California salmon returns track deteriorating freshwater and marine conditions

Climate change and human activities are pushing aquatic ecosystems into novel states; therefore, it is crucial to understand how ecosystem conditions affect fish. We quantified relationships between ecological conditions and recruitment for three California Chinook salmon stocks. Using linear and generalized additive models across rolling time windows, we identified indicators of freshwater and marine conditions that were persistently or recently important to recruitment (i.e., adult returns). We found that recruitment was higher following years with abundant natural area spawners, high outmigration flows, cool outmigration and incubation temperatures, low marine predator buffering, and positive NPGO phases. Inspection of these indicators across time revealed that ecosystem conditions have deteriorated over the past two decades, characterized by rising watershed temperatures, droughts, pulses of low marine predator buffering, and negative NPGO phases. Our results bolster prior research suggesting California’s salmon production system of worsening climate conditions and degraded habitats exemplifies a shifted ecological baseline. Ultimately, our work contributes to understanding why California’s salmon have performed poorly recently and an ecosystem perspective that informs decisions and expectations.