What Is a Fishfinder and How Does Sonar Work?

A fishfinder is an electronic device that uses sound waves to map the underwater world. It sends sound pulses into the water, measures their return time, and builds a picture on the display of the bottom's shape, depth, and fish. Fishfinding is based on the same SONAR technology (Sound Navigation Ranging) that navies have used since the early 20th century. Modern fishfinders give boaters and anglers a clear view beneath the surface, significantly improving both safety and fishing success.

What is a fishfinder and what is it used for?

A fishfinder is a boater's and angler's most important electronic aid for understanding the underwater environment. The device consists of two main components: the transducer, which sends and receives sound waves, and the display unit, which interprets the signals into a visual picture. The transducer is typically mounted on the transom, through the hull, or on an electric motor shaft, depending on the boat type and intended use.

A fishfinder's uses fall into three main categories. The first and most important is measuring depth, which is essential for safe navigation, especially in unfamiliar waters and shallow archipelago channels. The second use is locating fish, which makes fishing considerably more efficient by cutting down on wasted searching time. The third use is mapping the bottom, which lets you identify interesting structures such as rock piles, sunken logs, depth changes, and vegetation areas.

Fishfinders are used both during the open-water season from a boat and in winter for ice fishing. Modern devices often combine the fishfinder with a chartplotter into a multifunction display that shows both the navigation chart and the sonar picture on the same screen. This combination is now the most common solution, since it gives the best overall picture of the boat's position and the underwater world.

How does a fishfinder actually work?

A fishfinder's operating principle is simple but effective. The transducer converts an electrical pulse into a sound wave that travels through water at about 1,500 meters per second. When the sound wave hits a target, such as the bottom, a fish, or vegetation, part of it reflects back to the transducer. The device measures the signal's travel time and calculates the target's distance using a precise mathematical formula.

The sonar process proceeds as follows:

1. The transducer sends a sound pulse downward in a cone-shaped beam

2. The sound wave travels through the water and hits targets along the way

3. The reflected signal returns to the transducer

4. The device calculates distance based on the elapsed time

5. The processor builds image data from the signal strength

6. The display presents the data graphically

On the display, the picture typically scrolls from right to left, so the newest data always appears at the right edge. This means that as the boat moves, the screen continuously draws an updating cross-section of the underwater world. The faster the boat moves, the faster the picture scrolls.

A fishfinder scans continuously, so data keeps scrolling across the screen even if the device isn't moving. To get an accurate picture of the bottom contours, troll or reel slowly and steadily.

Sound travels slightly faster in salt water than in fresh water, which has a small effect on sonar range in different bodies of water. In the lakes and coastal waters most commonly used in Finland, however, the difference is practically negligible in everyday use.

Fishfinder frequencies and what they mean

A fishfinder's frequency choice has a decisive effect on the kind of information the device produces. Frequency is given in kilohertz (kHz), and different frequencies suit different purposes. A low frequency penetrates deeper but produces a coarser picture, while a high frequency gives a sharp picture at shallower depths. Fishfinder frequencies are well above the hearing range of both humans and fish, so they don't scare fish away.

CHIRP technology (Compressed High-Intensity Radiated Pulse) represents a major advance in sonar. A traditional fishfinder sends pulses at a single frequency, but a CHIRP unit sends multiple frequencies simultaneously across a wide frequency band. This significantly improves target separation and reduces interference. CHIRP frequencies are typically grouped into three classes: low (under 80 kHz), mid (80–160 kHz), and high (150–240 kHz).

In devices equipped with CHIRP technology, such as Lowrance fishfinders, target separation is significantly improved. In practice, this means you can pick out an individual fish from a school, or see a jig lure clearly on the display.

Fishfinder transducer types and installation

The fishfinder's transducer is a critical component whose correct selection and installation directly affect image quality. There are several transducer types, and the right choice depends on the boat's material, shape, and intended use.

A transom transducer is the most common and easiest to install. It's mounted on the boat's transom so the transducer face sits at or slightly below the waterline. A transom transducer is especially suited to outboard-powered boats and smaller craft. Its advantage is easy installation without modifying the hull.

A through-hull transducer is installed through the boat's bottom, offering the best possible image quality since the transducer is in constant direct contact with the water. This installation method is recommended for larger boats and sailboats. Through-hull transducers are often made of bronze, which withstands salt-water corrosion well. Check out Raymarine's transducers, which include a wide range of through-hull models.

An in-hull transducer is installed inside the boat's hull, so the signal passes through the fiberglass or plastic. This is a convenient solution when you don't want to drill holes in the hull. In-hull mounting, however, isn't suitable for wooden boats, cored (sandwich) construction, or fiberglass that's too thick, since the signal won't pass through these materials.

