Should I Stay or Should I Go? The Influence of Genetic Origin on Emigration Behavior and Physiology of Resident and Anadromous Juvenile Oncorhynchus mykiss

We investigated the interaction among genetically identified origin, behavioral tendency to emigrate, and Na+, K+-ATPase enzyme activity in recently diverged subpopulations of resident (above-barrier) rainbow trout Oncorhynchus mykiss and steelhead (anadromous rainbow trout) in Scott Creek, California. Genetic assignment tests found that the frequency of resident and anadromous origin fish varied by sampling location within the watershed. Individuals immediately below barriers assigned to both above-barrier (37%) and anadromous (63%) subpopulations, and distinct differences in size and age were observed. However, the majority of downstream migration behavior occurred in fish of anadromous ancestry, which represented 97% of the fish sampled as outmigrating smolts. Nonmigratory fish of both life history types and origins typically had low Na+, K+-ATPase activity levels throughout most of the year, but significantly elevated levels were observed in individuals from both groups during the spring smolt migration period. Conversely, many fish sampled in the upper watershed with anadromous genotypes were greater than typical smolt size thresholds for this population yet appeared unlikely tomigrate based on low Na+, K+-ATPase activity. Life history pathways of O. mykiss in this population are strongly influenced but not entirely determined by origin in the resident or anadromous subpopulation, and this relationship has implications for recovery of populations impacted by dams, water diversions, and residualized hatchery fish. Further, this work demonstrates the need to consider resident rainbow trout as potentially important resources for recovery of threatened and endangered steelhead populations.