Fish Finder Fundamentals
Shopping for a new fish finder can be a daunting task with the number of manufacturers out there and the number of different models available. When fisherman come in to Mark’s Marine looking for a new fish finder the first question we always ask is, “Where do you do most of your fishing?” We ask so we can make sure the fisherman leaves the store with the right transducer and display for the body of water he is spending most of his time fishing.
Transducers operate at specific frequencies and cone angles or beam widths. The sonar signal leaves the transducer in a cone or fan shape and gets wider the deeper it goes. These transducer properties make them good at some things and not so good at other things. Higher frequencies do not travel far in water and typically have narrow beam angles. Lower frequencies travel farther in water and typically have wider cone angles. Narrow cone angles are great for marking fish and wider cone angles are better for scouting for fish since their wide cone angles are covering a larger section of the water column under the transducer.
New fish finders can be purchased bundled with a 3-in-1 transducer or, as a separate, stand-alone head. The bundled, 3-in-1 transducers, generate 3 different types of sonar information: traditional, down imaging, and side imaging. Traditional sonar view is what most of us are used to seeing on our fish finders and displays the fish at various depths as nicely defined arches. These arches are formed when the sonar ping is reflected off the fish’s internal air bladder. Available traditional sonar frequencies are between 50kHz and 200kHz. Down and side imaging frequencies fall between 455kHz and 1200kHz. These much higher frequencies provide a very detailed image of the bottom but are only good for shallower water.
The traditional sonar and down scan data are rendered on the fish finder in the same perspective. Let’s call it a “Fishes Eye View.” When viewing the screen, think of the back of the boat is the far-right edge and yourself as being in the water on the driver's side of the boat, looking through the cone or beam towards the passenger side with time passed (not distance) scrolling to the left. The down scan perspective is the back of the boat is the top edge of the screen and you are viewing from above. A “Birds Eye View” with the bottom stretching out to the right and left and time scrolling towards the bottom. The dark strip down the middle is the water column directly under the boat, splayed flat, so the real-world 3D scene can be rendered on a 2D display.
These graphics from Humminbird do a really good job of demonstrating how the side imaging data is rendered onscreen. Bottom topography, downed trees, stumps, rocks, and suspended fish are clearly visible on the fish finder screen. In normal down and side imaging views, fish show up as light-colored specks, sometimes very “fish like/shaped” in shallow water. Traditional sonar shows the fish as an arch, or streak depending on boat and fish speed. This arch is created when the sonar signal is reflected off of the fish's internal air bladder.
A few years ago, Lowrance did something very innovative that no other fish finder manufacturer has done; they combined their traditional sonar view with the down scan view in what they call Fish Reveal. The combination of the two views makes it much easier to identify fish targets when they are suspended in vegetation or intermingled with rocks or debris. This is a standard feature on any of their units bundled with a 3-in-1 transducer.
Lowrance recently announced their new Fish Reveal Side Scan that shows the traditional fish arches in the side scan view. Again, making it very easy to see where the fish are hiding in the side scan view. This new feature does require purchasing a separate S3100 module and compatible transducer.