A trolling-motor transducer is mounted on the shaft of a bow-mounted electric motor and is a popular solution among anglers. It allows sonar scanning right from the bow of the boat, which is especially useful for precision fishing. Garmin's transducer range includes several models suited to trolling motors.

Pay special attention to correct transducer installation. An incorrectly installed transducer causes interference in the sonar picture, especially at higher speeds. The transducer must not be positioned at a strake or in the path of air bubbles. See our fishfinder installation guide, which helps you avoid the most common installation mistakes.

How do you read a fishfinder display?

Reading a fishfinder display takes practice, but you'll pick up the basics quickly with hands-on experience. The bottom appears at the lower edge of the display, and its thickness and color indicate how hard the bottom is. Fish typically appear as arches or lines, depending on the boat's speed and the fish's movement relative to the sonar beam.

Fish arches form when a fish swims through the cone-shaped sonar beam. The shape and thickness of the arch give clues about the fish's size: a thicker, taller arch generally means a bigger fish. If a fish only swims through the edge of the beam, only a partial arch or a plain line appears. Complete arches require the fish to swim through the entire beam at a suitable boat speed.

Fish symbols, or automatic fish identification, are generally best switched off. The device easily mistakes aquatic plant leaves, air bubbles, or other targets for fish, leading to false readings. Learning to interpret the raw sonar data teaches you to read the fishfinder more effectively and gives a more accurate picture of what's happening underwater.

On a color display, target strength is shown in different colors. Generally the strongest returns appear red or orange, medium-strength ones yellow, and the weakest ones blue. This helps distinguish hard targets from soft ones, and big fish from small ones.

Adjusting sensitivity and depth range

A fishfinder's most important setting is sensitivity (also called gain), which controls how readily the device registers returns. Getting sensitivity right is key to a clear sonar picture, and it varies with depth and conditions.

The basic principle is: the shallower you're fishing, the lower the sensitivity should be set. As you move into deeper water, sensitivity can be increased. A good sensitivity level for ice fishing is typically in the range of 50–75%, depending on depth.

A practical method for adjusting sensitivity: lower your lure to mid-water and adjust sensitivity so the lure appears as a thin line on the display. With sensitivity too high, all targets are drawn in the same strong color, making it hard to tell sizes apart. Excess noise and interference also appear on the display. With sensitivity too low, even small fish go undetected.

It's generally best to set the depth range manually rather than using automatic mode. If you're ice fishing at, say, seven meters, set the depth range to 10 meters. Automatic depth range can mistake a dense school of fish for the bottom, causing the display to jump distractingly between two depths. Manual adjustment gives a steadier, more reliable picture.

Other useful settings include scroll speed, which affects how quickly the picture updates, and the color palette, which can be changed to suit conditions. In bright daylight, a higher-contrast color palette works better.

Interpreting bottom composition from a fishfinder

Besides depth, a fishfinder also tells you about bottom composition, which is valuable information for anglers. The thickness and color of the bottom line reveal whether the bottom is hard or soft material. A thick, strongly colored bottom line indicates a hard bottom such as rock, gravel, or hard clay. A thin, paler line suggests a soft, muddy bottom.

Identifying bottom type matters for fishing, since different fish species favor different bottoms and structures. Perch often prefer hard bottoms near rocky areas, while bream favor soft, muddy bottoms while foraging. Pike-perch typically hunt along the borders between hard and soft bottoms, where smaller fish move.

Vegetation appears on the fishfinder as vertical lines or an indistinct mass above the bottom. Telling vegetation apart from fish can be tricky at first, but on a color display they usually show up in different colors. Plants often appear greenish, while fish show up yellow or orange. Baitfish schools often appear as clouds or dense clusters in mid-water.

Sunken logs, rock piles, and other structures appear as distinct shapes on top of the bottom. These are often the best fishing spots, since they offer fish shelter and food.

Choosing the right fishfinder

Choosing a fishfinder requires considering intended use, boat size, and budget. In the under-€300 price range you get a simple basic unit suited to occasional fishing and depth monitoring. These models typically have a smaller display and traditional frequencies without CHIRP technology.

In the mid-range €300–800 models, you already get CHIRP technology, a larger display, and often integrated GPS. This price range suits active anglers who want a more versatile device. Garmin fishfinders include several popular models in this range.

In units over €800 you get access to top-tier technology, such as DownScan and SideScan imaging, large touchscreens, and networking with other devices. Raymarine's multifunction displays represent this professional tier.

