I've been fishing for steelhead for many years and I've caught lots of summer and winter fish. This is a question I've always had. For the life of me, I can't figure it out. How do you tell the difference between summer and winter fish,other than the time of year! I caught a chrome bright hen yesterday on the upper sky, and a couple guys thought it was a late summer and another guy thought it was a early winter fish. the eggs were pretty tight. I know it is a fresh fish but I can't tell which strain. I know it's probably not a big deal , but I'm just curious to find out if there are any physical charactaristics that will help determine the answer to my question.
The fishing was real good yesterday, alot of fish caught from cable hole up. Thanks for any info.

Ah yes, this really gets to be a problem when mankind starts introducing both strains into the same river. You see most steelhead spawn during a much more compressed time than when they return - Skamania stock summer runs ripen almost exactly the same time as Chambers Creek winter runs for example, and those are the two stocks that have been plastered all over the place in years past. The result is the summer runs get later and the winter runs get earlier. For example, in the Bogy/Calawah they collect all the hatchery fish together, the ones that ripen first are "summer" runs and the ones that ripen later are "winter" runs - their spawning time maybe differs by a month. Probably happens everywhere else too, so if you caught a brat likely you got one of those mix and match jobs

In the natural world summer fish evolved to take advantage of snowmelt dominant systems, like the Columbia and upriver tribs, so they could get through the bad spots during high summer flows and spawn early in the fall before everything froze up. In these systems they dominated. Winter fish by contrast evolved to handle rivers that went crazy in the winter with rain and wild swings in meander geometry, by migrating during high winter flows and spawning when things settled down in the spring. In short run coastal streams like those on the OP they dominated. But every system had a few of one or the other that kept trying different trajectories, and a few real stable heavily wooded lowland streams even had runs that spawned with the salmon, all late fall and winter, although in those streams salmon seemed to dominate. So if you caught a wild (or anymore, "locally adapted") fish, it may have been a varient that was trying to explore a new trajectory and perhaps exploit a new niche. Certainly, in this year of extreme drought maybe some new trajectories are called for. I hope it was a wild fish, and you wished him luck on his way back into the water

The line between winter and summer steelhead is not absolute. Generally, a summer run enters freshwater many months before spawning and sexually immature, whereas a winter run enters a few weeks to few months before spawning and is already starting to sexually mature (males have an elongated lower jaw even when chrome, gonads are relatively developed). They pretty much all spawn late January into July. Not sure where Spawnout got his info on Columbia river fish. (Deschutes fish arrive June through December and usually spawn in late winter, early spring. They're all considered summer steelhead, by the way.)
I once heard several biologists admit in a discussion of steelhead run timing (on the Sandy River), that they cannot tell in many cases when a given steelhead in a given river will spawn (based on appearance). Thus, it can be hard to tell.
If the fish is hatchery origin, sometimes you can tell by the finclips which strain it is.

Woody -
Separating summers and winters; especially at this time of year can be difficult. The most reliable method on the Skykomish as well as most Puget Sound rivers (especially for the hatchery fish) is to look for parasitic copepods. This lice-like parasites attach themselves to the fish during the summer. Usually you can find them in the mouth (roof most common location), on the gill rakers, and at times at the base of the fins (most commonly the pectorals). The parasite is about the size of a grain of rice with a whitish to tanish coloration. Each summer ususally has a hand-ful to dozens of these parasites.

For fish that have been killed looking at the entrails also can helpful. The summer fish have generally exhausted their fat reserves thus will have paler flesh than a winter fish. Locally hatchery winter fish begin spawning in December where summer fish begin in January. Therefore this time of year "ripest" fish will be hatchery winters - not summers.

Things can get complicated - for example it is not uncommon to see summer fish on the lower rivers this time of year. Some of those fish will have copepods- however they will be dead (paler in color and beginning to shrivel). These fish have been in another river (say the Stilli) dropped out to the salt (kill the parasite) and then re-entering another river (Snohomish).

As you know this time of year it is common for a bunch of fish to show up at Reiter (especially following the first big rains). Most if not all of these fish are not fresh run fish but rather summer fish that have been scattered throughout the river (mostly upstream of Reiter) that are returning to "home" with the river rise. Except for the rare winter fish they are not just entering the river.

Tight lines
Smalma