It's definitely a school of "fish" ... but, the "size" of those blotches seem to indicate fish larger than the average Fall Shad size IMHO. Unless, of course, your Gizzard Shad DO get quite large in this "small lake".

It almost looks like a school of White Bass crashing the party of a school of Shad ... but, that could also be the school of "whatever" reacting to the boat going over them, and them diving to a slightly greater depth.

The only time I've seen a school of Shad "pinched" at one end was when I was Pushing jigs at Green River Lake while watching the screen on my Down Image unit .... and there were 5 "Crappie" at the narrowed end of the school. That was my first thought when I first looked at your screen image.