Life history diversity of Hoh River winter steelhead based on three decades of scale analysis

ABSTRACT Objective Many populations of wild steelhead Oncorhynchus mykiss along the west coast of North America have declined in abundance over the past several decades. This has led to increased interest in improving the monitoring of population parameters, such as abundance and diversity. The objective of this study was to describe demographic trends in age structure and life history diversity over the past three decades for wild winter-run steelhead returning to the Hoh River on the Olympic Peninsula, Washington. Methods We used age information derived from scale analysis (n = 5,420) to describe the trends in life history over run years 1994–2023. We define a life history as a unique combination of freshwater age, marine age, and spawning history. We generated three life history diversity indices for each run year—all returning fish, maiden spawners, and repeat-spawning steelhead—and tested for any temporal change across the study period. Results Over the entire study, we observed 35 unique life histories based on age at return of wild winter steelhead, and over half (63%) of the life histories observed were repeat spawners. We found that life history diversity has decreased over the past 30 years, and the decrease was primarily related to a decline in repeat spawner life histories and the oldest marine age of maiden spawners. Size at age, freshwater age, and marine age of the most common age-classes of maiden spawners have not changed over the study period. Annual survival rates for repeat spawners were positively related to the life history diversity index for repeat-spawning steelhead, highlighting the importance of conserving diversity in this population. Conclusions This study used scale age data to develop life history diversity indices for a population of wild adult steelhead and identified a demographic shift in repeat and maiden spawners. We observed a decline in the life history diversity of repeat spawners, which was related to a decrease in kelt survival. These results highlight that actions taken to minimize the mortality of migrating kelts should help to protect O. mykiss diversity and abundance and provide the greatest chance of meeting management objectives.