Selective mortality during early life history stages can have significant populationlevel consequences, yet critical periods when selective mortality occurs, the strength of selection, and under what environmental conditions can be difficult to identify. Here, we used otolith microstructure and chemistry to examine the factors potentially linked to selective mortality of juvenile fall-run Chinook salmon Oncorhynchus tshawytscha from California's Central Valley during early ocean residence. Back-calculated size and growth rates of the population were compared across 3 sample periods: as juveniles exited the San Francisco Bay estuary (estuary-exit), after their first month at sea (summer-ocean) and 5 mo after ocean entry (fall-ocean). We compared mortality dynamics during years of exceptional recruitment (addition of individuals to harvestable population; 2000 and 2001) to a year of poor recruitment (2005). Otoliths from 2005 were also analyzed for sulfur isotopes to discern hatchery from naturally spawned stock. Significant size and growth-rate selective mortality were detected during the first month at sea in the low recruitment year of 2005, but not in 2000 and 2001. Individuals that were larger and growing faster during freshwater and estuarine rearing were more likely to survive to summer and fall in the low recruitment year. There was a slight, but insignificant, increase in the proportion of hatchery to naturally spawned individuals from estuary-exit to fall-ocean, suggesting that fish from neither origin were overwhelmingly favored. Our results suggest that Central Valley Chinook salmon can be subject to significant size and growth-rate selective mortality resulting in low adult abundance, and this mortality appears independent of origin.

We present a novel approach to examine the relationship between pelagic juvenile fish and their environment in an open coastal system using a geospatial technique to relate water and otolith chemistries. We compared the chemistries of water and pelagic juvenile rockfish otoliths Sebastes jordani collected from a coastal upwelling system off central California during May and June 2009. To determine the presence and composition of an upwelling chemical signature, the elements Ba, Sr, and Mg expressed as ratios relative to Ca were quantified in the water and outer otolith margin using inductively coupled plasma mass spectrometry. Recently upwelled water, as indicated by a strong inverse relationship with water temperature, had an elevated Ba:Ca concentration. Using all 3 element ratios, cluster analysis and multivariate analysis of variance were used to identify 3 distinct chemical signatures for otolith and water samples. When mapped, these signatures displayed marked geospatial variability that we attributed to mesoscale upwelling dynamics. Canonical discriminant function analysis results indicated that the relative contribution of each element to the 3 signatures was similar between water and otoliths. Interestingly, otoliths containing upwelling signatures (high Ba:Ca) did not match spatially with upwelling signatures in the water. A directional-dependence analysis (spatial cross-covariance) revealed the highest covariance between water and otolith chemistries at a distance between 50 and 100 km apart, suggesting southern movement or transport of fish.