The survival of juvenile migrating salmon depends on the flow thresholds of watercourses.




PICTURE: Juvenile chinook salmon were tagged with acoustic transmitters, allowing scientists to track them as they migrated along the Sacramento River towards the sea. After

Credit: Photo by Alex McHuron

Juvenile salmon that migrate out to sea in the Sacramento River face a gauntlet of dangers in an environment drastically altered by humans, especially with respect to historical patterns of stream flow. Many studies have shown that juvenile salmon survival rates improve as the amount of water flowing downstream increases, but “more is better” is not a useful guideline for organisms. that manage competing demands for available water.

Now, fisheries scientists have identified key thresholds in the relationship between stream flow and salmon survival that can serve as actionable goals for water resource management in the Sacramento River. The new analysis, published on May 19 in Ecosphere, revealed non-linear effects in the flow-survival relationship, meaning that it changes in stages, with significant jumps in survival rates at two key stages.

A threshold defined in the article as the “historical average” flow of 10,712 cubic feet per second (cfs) is a particularly important target for resource managers, said first author Cyril Michel, project scientist at the Institute of UC Santa Cruz Marine Sciences. .

“We are seeing a substantial increase in salmon survival above this level, so if we can increase stream flow to this level during critical times of the year, it would really benefit salmon populations.” , said Michel.

The researchers analyzed migratory survival data from 2,436 juvenile chinook salmon tagged with acoustic transmitters and followed for years with different water flows, from 2013 to 2019. After identifying key thresholds, the team then used historical data on river flows and salmon migration patterns to run simulations of different management actions.

“We wanted to see how much salmon populations would benefit if we had adopted flow rates that matched that 10,712 cfm threshold,” said Michel. “We have found that we can dramatically increase survival, sometimes doubling or tripling survival rates in any given year, without having to spend too much water. It’s a reasonable goal that won’t blow the whistle. bank most years. “

Juvenile salmon migrate to the sea in the spring, which was historically a time of high flow in the Sacramento River. Now, however, dams and water diversions combined with seasonal reductions in tributary flows result in spring flows that tend to be the lowest of the year.

“Because of the way we dug the central valley, the salmon now have to migrate to the low flows. We therefore propose to put in place impulses in the spring to bring the river back to historical conditions for short periods, ”Michel mentioned.

He noted that plans to implement impulse flows in the Sacramento River are currently being developed under environmental permits renegotiated last year for the Central Valley Project, the massive federal management project of the water that includes the Sacramento River. An interagency group of scientists agreed to use the 10,712 cfs threshold as a target for these pulse flows, Michel said.

“We are delighted that it can actually happen this year or next year, and we will be monitoring survival rates to see how successful it is,” he said. “This could be a tool that will be used for many years to come, with real benefits for salmon populations.”

Further research into the optimal timing of pulse flows could improve their implementation, allowing adaptive management in response to environmental conditions, he added.

The study identified two other thresholds, defined as “minimum” (4,259 ft3) and “high” (22,872 ft3). Below the minimum threshold, only 3% of tagged salmon survived the migration. Survival was 18.9% between the minimum and historical mean, 50.8% between the historical average and high and 35.3% above the high threshold.

The results suggest that the main mechanism behind flow thresholds is related to the speed at which migrating fish are able to move through the river and overcome dangers along the way. Fish travel times during flows between the historical mean and high thresholds were significantly shorter than for fish undergoing all other flows.

“In most years it is best for juvenile salmon to spawn in the upper part of the river, where it is safer, and then move to the lower part of the river as quickly as possible to reduce their exposure to predators. and other stressors, ”explained Michel.

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The research team includes scientists affiliated with UC Santa Cruz and NOAA’s National Marine Fisheries Service (NMFS). The UCSC Institute of Marine Sciences Collaborative Fisheries Program supports research collaborations with the NMFS Southwest Fisheries Science Center laboratory in Santa Cruz, located on the UCSC Coastal Sciences Campus.

Besides Michel, the co-authors of the article include Jeremy Notch and Flora Cordoleani at UCSC and Arnold Ammann and Eric Danner at NOAA Fisheries. Funding and resources for the study were provided by the US Bureau of Reclamation, the US Fish and Wildlife Service, and the Central Valley Project Improvement Act.

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