I often get comments from anglers that caught large rainbows in the Colorado or Blue and they wonder if it is one of the whirling-disease resistant fish that we have been slowly getting established. I started stocking large numbers of very small WD-resistant rainbows (a strain that we call HXC's) into the Colorado and other rivers starting in 2010. Those fish have been growing 5-7" annually. The 2010 year class in the Colorado is now in the neighborhood of 14" and I'm hopeful that we may see wild recruitment in 2013 for the first time in decades. So, if the fish in question is is larger than 16", chances are very good that it was stocked privately. There are a handful of private landowners along some of the rivers that do stock fish into their reaches of river. We see these privately stocked fish show up in far-flung locations, both upstream and down. The other source of "non-wild" rainbows into the rivers is if one of the large reservoirs where we stock 10" catchable rainbow trout happens to dump some of those fish as it is spilling.
We're trying to manage the genetics and usher in a new golden era, if you will, of wild, self-sustaining rainbow trout in our rivers. This takes time and patience but I am confident that we are on the verge of seeing this effort pay off, big time. When highly domesticated fish move into these areas and mix with the HXC's, it may hinder this process if they manage to inject their genetics into the mix. So if a large number of these fish turn up in our sampling reaches, it presents a bit of a concern.
There are some other ways to tell if the fish you caught is not wild. Often the stocked fish are skinnier and have less color, such as the example below.
Now, take a look at this dorsal fin:
This is the dead giveaway that this fish was raised to a large size in a hatchery. Wild fish (or fish stocked at a very small size, therefore virtually wild) have a normal dorsal fin with straight fin rays and it's obvious when you lift the fin.
Once you get in the habit of looking closely at the dorsal fin on the fish that you catch, it becomes very instructive.