Big gizzard shad look a lot like crappie on traditional sonar. That might be what you're seeing, especially this time of year.
As others have said, unless you are pulling some heavier than normal jigs or adding a split shot above your jig, I doubt you're getting to 12 ft. at 1.0 MPH. As reference, my 1/16 oz. curly tail jigs run about 6 ft at 1.0 with 4 lb test mono.
To get an education on depth at a certain speed and weight, find a flat to test your current setup then you'll know for sure what speed equals what depth.

Chris