Environmental and Genotypic Influences on Size and Survival of Juvenile Rainbow Trout in Seasonal Ecosystems

Natural selection favours traits that maximize survival and overall fitness, and can be strongest at life stages with high mortality rates.  Juvenile fish face two major sources of mortality in seasonal environments: predation and winter starvation.  Both of these mortality agents are negatively correlated with body size in early life stages. Consequently, traits associated with the processes, tradeoffs and outcomes of growth should be under significant selection depending on local environmental factors.  The objective of this research was to better understand short term and long term selection on traits associated with growth and survival in seasonal ecosystems.  I tested for environmental and genotypic influences on growth using whole-lake common garden experiments on two wild strains of juvenile rainbow trout across an altitudinal gradient.  I assessed three traits that could be functionally adaptive in seasonal ecosystems: fall body size, energy allocation, and foraging activity. I found evidence of genetic differentiation in the growth rates between strains but otherwise similar phenotypic responses across environments. In high altitude lakes, with suboptimal growth conditions and high winter mortality rates, there was no evidence that higher growth rates positively influenced first year survival.  Patterns in size and survival were influenced by an interaction between environment and genotype and suggest the presence of ecological and physiological tradeoffs in the processes driving these outcomes in seasonal ecosystems.