sores on koi

3 days ago I noticed a sore on my large koi, today there is one on another koi.
Both koi are white females, one a butterfly. Both are about 20" and in great
health. They are on the side of one and the top of another. the one with the
sore on its back has a large shred of skin following. It looks exactly like
they rubbed on some of the rocks. There is no preditor behavior and both are
being social and eating like the pigs they are. My question is....why would
they get scraped on the pond for the first time within 3 days of each other,
and in a pond they have been in over 5 years? Do female koi go after each
other for territories or males ....or any strange thing like that??? Would the
channel catfish ( who is their size) gone after and scared them for some reason
??? Any ideas They have spawned in this pond for several years and never
got scraped up. Jerri

http://www.fringeweb.com/Ponds/JerrisPond