Ecological and Evolutionary Significance of Freshwater Maturation in Pacific and Atlantic Salmonines

Populations of some Pacific and Atlantic salmon species contain a mixture of anadromous and non-anadromous individuals.  Because of an increased interest in potential demographic and genetic advantages of populations exhibiting mixed migratory strategies, we provide a framework for investigating the ecological and evolutionary significance of freshwater maturation in salmonines.  For convenience, we first independently consider hypotheses related to ultimate and proximate factors underlying freshwater maturation.  We evaluate several evolutionary hypotheses to explain patterns of freshwater maturation among and within salmonines including phylogenetic history, irreversible evolutionary transitions within species (e.g., vicariance), geographic variation in selective pressures, and contemporary reductions in genetic variability in reactions norms (e.g., effects of domestication selection).  Ecological hypotheses for proximate mechanisms underlying freshwater maturation include environmental constraints to freshwater maturation, interspecific interactions, and environmental stochasticity (e.g., dynamic vs. stable environments).  We then consider the interaction of ecological and evolutionary processes underlying patterns of freshwater maturation and discuss how they influence demographic and genetic resilience of populations.  Our goals are to elucidate patterns of freshwater maturation among and within species of salmonines, identify potential conservation actions that promote population recovery and resiliency by facilitating diversity in life history expression, and identify critical knowledge gaps.