Besides appearance, winter fish will be totally ripe and ready to spawn meaning egg/milt sacks will be full. Summer fish will be roughly half ripe meaning eggs/milt will be a little over half mature. Summer fish spawn late winter/early spring, whereas winter hatchery fish will be pretty much ready to spawn on arrival.

There's nothing prettier than a fresh winter steel
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"I have a fair idea of what to expect from the river, and usually, because I fish it that way, the river gives me approximately what I expect of it. But sooner or later something always comes up to change the set of my ways..."
- Roderick Haig-Brown