Topic 18 - Fish school display during mooring and brief stops

How do fish schools get displayed on the fish finder screen when the boat is not in motion? Lets think of an example where your boat is at a standstill over an area with large rocks.

The fish finder is in operation, and the image is sequentially streaming from right to left. In the picture below to the right, we can see that at first there is no indication of fish schools and then a fish school appears, swimming over the rocky area.

For the sake of argument, lets assume that the fish school remains in the same position while the boat is still. We will see the returning echo from the fish school appear continuously from the right to the left, it will keep on displaying the fish school in the same manner as long as it stays in the same position.

Even though this fish school in reality looks like a large lump, on the fish finder screen it looks like a large fish school. The rough seabed will also appear as a smooth line since the boat is not moving. If one stays in this position long enough, the fish school fills the entire fish finder screen and it is easy to misinterpret as one enormous fish school.