Life history diversity of Oncorhynchus mykiss in the Elwha River during and following dam removal

Intensively monitored watersheds provide long-term data sets for evaluating land management activities and can also identify causal mechanisms for population-level changes in aquatic species including life history diversity in salmonids. The influence of dam removal (ecosystem restoration), conservation hatchery practices, and an in-river fishing moratorium (i.e., management actions) on life history diversity in salmonids has not been fully evaluated. We used scale and genetic samples collected during intensive monitoring following these management actions in the Elwha River to determine their influence on the expression of life history traits in Oncorhynchus mykiss. We evaluated adult migratory patterns, age at migration, and degree of repeat spawning to determine if they changed through time, in both hatchery- and natural-origin individuals. We also assessed if genetic diversity shifted through time, reflecting demographic changes after these management actions. Using age data from scales, together with run timing data, we identified 39 different O. mykiss life history (hatchery- and natural-origin) strategies in the Elwha River from 2013 to 2024 based on age at migratory events. The number of life history strategies identified increased from 18 strategies during dam removal impacts to 38 strategies after dam removal impacts; however, this difference was largely due to differences in temporal and spatial sampling effort through time. Life history diversity and the increase from during- to post-dam removal impacts were much greater for natural-origin than for hatchery-origin O. mykiss . Although life history diversity appeared to increase over time, overall genetic structure decreased, and diversity was largely unchanged. Resident and anadromous individuals were observed, with anadromous individuals displaying winter and summer migration timing. Winter steelhead smolt age, time spent at sea, total age, and repeat spawning life histories varied by period and origin. Intensive monitoring allowed us to document the combined positive influence of large-scale dam removal, conservation hatchery supplementation, and a fishing moratorium on life history diversification in O. mykiss in the Elwha River.