I have a Humminbird Fishing Buddy depth finder I use on my portable Bass Baby boat. I've noticed something unusual but don't understand what is causing it.

I have been fishing this same small reservoir several times and see the same thing happen. During the day I get lots of fish ID returns over deep water as well as what I thought was either weeds or a denser cold water thermocline. it seems too early for weeds to me.

Then after the sun goes down the returns occur in mid-depth. Same numerous fish ID returns and also what looks like a thermocline. Here is a picture after the sun went down and the returns moved up.