Salmon becoming river ‘ghosts’ due to deadly droughts and violent floods

Salmon are becoming river 'ghosts' as brutal droughts and violent floods cause unprecedented losses on their treacherous journey to the Pacific Ocean, scientists say.

A major study led by the University of Essex, NOAA Fisheries, University of California, Davis, and Cramer Fish Sciences found that young Californian Chinook salmon face a deadly double threat from extreme weather and the destruction of historical wetland habitats they rely on.

The study emphasised how deadly droughts are for young fish and how they thrive in wetter conditions.

However, the results also indicated that in modern, simplified rivers, extreme flows during winter storms can be devastating too.

Decades of engineering in California's 1,100 square mile Sacramento-San Joaquin River Delta have created an "ecological trap" by carving the Delta into a series of fast-flowing canals.

The paper published in Global Change Biology contrasted juvenile salmon habitat use during the multi-year drought of 2012-2016 with the massive floods of 2016-17 that were associated with millions of dollars of damage to roads and infrastructure.

It showed that the altered river system simply could not support the smallest fish at extreme high flows, with high numbers shot out to sea in early 2017 instead of guiding them through the freshwater floodplains and wetlands they need to grow and survive.

The team described these lost fish as river "ghosts" because they die unseen, their fate hidden beneath the water.

Lead author Dr Anna Sturrock, from Essex's School of Life Sciences, said: "The hero of the tale, the 'early migrants', were a bit of a mystery before.

"They're simply too small to track with traditional tags when they leave their rivers.

"By turning to natural chemical tags that are more often used to identify the origin of bones found in archaeological digs, we could track the lifetime movements of these tiny fish and identify the key mortality hotspots."

By analysing the chemical composition of otoliths, tiny ear stones that preserve a chemical record of each fish's life, alongside their eye lens isotopes, the team reconstructed where each salmon had travelled and grown.

By sampling the same cohort across their entire life cycle, they could also infer where and when they were being lost.

The study showed early migrants became rarer at every stage of the journey.

On average, the early migrants made up about 80% of the juvenile salmon entering the Delta, but only 26% leaving it and just 15% of the adults that returned to spawn.

In the extreme climate years, young fish either face low flows and rising temperatures or are swept downstream by powerful floods into hostile environments with a slim chance of survival.

"In extreme climate years, juvenile salmon run out of options, and climate models predict these harsh conditions will only become more frequent," said Rachel Johnson, senior author of the study and scientist with NOAA Fisheries.

Despite heavy losses, some fish from every 'migratory group' still made it back to reproduce, showing why having different types of salmon is so important.

These different groups take slightly different routes and leave at different times, which helps the species survive when conditions change, but researchers warn that as weather becomes more extreme, losing this diversity makes the whole population more likely to collapse.

The researchers say that restoration actions need to mimic that diversity and to be made "climate-ready", with habitats restored across the full migratory route so salmon have safe places to grow, shelter and survive, whatever the weather throws at them.

Dr Sturrock added: "The impacts of 'whiplash weather' are being felt all around the world, impacting both human and natural systems.

"Salmon didn't evolve to bet everything on a single strategy.

"Historically, the Delta offered multiple pathways and places to grow, which allowed different migratory groups to succeed in different years.

"Restoring that diversity of habitats is essential if we want salmon populations to remain resilient in the face of increasingly extreme and unpredictable climate conditions."