A 15-Day Cold-Water Window Decides Which Salmon Eggs Survive, Study Finds

Cold water during the first 15 days after fertilization is especially important for the survival of Sacramento River winter-run Chinook salmon eggs, according to research published in the journal Science Advances. California's most endangered salmon now survives as a single population in the Sacramento River, after Shasta Dam blocked its access to colder, higher-elevation spawning grounds on rivers draining Mount Shasta, leaving the fish to spawn in the warmer, low-lying Sacramento River, where returning adults have averaged only a few thousand in recent decades.

"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 conducted the research as a postdoctoral scholar at UC Davis with scientists from UC Davis, NOAA Fisheries, UC Santa Cruz, the University of California, Los Angeles (UCLA) and the Norwegian Institute for Nature Research. During warm, low-water years, the cold water released from Shasta Reservoir barely lasted through this critical 15-day window, sharply reducing egg survival - a pattern researchers say may become more common as the climate warms.

For the first time, researchers combined spawning surveys, river temperature models and measurements from salmon otoliths, or ear bones, using an ion microprobe at a UCLA laboratory to measure oxygen isotopes that record 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 of 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 responding by concentrating scarce cold water on only part of the spawning season to favor a smaller number of nests, warning that "selecting the winners at this life stage can have unintended consequences" by narrowing later survival chances. Instead, the results support approaches such as reintroducing salmon to historical habitat. The research was funded by the U.S. Bureau of Reclamation and the California State Water Board.