Fish Finders: How Sonar Works, the Types, and Which Unit to Buy

A fish finder is a sonar. A transducer under the boat sends a pulse of sound into the water, times the echo, and the screen draws what came back. Sound travels about 1,500 m (4,900 ft) a second in water, so the echo from a fish 30 ft down is back in about a hundredth of a second. A first unit costs about $120; live sonar that shows fish swimming starts at $999, and the best fish finder guide gives the pick at every price between.

Lowrance’s first portable fish finder, the FISH-LO-K-TOR or “Little Green Box”, went on sale in 1959. In 1975 it appeared in Jaws.

Start here

• Best fish finder: the pick at each price from $120 to $3,000, what every step adds, and the Black Friday price to wait for.
• How to read a fish finder: arches and dots, hard and soft bottom, bait, the thermocline and the settings.
• Bass fishing: the fish that drove the live-sonar boom, and where it holds season by season.
• Fishing gear: the rods, reels and line the electronics are meant to put in the right place.

Looking for the sliding-sinker bait rig instead? That is the fish finder rig.

A landing net over the water. Photo: etitarenko / Depositphotos.

How does a fish finder work?

Every fish finder runs the same cycle, many times a second. The display tells the transducer to fire a short pulse, a “ping”. The transducer turns that electrical pulse into sound. The sound spreads down through the water until it hits something that is not water: the bottom, a rock, a weed bed or a fish. Part of it bounces back, and the transducer turns the echo back into an electrical signal.

The display then does the arithmetic. It knows how long the echo took and how fast sound moves in water, so it knows how far away the target was. NOAA puts it in one line: “By measuring how long it takes for an echo to return, scientists can figure out how deep an object is.” The strength of the echo becomes a color. Hard bottom and big fish send back strong echoes and get the hottest color; mud and small bait send back weak ones.

Each ping becomes one thin column of pixels at the right edge of the screen, and the older columns scroll left. That is why a standard screen is a record of the last minute, not a live picture. The how to read a fish finder page builds on that one fact.

Sonar, depth finder, fish finder: what is the difference?

They are the same machine. Sonar is the method, sound navigation and ranging. A depth finder, or depth sounder, uses it to show one number. HawkEye’s DepthTrax 2BX dash gauge, for example, reads to 600 ft (180 m) and sounds an alarm at a depth you set. A fish finder draws every echo as a picture, so the bottom, the weeds and the fish appear around that number.

That difference is how the fishfinder was born. The echo sounders of the 1940s heard the strong echo from the seabed. In 1949 Japan’s Furuno Electric commercialized what the IEEE recognizes as the world’s first fish finder, with an amplifier sensitive enough to pick up “weak echo signals reflected from fish schools.” Today every fish finder shows the depth, and on a boat the depth finder most anglers buy is a $120–200 fish finder.

A fish finder on the bow at sunset. Photo: FedBul / Depositphotos.

Does a fish finder show actual fish?

It shows echoes, and an echo is a fish only when its shape and place say so. On standard 2D sonar a fish draws an arch as it passes through the beam. On imaging sonar it is a bright dot, often with a shadow. On live sonar it is a mark that moves, and you can watch it turn toward your lure. The screen does not tell you the species; you read that from the depth, the structure and the season.

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Which frequency, and how wide a cone?

Sound leaves a standard transducer in a cone. The lower the frequency, the wider the cone and the deeper it reaches, at the cost of detail. The higher the frequency, the narrower and sharper the picture. Lowrance puts the cone of most traditional 200 kHz transducers at under 20 degrees. Garmin’s dual-beam transducers make the trade in one housing, at 15 and 45 degrees.

So at 200 kHz you see a circle of bottom about a third as wide as the water is deep; a 45-degree beam sees about four-fifths. In 30 ft (9 m) of water the narrow beam looks at a circle of about 10 ft (3 m), and a bass 15 ft off the side of the boat never shows. In 20 ft (6.1 m) of water Garmin’s wide beam covers about 16.6 ft of bottom, the narrow one about 5.3 ft.

How deep does a fish finder read? Far deeper than most fishing. Retailers list the $159.99 Striker 4 at 1,600 ft (488 m) in fresh water and 750 ft (229 m) in salt, and salt water always gets the smaller figure.

Shallow water is the harder case. Lowrance’s own example: in less than 10 ft (3 m) of water, a 200 kHz cone covers less than 3 ft of bottom. That is where side imaging and live sonar earn their keep, because they look out to the side and ahead instead of straight down.

CHIRP sonar sweeps a band of frequencies in each pulse instead of firing one. Lowrance’s high CHIRP band, for example, sweeps 140–250 kHz in every pulse. The sweep separates targets that a single frequency would blur into one arch, so two fish close together are drawn as two.

What are the types of fish finder sonar?

Five kinds of sonar sit inside most of today’s units, and the brand names hide them. A Garmin calls its imaging ClearVü and SideVü, a Humminbird Down Imaging, Side Imaging and MEGA, a Lowrance DownScan, SideScan and Active Imaging.

What is the difference between down imaging and side imaging?

Both use a thin, high-frequency beam that is narrow front to back and wide side to side. Each ping draws a slice, and the slices stack into something close to a photograph. Lowrance launched its DownScan Imaging in 2009 as “a photo-like view beneath the boat.”

Down imaging points that slice straight down. It shows what you are driving over: a brush pile looks like a brush pile, and a fish is a dot beside it. Side imaging points two slices out to the sides, like an aerial photo of the bottom on both sides of the boat. It finds a laydown, a rock pile or a school of crappie before you drive over it, and it is the type that pays on big water.

