coded wire tag / it is a tiny wire with a numerical on it that is imbedded in the fishes nose so to speak when they are fry.

Another possibility is the volunteers that use remote site incubators ( RSI ) which get eggs and hatch them and release the fry. They use bio rings rather than gravel and but the bio rings are smooth. Still to much flow will make them move around . These fry are not marked before release.

One final thought is that there is a difference between a wild ad fin and hatchery. Bit of an old wives tale for normal timed Coho. Now the late timed wild Coho do go against that with a rather large ad fin but that tends to be proportional to size and predominantly West Fork & Middle Fork Satsop fish that return mid Dec through Jan.