Ice-fishing kits are available separately for winter fishing, typically including a small portable unit, a battery, and an ice transducer as a ready-made package.

The most common usage mistakes in fishfinding

Using a fishfinder effectively means avoiding a few common mistakes. These issues come up especially often with beginners, but recognizing them helps you improve faster.

1. Going too fast: At high speed, air bubbles (cavitation) form under the transducer, disturbing or completely blocking the sonar picture. The depth reading may disappear, and interference lines appear on the screen. Slow down if the sonar picture drops out or becomes disturbed.

2. Wrong sensitivity setting: Sensitivity that's too high shows excess noise and false fish readings. Sensitivity that's too low leaves real fish and structures invisible. Actively adjust sensitivity to match depth and conditions.

3. Using the fish-symbol feature: Automatic fish identification interprets almost anything as a fish, leading to unrealistic readings. Turn off fish symbols and learn to read the raw sonar data.

4. Incorrectly installed transducer: Transducer installation mistakes are the most common cause of a poor sonar picture. The transducer must not be positioned at a strake or in the path of air bubbles, and it should be level with the boat's bottom. Study the installation instructions carefully.

5. Automatic depth range: A dense school of fish or vegetation can confuse automatic depth range, causing the display to jump around uncontrollably. Use manual depth range for a steadier picture.

6. Understanding beam angle: A fishfinder shows all targets within the cone as a two-dimensional picture. A wide beam angle covers a large area but places targets at the same level even if they're actually off to the side of each other. This can lead to misjudging a fish's exact location.

Frequently asked questions about fishfinders

What is a CHIRP fishfinder and how does it differ from a traditional one?

A CHIRP fishfinder transmits multiple frequencies simultaneously, while a traditional unit uses only one frequency. This significantly improves target separation: with CHIRP technology you can distinguish individual fish from a school, or see near-bottom targets much more clearly. CHIRP is now a standard feature on almost all modern units.

How deep can a fishfinder reach?

Maximum depth depends on frequency and the unit's power. Low-frequency (50 kHz) professional sonar units can reach as deep as 750 meters, while typical recreational units at 200 kHz work reliably down to about 150 meters. In Finnish lakes and coastal waters, a depth capacity of around 200 meters is generally more than enough.

Can a fishfinder identify fish species?

A fishfinder doesn't directly tell you the fish species, but an experienced user learns to recognize species from their typical behavior and how they appear on the sonar picture. For example, pike-perch typically appear as single arches near the bottom, perch as smaller schools near rocky structure, and pike as a large single target in mid-water or at the edge of vegetation.

Does a fishfinder work for ice fishing in winter?

Yes, it works excellently. Many fishfinders are also designed for ice-fishing use with a separate ice transducer. Ice-fishing fishfinder kits usually include a battery, a carrying case, and an ice transducer. In winter, a fishfinder is especially useful because you can see fish reacting to your jig in real time and adjust your technique accordingly.

What's the difference between DownScan and SideScan sonar?

DownScan scans straight down and produces a photo-like, detailed picture of the bottom and structures directly under the boat. SideScan, or side-imaging sonar, scans to the sides and shows a wider area on both sides of the boat, out to as much as 180 meters. Both technologies complement traditional sonar and are especially useful for finding structures and fish over large areas.

How do you maintain a fishfinder?

Maintaining a fishfinder is simple. Clean the transducer regularly with a soft cloth and fresh water, especially after use in salt water. Check the transducer mount at the start of the season. Update the device's software from the manufacturer's website, since updates often improve performance and add new features.

Fishfinders from Marinekauppa

A fishfinder is an essential tool for every boater and angler who wants to understand the underwater world. Used correctly, it improves both navigational safety and fishing success. The key is learning the device's settings and how to read the display, since simply owning the unit doesn't guarantee results – skill comes through practical experience.

In Marinekauppa's extensive range you'll find quality fishfinders from leading manufacturers such as Lowrance, Garmin, and Raymarine. A wide transducer selection makes it easy to find the right solution for every boat. Also see our fishfinder installation guide, which helps you get the most out of your unit.

Sources

Totalvene.fi – Introduction to sonar technology

Garmin – How CHIRP technology works

Lowrance – Fishfinder user guide and transducer technical data

Deeper Sonar – Guide to interpreting your fishfinder

Disclaimer

This guide provides general guidance only and does not replace the product manufacturer's operating, installation, or service instructions. Marinekauppa is not responsible for actions taken based on this guide or their consequences. Always check product-specific instructions and consult a qualified professional if needed.