Because they disappear... I find the exact same thing in open water during the summer. Absolutely slamming them from about 2hrs before sunset until just after sunset then zip, it's like a lightswitch. I've gone deeper, shallower, you name it..
We took a measurement with a fishing rod, right around 38 inches. No girth measurements taken. It also made the rounds at the local MNR office from what I've heard.