Young salmon in California are disappearing at alarming rates as their once-reliable migration routes grow increasingly dangerous.
According to Earth.com, scientists studying salmon in the Sacramento–San Joaquin River Delta found that many juveniles are dying before completing a critical stage of their migration to the ocean.
The Delta, spanning about 1,100 square miles, once had a mix of gentler channels, wetlands, and floodplains where young salmon could rest, eat, and grow. Over time, much of that habitat has been transformed into straighter, faster-moving waterways.
In dry years, shrinking channels can trap salmon in warm, shallow water. In wet years, powerful flows can sweep them out to sea before they're ready.
To better understand these risks, a research team led by Dr. Anna Sturrock analyzed chemical signals in salmon ear stones — known as otoliths — and eye lenses to reconstruct their movements.
The findings, published in the journal Global Change Biology, were stark. Early migrants made up about 80% of juvenile salmon entering the Delta, but only 26% made it out, and just 15% survived long enough to return as spawning adults.
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Researchers were so struck by the losses that they dubbed many of the missing fish "river ghosts" — salmon that vanish while passing through the Delta, unseen beneath the water.
The issue extends far beyond a single species. Salmon play a vital role in river ecosystems, transporting nutrients and acting as a food source for birds, mammals, and other aquatic life. They also hold deep cultural and economic importance for fishing communities and Tribal nations.
When juveniles survive, fewer adults return to spawn, weakening future populations and reducing the species' ability to withstand increasingly extreme weather.
"In extreme climate years, juvenile salmon run out of options, and climate models predict these harsh conditions will only become more frequent," said senior author Rachel Johnson, per Earth.com.
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One of the biggest challenges is the loss of diversity in the river system itself. Historically, salmon relied on a range of migration timings and pathways, a natural insurance policy that helped populations survive tough conditions.
But as wetlands disappear and rivers become more uniform, that safety net is shrinking.
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