Modeling terrestrial carbon sources for juvenile Chinook salmon in the Merced River, California

This study makes inferences about primary carbon sources for juvenile Chinook salmon (Oncorhynchus tshawytscha) in a highly regulated stream by measuring stable isotope carbon ratios in muscle tissues of net-penned hatchery salmon over three rearing periods and four locations. We used a stable isotope two-end member mixing model within an innovative hierarchical nested Bayesian framework to assess the relative contribution of allochthonous sources for juvenile Chinook salmon in the Merced River, California. The analyses revealed that the contribution of terrestrial carbon sources for juvenile fish was highly variable both spatially and temporally, ranging from 3% to 27% across years and sites. The percent of biomass accounted for by allochthonous carbon suggested that autochthonous carbon was the primary source supporting juvenile Chinook salmon. In 2011 (wet year), our findings were fully compatible with River Continuum Concept since juvenile Chinook salmon allochthony decreased downstream. In 2012 and 2013 (dry years), juvenile Chinook salmon allochthony longitudinal patterns were compatible with the Riverine Productivity Model (foodweb driven primarily by local production), indicating the importance of terrestrial organic carbon contributions from local sources downstream. The primary outcome of this research was that juvenile Chinook salmon utilized both aquatic and terrestrial primary carbon sources in this system and this changed longitudinally through time.