What a Long Strange Trip It's Been: Growth History and Variability in Age at Return of Steelhead

Anadromy results from two life history transitions: from a freshwater environment to a marine environment during the juvenile life stage and the subsequent return to a freshwater environment as an adult. Variability in age during these transitions (age at outmigration and age at return, respectively) leads to substantial life history variation within and among populations of anadromous fishes. Prior research has revealed that a combination of genetic thresholds and individual fish condition relative to these thresholds contribute to plasticity in the age and timing of outmigration for juvenile steelhead Oncorhynchus mykiss. Fewer studies have examined patterns and mechanisms of variability in the age at return of steelhead. Consequently, we investigated the relationship between growth history and variability in age at return of adult steelhead from the Willamette River Basin, Oregon and Scott Creek, California. Previous growth history was determined from specific growth increments estimated from scales. Understanding the mechanisms affecting variability in both age at outmigration and age at return is critical, as the resulting life history diversity affects adult returns by buffering anadromous populations from stochastic events and increasing population stability.