Here is a basic refresher course on how a sonar makes a fish arch. Now remember there are lots variables that effect how a fish arch is displayed such as to name a few boat speed, distance from transducer, location in sonar cone, frequency and size of fish. If you want to learn more about the variables Doc has an excellent tutorial DVD on this subject and many more at the following link -> https://doctorsonar.com/collections/dvds/products/doctor-sonar-educational-dvd-understanding-sonar-interpre