I'm with Young Salt on this issue. If I catch a well-colored December hatchery buck I quickly check to see if it's got the "bugs," or mites down its throat. If it does then most often I'll release it because it's probably a summer-run fish. If there are no bugs then it gets conked because it hasn't been in the river that long. The bugs jump on the summer-runs during their time spent in the river. Also, I heard that colder water inhibits the activity level of these parasites, thus steelhead that enter the river later in the year, while the temperature is rapidly dropping, will not get attacked by this parasite. By definition a winter-run enters the river from November onwards. So a "dark" December Buck could be a fish that has spent a month in the river or been there all summer. The bugs will tell you how long. Disclaimer: Like Salmo G. said there exceptions to everything, so don't quote me on any of this stuff! This is what I go by. My partner just clubs them all.