Salmonid Life History Variation in Relation to Biophysical Complexity of Natal Rivers

We used a remotely sensed data base called RAP (Riverscape Analysis Project) to obtain a suite of metrics that describe physical complexity of 1500 Pacific Rim salmon rivers.  We then related these metrics to life history variation in resident and andromous salmonids in a suite of pristine salmon rivers (Salmonid Rivers Observatory Network) where we collected long term, detailed population measurements and physical data to calibrate the remote sensing.  Phenotypic variation was highest in complex, floodplain rivers.  For example, anadromous steelhead trout predominated in single thread tundra streams (oligotrophic) whereas four additional life history types were present in the large anastomosing systems that are highly subsidized by huge salmon runs.  Large bodied resident rainbows predominated in constrained, spring-sourced rivers (low complexity) with huge nutrient subsidies from salmon runs.  Oncorhynchus mykiss heterozygosity was negatively correlated with complexity metrics suggesting lower gene flow (straying) in complex but highly connected river systems, perhaps due to better homing.  We observed similar responses in O. keta and O. nerka and in Salmo trutta, the latter being an introduced species in a Patagonian river analogous to SaRON rivers. In all cases, overall river productivity, notably including supply of marine nutrients from spawners, played a key role in expression of life history variants, in addition to amount of physical habitat.  Life history variation in salmonids probably increases resilience and ability to inhabit a wide range of river systems.  Uncertainty about the genetic basis (e.g., gene expression versus structural changes in the DNA) remains problematic in understanding evolution of life history variation but recent advances in DNA sequencing technology portend new insights. In any case, understanding the evolutionary ecology of  life history variation is crucial for predicting effects of environmental change on salmon adaptive potential and productivity.