I do agree with Kenny about pathogens and other things that cause issues with winter time fish health.
Stressors are significant. If fish, cold-blooded as they are, have to try to move quickly in cold water to escape from being eaten, and do that over and over, that's definitely a stressor. Heck, that's a stressor in summer as well. But in winter, if a fish is compromised, bruised, injured, poked, chased into brush or something, it out protective layer can be compromised. If slime is scraped, skin injured, muscles bruised, that opens the door to bacterial attack. In cold water, fish can't heal as readily as they can in warm water, so an infection can spread over a fairly short period of time. When infection manifests on the surface of the skin, and it doesn't heal quickly, saprolegnia attacks. Saprolegnia is that gray, mossy-looking fungus that attaches to the fish. When the fungus hits, the fish is pretty much gonna die. It's not the fungus among us, it's the chase/stressors of environment stuff that starts the process.
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