I hazarded a guess on your post on the Main Forum (sensitivity too high) ... but, on second look, here, I have a question : were you moving or sitting still ?? I know that it will draw out any signal across the screen, if you're not moving much ... even turning fish arches into straight lines.
I don't know much about HB's, or 2d sonar for that matter ... so I'm just guessing based on what I've seen/read on here, and having used 2d sonar (black/white) units since the 1960's. But I never could read them with any accuracy, so they were basically just used to tell me depth (and the occasional piece of cover I might cross over). I'm currently using a Down Image unit, which pretty much shows me a "picture" of what's there.