Extreme variability in abundance of California salmon populations is often ascribed to ocean conditions, yet relatively little is known about their marine life-history. To investigate which ocean conditions influence their distribution and abundance, we surveyed juvenile Chinook salmon (Oncorhynchus tshawytscha) within the California Current (central California (37o 30’ N) to Newport, Oregon (44o 00’ N)) for a two-week period over three summers (2010-2012). At each station, we measured chlorophyll a as an indicator of primary productivity, acoustic-based metrics of zooplankton density as an indicator of potential prey availability, and physical characteristics such as bottom depth, temperature, and salinity. We also measured fork lengths and collected genetic samples from each salmon that was caught. Genetic stock identification revealed that the majority of juvenile salmon were from the Central Valley and the Klamath Basin (91-98%). We constructed generalized logistic-linear negative binomial hurdle models and chose the best model(s) using AIC to determine which covariates influenced salmon presence and, at locations where salmon were present, determined the variables that influenced their abundance. The probability of salmon presence was highest in shallower waters with high chlorophyll a concentration and close to an individual’s natal river. Catch abundance was primarily influenced by year, mean fork length, and proximity to natal rivers. At the scale of sampling stations, presence and abundance was not related to acoustic indices of zooplankton density. In the weeks to months following ocean entry, California’s juvenile Chinook salmon population appears to be primarily constrained to coastal waters near natal river outlets.
Hassrick, J. L., Henderson, M. J., Huff, D. D., Sydeman, W. J., Sabal, M. C., Harding, J. A., Ammann, A. J., Crandall, E. D., Bjorkstedt, E. P., Garza, J. C. and Hayes, S. A. (2016), Early ocean distribution of juvenile Chinook salmon in an upwelling ecosystem. Fish. Oceanogr., 25: 133–146. doi:10.1111/fog.12141