Encountered something very similar on a local community lake last week. Came upon a massive school of IDK what in 20 fow. School was from the bottom to @ 15 fow and extended from 10' behind me to 40' forward according to the livescope. I tried everything I had including jigging spoons but did not connect with anything. Several blips appeared to be 10-12". I do know there are gizzards in this lake, and some of them are big. It was a huge school. Made me wish I had a cast net just to see.
Sure sounds like shad, they are known to bunch up in cold water.