How Sonar Detects Fish

What is Sonar?

Sound Navigation and Ranging, or SONAR, is a technology that helps you explore and chart the sea since sound waves travel farther in water than do radar or light waves. This is also why submarines use sonar instead of radar.

Sonar will use sound waves to detect objects in water. A sonar device will emit pulses of sound and then measure the return strength. Anything that has a different density(how solid) from the surrounding water (e.g., fish, plankton, air bubbles, or even the sea floor) can give off a signal. Although extremely useful to scientist and fisherman alike, there are some downsides to sonar.

How Sonar Detects Fish

Fish would waste a lot of energy if they only used constant swimming to keep their present water depth. Many fish, on the other hand, use their swim bladder, a dorsally located gas-filled organ that controls their stability and buoyancy in the water column.

This swim bladder provides an excellent contrast from the water that sonar sound waves can detect. The larger the fish, the larger the swim bladder thus a larger return echo.

A sonar device can detect and display the density and movement of a school of fish, either at an angle of 360 degrees or 180 degrees.

In Japan, the system is employed on a wide range of seiners and bonito boats, to name a few.

In Northern Europe, sonars are used very frequently with trawlers. Miniaturized general-purpose sonars are increasingly being used on smaller fishing boats and pleasure boats in recent years.

Fish Finding Sonar

Fish finding sonar units send and receive signals, many times per second. They focus sound into a beam that is sent by a transducer. These devices include visual displays that print the echoes.

The bottom is drawn as a continuous line across the display. Additionally, any objects in the water between the surface and bottom may also appear.

Fish finders operate using high frequencies of sound, usually 20-200 kHz (20-200,000​ cycles per second). This helps easily identify targets and can even display two fish as two different echoes or arches. These frequencies’ are not heard by humans

Transmitters that produce lower frequencies (50 kHz, for example) can penetrate deeper into the water but may not return a signal about an individual fish. emitting more energy from the transducer will make it more likely to get a signal back in deep water.

Because the boat and fish move, arches are produced on the visual display. A beam of sound is concentrated when it is transmitted via the transducer. As the sound penetrates deeper water, it diffracts and expands. The transmitted sound would appear like a traffic cone with a sharp top and a broad base if plotted.

The fish swimming inside this cone may make some of the sound return to the transducer. The reflected sound, also called an echo, will appear on sonar’s chart display. The shape or formation of a school of fish will depend on how much of the school is within the transducer’s cone.

Different Types of Sonar Used on Fishing Boats

Searchlight Sonar (PPI Sonar), Sector Scan Sonar, and Scanning Sonar are the three major types of sonar.

Searchlight Sonar (PPI Sonar)

The searchlight sonar is a 360-degree sonar system that may show underwater data in a full circle around the boat. The sensor can continuously rotate to display information such as schools of fish and sea currents in a 360-degree radius all around the boat.

It’s similar to looking for things on board your boat with a flashlight. It is frequently seen on little fishing boats, although it is increasingly utilized on recreational vessels these days.

The sonar is usually shown on the screen as a dot in the middle (the boat), with echoes circling around it in a circle.

The Searchlight sonar sends ultrasonic waves to the seabed below, where the echoes are reflected back to the boats transducer.

The sensor’s angle changes when the next ultrasonic wave is sent out. As soon as the ultrasonic waves leave, the sonar switches to receiving mode and listens for bouncing back echoes. Much like how a radar systems works.

The searchlight sonar has a detection range of 6 degrees. By emitting ultrasound and then rotating the beam sequentially, it can detect schools of fish with fairly good precision; however, because its detection angle is very narrow, some areas might go undetected as the sensor takes time to make a full rotation.

Although the Searchlight sonar’s detection speed is relatively slow, the sensor can be miniaturized to fit on small boats.

Scanning Sonar

The sonar emits ultrasonic waves in all directions 360 degrees simultaneously. The returning echoes are then detected and displayed instantly.

Scanning sonar will emit ultrasonic waves in all directions at the same time, 360 degrees. Then instantly the returning echoes are displayed on a monitor.

The detection speed is much faster than that of the Searchlight sonar, and has the added benefit of detecting all of the surroundings at once. This is great for detecting the moments of fish that can swim at very high speeds, such as the tuna fish.

The scanning sonar coming in different scanning capabilities:

Full-circle Scanning Sonar

The system emits ultrasonic waves in all directions around the ship in a single burst, and it can quickly identify and show everything that is going on around the vessel.

Half-circle Scanning Sonar

The sensors at the bottom of the ship use ultrasonic waves to instantaneously search an area that is 180 degrees under the ship.

Sector Scanning Sonar

The principle is the same as that of the searchlight sonar, but instead of scanning in one direction, it scans across 45 degrees. The sector scanning sonar searches in 45-degree steps, so it is 4 to 7 times faster than the searchlight sonar.


Sonar is a great tool for detecting fish, and it can be used in a variety of ways depending on the type of sonar you have. Searchlight sonar is good for small boats, while scanning sonar is better for larger boats or those that need to detect fish quickly. Sector scanning sonar is somewhere in between, and can be a good option depending on your needs.

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