Preparing to tag a juvenile salmon. Photo: Cyril Michel
New research indicates that survival of juvenile Chinook salmon in the Sacramento River system can be significantly boosted by achieving key thresholds for river flow.
The findings, published in the journal Ecosphere
, add important context to the general scientific understanding that more water in the river improves fish survival. Previous studies, the authors explain in their paper, have demonstrated that more juvenile salmon migrating toward the sea complete their journey when the Sacramento River system contains more water. Just how much water has been the source of much controversy
among user groups. “These studies have not explored the potential nonlinearities between flow and survival, giving resource managers the difficult task of designing flows intended to help salmon without clear guidance on flow targets,” write the authors, led by Cyril Michel of the Institute of Marine Sciences at UC Santa Cruz. Studying migrating Chinook salmon smolts under a wide range of hydrologic conditions, they found that fewer than three percent of fish carrying acoustic tags were detected after release at flows weaker than about 4,260 cubic feet per second. However, survival jumped to 19 percent at flows ranging between 4,260 and 10,700 cfs; they jumped again to more than 50 percent when the river was carrying between 10,700 and 22,870 cfs. However, fish survival did not improve at even greater flows. “It looks like survival plateaus beyond a certain point,” Michel says. “You can get a lot more bang for your buck if you aim for these thresholds.” Notably, the researchers calculated that achieving these key flow rates could be possible in most years without adding any extra water to the system by holding reservoir water back at times to release in pulses during heavy migration periods. However, Michel and his team did conclude that just a little extra water—about 150,000 acre-feet per year dedicated to the river system—would be helpful. Michel says he hopes his research will provide a productive guideline for agencies that manage reservoir releases and Delta pumping.