Depth sounders (also known as fish finders and echo sounders) are electronic units that are used to determine the water depth and bottom conditions, and the location of fish schools and individual fish targets. They have come a long way from the days when they were primarily used to determine depth by printing on graphing paper. Today, the one unit can incorporate colour LCD screens, GPS with maps and chart plotting capability, dual frequencies (200/50 kHz), high wattage output, and even fish radar and electronic compass.
How they WorkWhen the transducer sends out a sound wave (frequency) through the water, it is reflected by objects in the water. The harder the object - the stronger the echo will return, and therefore, it will show up darker on the display. For example, if you are fishing over a hard rocky bottom, the screen will show the bottom contour in a dark colour. In some instances, it will even display vegetation that is directly above the sea bed. When the sound wave heads towards a fish, it’s the air bladder of the fish that returns the echo back to the fish finder. Larger fish will have larger air bladders and therefore, return stronger echo signals. Bait fish usually show up on the display as a ball because the echo is being returned on their collective air bladders. The air bladder on a single bait fish is not significant enough to return an echo, but as a school, their air bladders can be picked up.