The heavy management of river systems in California is causing a compression in the migration timing of Chinook salmon to the point that they crowd their habitats. As a result, they might miss the best window for entering the ocean to grow into adults. Currently, less than 3% of wetland habitat remains in the Sacramento-San Joaquin Delta.
The study outlines how flow alteration and habitat loss have effectively homogenised the survival opportunities of salmon in this highly managed river system. This diminishes the ‘portfolio effect,’ where a broader variety of salmon migration strategies helps the fish cope with the changing environmental conditions.
“You never know what’s going to be a winning strategy in the future,” said Anna Sturrock of the University of California, Davis, and lead author of the research. “Keeping options on the table is the best strategy, but that is not what we see happening.”
Biologists analysed two decades of salmon migration data, tracking seven generations of Chinook salmon in the Stanislaus River. They used chemical signals in their ear bones, called otoliths. Otoliths grow in proportion with the salmon and reflect the chemistry of the surrounding water, so they can be used to trace the way fish travel to the sea and gauge their size when they move through habitats.
The use of otoliths made it possible to track very young juvenile salmon that are too small to fit with the electronic tags. This approach revealed that large numbers of migrating fry can survive to adulthood, as long as they can find freshwater rearing habitats where they can grow along the way.
“That tells us there is this other life history strategy that may be really important,” said Rachel Johnson, a research fisheries biologist at NOAA Fisheries’ Southwest Fisheries Science Centre and senior author of the study. “Tracking the smaller fish through their otoliths provides important new insights into Chinook salmon dynamics that have otherwise been missing from the picture.”
The researchers also discovered that the lower flows released from dams tend to reduce fish production. This is likely due to reduced access to floodplain habitats and lower food production in rivers. The study states that even minor steps to restore some of the natural fluctuations in river flow could benefit salmon by helping maintain some of their valuable diversity.
“As the climate gets more unpredictable, we need to think about incorporating bet-hedging into river management rather than manipulating the environment in ways that limit options for fish,” Sturrock added. “The more options that are left on—or added to—the table the better chance that some fish will be in the right place at the right time.”
To read the full paper, Unnatural selection of salmon life histories in a modified riverscape, click here.
Photography by Rachel Johnson, NOAA Fisheries / University of California, Davis
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