indicate intrinsic smolt characteristics and early-ocean conditions were the most supported predictors of survival. Although other broad-scale factors were also significant, these results suggest earlyocean conditions are stronger drivers of survival. However, there is evidence that mortality extends beyond ocean entry, with conditions during the remainder of the first year of residence acting as further constraints on return rates.
Snake River Steelhead Marine Survival: Influential Periods and Conditions
Returns of adult natural-origin Snake River steelhead have declined substantially over the past few decades, driven by low smolt-to-adult survival. Previous studies have identified the first year of ocean residence as the most limiting, influenced by intrinsic smolt characteristics, coastal conditions during outmigration, and environmental conditions during later ocean residence. We examined the relative importance of these factors by analyzing survival rates of ~28,000 emigrating natural-origin Snake River steelhead between the years 2006 and 2021. We estimated smolt-to-adult survival from Bonneville Dam to Bonneville Dam. We used logistic regression models to evaluate the influence of intrinsic characteristics and ocean conditions across three temporal scales: early-ocean entry, first summer/fall, and first winter. Model results indicate intrinsic smolt characteristics and early-ocean conditions were the most supported predictors of survival. Although other broad-scale factors were also significant, these results suggest earlyocean conditions are stronger drivers of survival. However, there is evidence that mortality extends beyond ocean entry, with conditions during the remainder of the first year of residence acting as further constraints on return rates.