A new study links widespread deficiency of vitamin B1, or thiamine, among California Chinook salmon to their deaths. This adds yet another challenge for this iconic species whose population is already imperiled by climate change, habitat loss and overfishing.
Salmon serve a critical ecological role in the Pacific Northwest, transporting marine nutrients from the ocean to the rivers where they were born when they return to lay eggs. The dual life in the ocean and rivers leaves them vulnerable to disturbances in both habitats: from ocean acidification and shifting food webs to rivers being blocked by dams and made too warm by climate change.
“They have like a hundred challenges that they’re facing,” Nate Mantua, a scientist with the National Oceanic and Atmospheric Administration (NOAA), told Mongabay in a video interview. “If you add thiamine deficiency on to all these other big problems, you start to really threaten the species, its ability to exist.”
Mantua and his colleagues began investigating thiamine deficiency in 2020 when several California salmon hatcheries began reporting odd behaviors in the fish, including swimming in circles and dying in large numbers. The researchers confirmed thiamine’s role after sick fish placed in a thiamine-enriched bath got better within hours.
To better understand the scale of the problem, the researchers collected eggs and embryos from 135 spawning Chinook salmon (Oncorhynchus tshawytscha). They injected thiamine into a treatment group of female salmon 4-10 weeks before they laid eggs while the control group received saline. No baby fish from the treated group died, while an average of 23% of babies per female died in the control group, indicating thiamine deficiency.
Researchers began to suspect an anchovy-heavy diet was to blame when they examined the guts of ocean-caught Chinook salmon from 2020-22. They found anchovies almost exclusively, suggesting the thiamine deficiency in salmon likely stems from eating too many anchovies.
“They’re loaded with lipids, they’re loaded with energy,” Mantua said. “So it’s kind of like just eating a Big Mac every day.”
Anchovies contain an enzyme called thiaminase in their digestive tract that scrubs out thiamine from the salmon’s gut, making them sick. This wouldn’t be a problem with a more diverse diet.
However, overfishing and several extreme ocean heat waves have decimated many salmon prey species including herring, krill, sardines and squid. Meanwhile, anchovies have experienced a population boom, making them the near-exclusive targets for salmon, Mantua said.
Treating every spawning salmon with thiamine is logistically impossible, Mantua said, but we can support them in other ways, like removing dams and reducing fishing pressure on other important forage species including herring and squid.
“And if we can relieve some of those kind of just chronic hurdles, they can do really well, even when something like this [thiamine deficiency] comes along.”
Banner image: Chinook salmon. Image courtesy of Eiko Jones via Pacific Wild.
This story first appeared on Mongabay
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