Extending Spatial Resolution in Juvenile Salmon Migration Studies Using Otolith Microchemistry and GIS Prediction

The development of divergent life history strategies in salmon is the result of variation in individual migratory behavior and the effects of environmental fluctuation. How these divergent life history strategies confer fitness advantages to individuals within the population, and the effect of environmental fluctuation on the spatial distribution of these divergent strategies, is fundamentally important to understanding and managing salmonid populations. Recent research has shown increased variation in the migration strategies of juvenile wild Snake River fall Chinook salmon (Oncorhynchus tshwasytscha). In particular, representation of juvenile migrants at older ages has increased in a population that historically has been dominated by sub-yearling juvenile migrants. Understanding the spatial distribution of migration strategy has implications for management and hydropower operations of this population. Reconstructing the spatial distribution of juvenile strategies and natal origins of wild, ESA listed populations can be difficult, however, when tagging large numbers of individuals is not a feasible option. We used otolith microchemistry, and geospatial prediction, to reconstruct the life history and migration path of individual returning adult Fall Chinook from return years 2006-2008. Our goal is to understand the spatial distribution and fitness consequences of alternate life history strategies among juvenile Fall Chinook salmon in the Snake River. Our results indicate that the spatial resolution of life history reconstruction from otoliths can be improved using geospatial prediction based on watershed lithology. Our results show significant spatial differences in the geochemistry of key O. tshawytscha spawning, rearing and overwintering locations which can be recovered from adult otoliths. Using these chemical differences we examined spatial differences in the juvenile stage migration strategy of returning, wild fish. Our investigation indicates distinct differences in the timing of downstream movement and ocean entry between juvenile source locations.