Here is more info regarding the "Fish Symbol" (I called it Fish ID, which was wrong - sorry). I copy/pasted this from Wikipdeia. Here is the full link:
http://en.wikipedia.org/wiki/Fishfinder
Fish symbols in the screen of the fishfinder
Fishfinders usually have a "Fish Symbol" feature, which will display a FISH symbol for almost any echo not connected to the bottom or the surface. More specialized units use a detection of a special fingerprint in echosignals. With those you can find living fish and fish that have recently died. With these units also in some cases fish spawn will be shown as a large fish. Some kinds of seafish will be not marked as fish, but you will see an echo.
Fish ArchesWith the Fish Symbol feature disabled, an angler can learn to distinguish between fish, vegetation, schools of baitfish or forage fish, debris, etc. Fish will usually appear on the screen as an arch. This is because the distance between the fish and the transducer changes as the boat passes over the fish (or the fish swims under the boat). When the fish enters the leading edge of the sonar beam, a display pixel is turned on. As the fish swims toward the center of the beam, the distance to the fish decreases, turning on pixels at shallower depths. When the fish swims directly under the transducer, it is closer to the boat so the stronger signal shows a thicker line. As the fish swims away from the transducer, the distance increases, which shows as progressively deeper pixels.
When threatened, baitfish form a tightly packed school, as the individuals seek safety in the center of the school. This typically looks like an irregularly shaped ball or thumbprint on the fishfinder screen. When no predators are nearby, a school of baitfish frequently appears as a thin horizontal line across the screen, at the depth where the temperature and oxygen levels are optimal.