First 15 Days Decide Survival for California's Most Endangered Salmon, Study Finds

Cool river temperatures in the first 15 days after their eggs are fertilized are crucial for the survival of Sacramento River winter-run Chinook salmon, California's most endangered salmon population, according to new research published in Science Advances.

The species has a single surviving population, with an average of only a few thousand returning fish in recent decades. Shasta Dam blocks their original habitat in cold mountain rivers draining Mount Shasta, leaving the salmon to spawn in the lower Sacramento River, which is heated by the summer sun. In warm years with less water available, cold water released from Shasta Reservoir barely covers this critical 15-day thermal window, sharply limiting survival.

"Salmon may spawn throughout the summer season, but our results suggest that only a small fraction of those spawning events ultimately produced the juveniles that survived," said Kohma Arai, who authored the research as a postdoctoral scholar at UC Davis with scientists from UC Davis, NOAA Fisheries, UC Santa Cruz, UCLA and the Norwegian Institute for Nature Research. The team combined, for the first time, salmon spawning surveys, river temperature models and analysis of salmon otoliths, or ear bones — using an ion microprobe at a university laboratory to measure oxygen isotopes that reveal the temperatures fish experienced inside their eggs.

"We learned that even when surveys show many salmon spawning over an extended season, only those eggs that experience the right temperatures are likely to survive," said Rachel Johnson, a research scientist at NOAA's Southwest Fisheries Science Center and UC Davis who helped lead the research. "In some years, that number can be shockingly small." Johnson cautioned against using the findings to direct limited cold water to only part of the spawning season, saying that narrowing survival to a smaller number of nests, or redds, could reduce the population's chances of encountering favourable conditions later in life.

The findings instead argue for strategies such as reintroducing salmon to their historical habitat, which would give fish more options to survive, the researchers said.