Resolving natal origins and life history of wild fall Chinook salmon (Oncorhynchus tshawytscha) using otolith microchemistry

Background/Question/Methods The emergence and maintenance of life history strategies in salmon populations is the combined result of variation in individual migratory behaviors and fluctuating environmental conditions. It is therefore of fundamental interest to understand how different migratory strategies confer fitness advantages to individuals within a population and how this is influenced by changing environmental conditions over large geographic scales. Recent studies have documented increased variation in the migration strategies of juvenile wild Snake River fall Chinook salmon (Oncorhynchus tshawytscha), wherein there is increased representation of juvenile migrants at older ages in a population that was historically represented by underyearling juvenile migrants. Reconstructing the juvenile strategies and habitat origins of successfully returning adult fish can be difficult when tagging a significant fraction of individuals is not a feasible option. The objectives of this study are 1) to understand spatially explicit differences in migration timing for the remaining wild Chinook salmon in the Snake River basin and 2) to quantify how juvenile migration strategies are related to adult survival. Results/Conclusions To achieve our goal of understanding the spatial representation and fitness consequences of alternate life history strategies of juvenile fall Chinook salmon in the Snake River, we are using the microchemistry of otolith (earstone) increments to reconstruct the natal habitats and migration patterns of individual fish. Our results show significant spatial differences in the geochemistry of key O. tshawytscha spawning locations. These chemical signatures are recoverable in the otoliths of returning wild fish and inform us of site-specific migratory strategies. We are currently analyzing returning adults from 2006 – 2008 in order to develop a spatially explicit understanding of juvenile migration strategy that could inform regional management practices and hydropower decisions.