NOAA Fisheries West Coast Region, EurekAlert!
The manipulation of rivers in California is jeopardizing the resilience of native Chinook salmon. It compresses their migration timing to the point that they crowd their habitats. They may miss the best window for entering the ocean and growing into adults, new research shows.
The good news is that even small steps to improve their access to habitat and restore natural flows could boost their survival.
The curtailment of high winter river flows by dams means that they no longer provide the cue for the smallest fish to begin their migration to the ocean. The loss of wetlands in the Sacramento-San Joaquin Delta leaves little of the refuge habitat they need to grow along the way. Meanwhile later-migrating fish suffer from rising summer temperatures that reduce their survival even though they migrate at a larger size.
Fish that begin their migration in mid-spring are the ones that survive best and dominate adult salmon returns to rivers such as the Stanislaus. These results were cited in a study published this week in Global Change Biology. Flow alteration and habitat loss have in effect homogenized the survival opportunities of salmon in this highly managed river system, researchers wrote.
That diminishes what is called the “portfolio effect,” where a diversity of salmon migration strategies help the fish cope with changing environmental conditions. This is similar to how diversified investments help buffer your financial portfolio against jolts in the stock market. Chinook salmon in California evolved diverse migration timing to handle the wide variation in climate, ocean, and river conditions in the Central Valley region. This is also important as climate change and rapidly changing, “whiplash weather” patterns further alter the picture.
“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 that also included scientists from several other agencies and universities. “Keeping options on the table is the best strategy, but that is not what we see happening.”
The research also found 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.