Growbug said:
Been seeing a lot of these higher up the river, but they all seem to be around the 5-6" mark. Do they get any larger up there? Also, one of my neighbours said that there are a few "SeaRun Cutthroat" up there. What are these? Are they like Steelies are to Rainbows? LARGE Cutthroat with an attitude?
Above Willamette Falls, you won't have cutthroat trout that are searun. They may go down, but they don't come back.
Willamette Valley cutthroat are native and generally not the result of stock breeding. There has been crossbreeding between rainbow and cutthroat, but us fisherman will call a cutthroat trout by the coloration and teeth rather than genetics. Willamette Valley cutthroat exhibit all the life patterns of cutthroat elsewhere with the exception of being searun: local non-migratory, migratory along a stream/river, and migratory to/from a lake/impoundment. The lower Willamette has a deadly parasite that is active due to higher water temperature and the parasite prevents cutthroat from spending much time in the lower Willamette, thus migratory patterns do not cross the lower Willamette (say from Salem downstream).
Generally speaking, the closer you are to the headwaters of a stream, the smaller the cutthroat are (except if when you encounter a spawning run). Food resources are just not as rich in the smaller bodies of water, but if the water is free-flowing to a larger body, you'll likely see some large fish visiting spawning grounds.
Cutthroat trout tolerate higher water temperatures than rainbow trout, so occupy a niche in the Willamette Valley ecology where the more aggressive rainbows can't survive. This niche has allowed them to readily overcome habitat destruction and overfishing without much intervention from humans.
This past summer, OFF had many reports/pictures of largish cutthroat in the lower Mollala River, before the BLM land. Several 12-13" fish were mentioned. I visited the BLM land, probably about halfway up, one August evening to tinker with flies, bobbers, and jigs -- didn't see anything larger than 6", but the two hours spent there allowed me to test some techniques that I'm reserving for times when spinners quit working.
One last observation: water mineral content and diet will make some Willamette Valley cutthroat take on a coppery hue; they're still just regular cutthroat trout, not some exotic species. Mollala River cutthroat don't exhibit much of this coloration.