Fish out of breath: Assessing, developing, and validating physiological bioindicators of hypoxia across the Delta

Project Number
Project Date Range
Funding Agency
Delta Stewardship Council, Delta Science Program
Focus Area(s)
Education, Training and Public Information

In November 2021, nearly an entire run of spawning adult Chinook Salmon died in the San Francisco Estuary — just one of several recent local mass mortality events. Warmer water holds less oxygen, and often falls into hypoxic conditions. Either change alone would stress fish, but together they are often deadly.


This study will illuminate how Chinook salmon smolts and juvenile Delta smelt tolerate hypoxia across a range of temperatures. These resulting metabolic data will then be incorporated with existing environmental monitoring data to form a model called the metabolic index, which can provide spatial and temporal predictions of viable habitats for these two endangered species. Moreover, the metabolic indices will provide managers with species-specific tools to optimize ongoing conservation efforts such as wetland restoration and fish release programs. 


This study will also examine whether fish otoliths (structure within the fish’s inner ear) can be used to retroactively determine past hypoxia exposure. Once validated, this tool can help managers identify whether fishes across the Delta have experienced sub-lethal hypoxic exposure, and potentially at which point in their life said exposure occurred. 


Principal Investigators
Garfield Kwan
Scripps Institution of Oceanography at University of California, San Diego