If I had to guess the ones on top are 30" + with the bigger ones on the bottom that surface and literally push these guys out of their way. I call it a canal but it is a well flowing part of the ICW that was originally a fueling dock and pings 10' to 18' on my depth finder. They sit there and pop on the shrimp that go by and is actually quite entertaining to observe.
Here's a couple bigger ones that pushed their way to the top and the smaller 30" ones give em a wide berth. Hard to put anything in the pic for size reference without including a channel marker- sorry.