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I believe it's some type of bacterial infection. Not uncommon, mostly see it on lake trout this time of year. Don't see it as much during the summer, although that's just observational, I don't think we've ever kept data on it. Potentially related to stressors from water temp changes in combination with disruption of skin/mucus - say from spawning and digging gobies out of sediment all winter long

There's a ton of different bacterial infections out there and I'm not a disease expert. But it looks sort of similar to the "winter ulcers" found in farmed trout and salmon

veteriankey.com/wp-content/uploads/2016/..._En_6_Fig14_HTML.jpg

Wow that a pretty severe case but yeah I would have to agree with the Ulcers reference. Nice to know as this has been the first time I have ever seen or noticed it that I can recall. Thanks for the reply and insight.