Life History Responses of Oncorhynchus Mykiss to Selective Processes in Early Life: Implications for Resiliency

Oncorhynchus mykiss has an exceptionally diverse portfolio of life histories, with anadromy and localized freshwater residency being end members within a spectrum of migratory strategies.  Many populations consist of interbreeding anadromous and resident individuals, with each form able to produce the other.  This diversity is thought to have contributed to the species’ resiliency across a broad geographic range, but the proximate mechanisms underlying life history expression are not well understood.  In this presentation, we focus on potential feedbacks between demography and life history expression in O. mykiss.  Using results from experimental stream channels we discuss implications of density-dependent selective processes in early life for subsequent life history expression.  Of particular note is our finding that increases in intraspecific competition during the fry stage result in increased selection for behaviorally dominant individuals with larger otoliths at emergence (an index of standard metabolic rate (SMR)).  Selection on SMR is important because SMR is also associated with life history expression in partially migratory salmonids, with anadromous individuals tending to have higher metabolic costs.  The implication is that increases in intraspecific competition may push the mean response of a cohort towards anadromy through selective processes acting on SMR.  In terms of resiliency, the ability of individuals to adopt either anadromous or resident strategies is adaptive if there are reliable cues for individuals to assess the relative fitness of alternative life histories.  Initial conditions related to cohort density may be one in a suite of cues that allow individuals to optimize life history decisions in the face of fluctuating ecological conditions.