On my home lake here in E Tx, we had some really freezing weather that lasted several days and in the process had a massive shad kill. The dead shad have settled on the bottom and the catfish are feasting on the dead shad so much that they don't want anything that moves at all. You can drop a cut shad on top of them and can't get bit.
Same thing for crappie. You can see them holding deep but, can't get them to bite.