Did you ever notice or wonder why the fin on a fish looks like its been clipped? That's because it might have been clipped. The New York State Department of Environmental Conservation clips one fin (usually Adipose) of every stocked Skamanian Steelhead. They do this at the hatchery before they release them into a new setting so they can tell them apart from the wild species. This provides a way for the DEC to keep an accurate account of natural reproduction in a creek system. In some cases, there can be a double clipping (one on the Adipose Fin and one on the Pectoral) as seen by the stocking project on Cattaraugus Creek in Western, NY. The Canadian DEC stocked Steelhead in Lake Ontario get a clip on the left Pectoral Fin. Various clips for various areas. The domestic rainbow strains that are stocked do have a fin clipped at all.
If you look closely at the photo of me with this nice "domestic" Rainbow Trout caught in Western, New York, you can see the dorsal fin is not clipped but rotted or torn. Knowing your different fins on the body of a fish and also knowing regionally what your DEC biologists are doing to mark fish in your area can be very helpful in identifying wild strains vs native.