The spawning run is very stressful to a steelhead and when they're near spawning time, their immune system is suppressed, so they are very prone to infection. What you are seeing are opportunistic fungal infections. The nates are usually genetically more immunocompetant (resistant) to the infectious agents in a particular river, and tend to suffer less frequently from these infections.
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If every fisherman would pick up one piece of trash, we'd have cleaner rivers and more access.