A unique adaptive behavioral trait that may once have helped winter-run Chinook salmon thrive in the Sacramento River system could now be working against the fish as they face extinction.
The trait – which cues fish when to spawn based on water temperature – isn’t syncing up with current conditions in the Sacramento River below Shasta Dam. A paper published in the June issue of San Francisco Estuary and Watershed Science
, reports that cooler springtime river temperatures seem to prompt earlier winter-run spawning while warmer temperatures push back the peak
spawning period by a week or two. Under historic conditions, when winter-run Chinook spawned in high-elevation streams now inaccessible to the fish, their behavioral adaptability would have helped the temperature-sensitive eggs and offspring in two ways—by ensuring “sufficient time for egg maturation in cool years, while secondarily preventing egg and alevin mortality in warm years,” write authors Eva Dusek Jennings, a quantitative ecologist with Cheva Consulting, and Noble Hendrix, a biometrician with QEDA Consulting. Excessively cold water significantly slows egg development, Jennings explains in an interview, while water much warmer than 56 degrees F will kill the eggs. But the winter-run Chinook no longer spawn in the river system in which they evolved: Shasta Dam forces them to spawn in the sun-baked Central Valley. The dam, and how it’s operated, has also reversed water temperature patterns during the summer spawning season. River water downstream of the dam tends to be colder in the summer than it is in autumn. That’s because the cold-water pool that forms deep in Lake Shasta flows out of the dam in the summer and is often gone by early fall. Temperatures may spike at this time, causing high mortality of incubating salmon eggs —especially if a warm spring prompted the fish to spawn later. “It’s a confounding situation for the fish,” Jennings says. The findings could help inform fish-friendlier operations of Shasta Dam; the scientists are now modeling and evaluating how different approaches to reservoir management could benefit imperiled salmon.