Spatial scales of mixing of salmon populations in the coastal ocean detected by genetic and isotopic signatures of origin

For the many species whose migratory behaviors include natal populations that migrate simultaneously over great distances, the extent to which natal populations co-mingle can influence their demography and evolution.  Associating with related individuals may contribute to an individuals’ success at finding suitable habitat, avoiding predators, homing, and the maintenance of locally adapted populations.  Despite its ecological and evolutionary importance, our understanding of whether groups of migrating individuals consist of separate or mixed populations remains limited.  This is largely due to our inability to identify the natal origin of individuals.  We developed natural tags to identify the spatial scales of mixing of Chinook salmon (Oncorhynchus tshawytscha) off the central California coast.  Thirteen microsatellite DNA loci were used to identify the Evolutionarily Significant Units (ESUs) of adults in the coastal ocean.  Strontium isotope (⁸⁷Sr/⁸⁶Sr) ratios, and daily growth bands recorded in otoliths (earbones) of Chinook salmon from all major natural and hatchery spawning sites in the California Central Valley (CV), can be used to identify river and hatchery of origin of CV Fall ESU adults in the ocean with high accuracy (94%-98%).  Adults from different ESUs were found to mix among regions off the central California coast.  However, at the smaller spatial scales (aggregations of fish) individuals from the same ESU were found together.  Individuals from the same river/hatchery also stay together three years after entering the ocean, indicating that there is a degree of permanence in the composition of aggregations perhaps established during their early freshwater phase.  These data provide critical information for salmon conservation and fisheries management.