Saprolegnia, a fungus

From U.S. Fish and Wildlife Service Pacific Southwest Region:

As the current run of late-fall Chinook salmon in the Sacrament River is winding down we see more and more fish that look like this. Once Chinook enter freshwater and begin their upstream migration they spend their energy getting ready to spawn and their immune system weakens. The white patches on the body are caused by Saprolegnia, a fungus, which is very common in our creeks and river 




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