I was told from a private hatchery manager that they are natural in the environment, when fish get stressed out they are subject to them.
A few of the causes for them getting stressed are crowding,warm water etc.
Kokanee are more vulnerable than other species and can't seem to get rid of them as Rainbows do.
Last year there was an algae bloom and die off of a lot of Kokanee which most likely led to high stress levels.
When Rainbows in my ponds had them they would jump out of the water trying to knock them off their gills and recovered from them.
Imo, the removal of large Predators at Blue doesn't help the situation, just like Wolves taking the weakest or diseased prey, Predators help keep fisheries healthy.