Riverine environments are inherently dynamic, and their faunal communities have evolved equally dynamic life-history strategies to successfully exploit these environments. Few species provide a better example of diverse life-histories as Oncorhynchus mykiss, a facultatively anadromous salmonid that can either remain in freshwater as a resident ‘rainbow trout’ or migrate to the ocean to rear as a ‘steelhead trout’. This diversity of life-history strategies is hypothesized to promote population resilience to environmental change. As populations in heavily altered and regulated watersheds are becoming increasingly homogenous in their life-history expression, it is necessary to understand how to manage these watersheds to promote life-history diversity and resilience. However, there is profound uncertainty as to how environmental conditions interact with life-history regulatory mechanisms, such as density dependence and intraspecific competition. We used a state-dependent fitness model to identify the optimal life-history strategies of a cohort of acoustically tagged O. mykiss. We then compared their optimal strategies to their realized phenotype and Omy05 migratory genotype to investigate how assumptions about the sources of life-history variation may influence decision-making uncertainty for managing highly plastic species in regulated systems.