I'm no expert, however the one thing I got out of the sonar class at crappie academy from the humminbird field rep is that the arch indicates the the fish has moved into and out of the sonar cone, either by swimming or the boat moving. Fish that are still in the sonar cone will be a line. He said the arches look good for marketing the units but it's not what you want to see, lines indicate that they are still under the boat.