Why are my channel catfish dying in my one acre pond but not my large-mouth bass or bluegill?

as of about 5 days ago we have been losing around two to four catfish a day of all sizes(less than a pound and up to 10 pounds)! the bass and bluegill are not dying..I live is south central kansas. also found one baby red-ear slider turtle dead the other day but that might not mean anything? we stocked the pond wit 50 cats and 30 bass last fall so about 9 months ago. the deepest point of the pond is around 8 foot, i did also find a small catfish today that looked like it had a ''yellow'' mucus on it and from what i saw, little clear mites? could the mites be the problem? please help!!! please and thankyou for you time and answers!