Ark

In the bottom of things like this you will find sediment build up (silt ect). You have less of a return and this will show as darker. What would interest me more is what was the water temp in and around this depression? Differences in water flow due to disturbed sediment and temp could give the return I'm seeing just above the depression (what you are taking for bait fish).

You may have just found a underwater spring. The only difference being that in the shots I've seen of springs the trail of higher temp water mixed with sediment is strung out more and they are mostly winter shots.