FNP,

Can't say for sure. The two most common causes that come to mind are healed injuries and deformities, the latter usually on hatchery fish. As another poster indicated, wild fish with deformities usually don't survive very long.

The strangest deformity (if you'd call it that) that I ever saw was a 6 pound albino winter steelhead. It was absolutely white all over, with the beginnings of the palest red rainbow stripe down its side. Surviving to the returning adult stage must have been against million to one odds.

Sincerely,

Salmo g.