The deaths in hot weather could be catch and release stress, HSB are very susceptable to catch and release mortality when the water temps are above 82 degrees.

The sores could be from a bacterial infection, fungal infection, or some sort of parasite.

Here is an old brochure on things that affect fish in Nebraska.

http://lamarck.unl.edu/hwml/pdf-files/bugging-fish3b.pdf