Humminbird’s MEGA Imaging and Lowrance’s Active Imaging HD run at up to 1.2 MHz, the top of recreational sonar. The higher the frequency, the finer the picture and the shorter the reach. The first MEGA Imaging saw about 125 ft (38 m), down or to either side; today’s MEGA SI+ reaches about 200 ft (60 m) a side.

What is live sonar, and what is 360?

Live sonar, or forward-facing sonar, is the type that shows fish moving in real time. Its transducer usually sits on the trolling motor and fans a thin sheet of sound ahead of the boat, then redraws the whole sheet continuously. A bass, a brush pile and your falling lure all move on the screen as they move in the water. 360 imaging is different again: a transducer that rotates to sweep a full circle around the boat, a map of everything nearby rather than a live view.

What does each capability cost?

Every price step buys one thing you can name, and the way in is often a brand you were not looking at. The table gives an entry unit for each capability at its October 2026 list price. The best fish finder guide gives the pick at each step and the sale prices to wait for.

The jump that matters most is from the $120 unit to the $160–200 one. GPS turns the brush pile you crossed at dawn into a waypoint you can drive back to, and down imaging turns a vague arch over a lump into a fish beside a stump.

Is a fish finder worth it? On water you do not know, yes: for $120–200 it shows the depth, the bottom and the fish, and with GPS it brings you back to the spot that paid. Outdoor Life’s Pete Robbins adds a reason that has nothing to do with fish: “A simple depth finder can also prevent you from running aground.” On a pond you have fished for years, the money does more as tackle.

Which fish finder should you buy?

1. Where do you fish from? From the bank, a castable sonar. From a kayak, a 4–7 in unit with a small power draw. From a boat, a unit on the console, the bow or both.
2. Do you fish cover or open water? For brush, rock and timber on big water, side imaging, from $299.99. On a farm pond, CHIRP and GPS are enough.
3. Do you need maps? On reservoirs and big lakes, yes: a unit with preloaded charts. On one lake you know, a unit that draws its own contours as you drive costs less.
4. Will you want live sonar within two years? Then buy a display that runs it now, or the Eagle Eye 9 Live, not a Striker or a HELIX you will have to replace.
5. Do you fish tournaments? Check the circuit’s live-sonar rule before you spend $2,000 on a transducer.
6. Ice? A flasher, or an ice bundle of a screen unit if you also fish open water.

A fish finder and rods on the deck. Photo: FedBul / Depositphotos.

Do you need GPS and maps?

GPS is the feature that pays first. A depth reading tells you where you are now; a waypoint brings you back to the hump, the creek channel or the brush pile next weekend. Every Garmin Striker from the $159.99 Striker 4 up has it, and so do the HELIX 5 G3 and every current Lowrance except the Eagle 4x.

Maps are the next step, and they come in two kinds. Preloaded charts show the contours of thousands of lakes before you launch. Garmin’s ECHOMAPs carry Navionics+; Humminbird’s XPLORE includes LakeMaster and CoastMaster charts with 1 ft (30 cm) contours. Lowrance’s Eagle carries C-MAP US Inland, with more than 17,000 lakes.

The other kind is the map you draw yourself. Garmin’s Quickdraw Contours, on the Striker Plus and Striker Vivid units, draws a 1 ft contour map from your own soundings as you drive. Humminbird’s AutoChart Live and Lowrance’s Genesis Live do the same, Genesis Live to half-foot contours. On a small lake no chart company has surveyed, your own map is the only map.

Lowrance’s GlobalMap 2000 was, by Lowrance’s own timeline, the first LCD to put GPS, mapping and sonar in one box, in 1995. Thirty-one years later a $159.99 Striker 4 has the GPS and the sonar, and still no chart.

How big a screen do you need?

Buy the screen for where you sit and how far your eyes are from it. A 3.5 in (8.9 cm) Striker 4 shows depth and arches clearly at arm’s length on a kayak. A 5 in screen shows bottom contour and structure, and a 7 in screen fits a chart and sonar side by side. Outdoor Life’s kayak reviewer calls a 5 or 7 in display “perfect for a kayak”. Past 7 in, the weight, the draw and the price grow faster than what you can see from a kayak seat.

On a boat with live sonar, more glass helps. Wired2Fish’s Shaye Baker writes that “for the big boat angler, a 9-inch unit is about as small as you’ll be able to go to still see adequate detail.” His trick for saving money is height. Raise a 9 in screen a foot or so on a mount, and from where you stand it looks as big as a flush-mounted 10 or 12 in.

After size, look at how the screen reads in the sun. Wired2Fish’s buyer’s guide singles out IPS screens for bright sun and polarized lenses, and lists physical buttons among the pros of its picks.

Where does the transducer go?

The transducer, not the screen, decides what a fish finder shows. Every maker’s installation guide starts from the same rule: it needs clean water, with no air bubbles running over it.

Humminbird’s installation guide sets the bracket 1/8 in (3 mm) above the bottom of the transom on an aluminum boat. On fiberglass the gap is 1/4 in. Rivet rows and strakes throw bubbles, so stay clear of them. Garmin rates a transom mount for good readings below about 30 knots (34 mph); for speed, Garmin points to an in-hull mount on solid fiberglass.

Does a fish finder come with a transducer? Most do: the Striker 4 and Vivid 4cv ship with both transom and trolling-motor hardware. A unit sold as a control head only, Humminbird’s “CHO”, or a bare ECHOMAP comes without one, so check the listing before you compare two prices.

Will a fish finder drain your battery?

Not an entry unit. Garmin rates the Striker 4 at 0.23 A, so a 12 V 10 Ah lithium kayak battery runs it for about 43 hours. A HELIX 5 G3 draws 0.615 A, about 16 hours on the same battery. A touchscreen Elite FS 7 draws up to 2.0 A and empties it in about 5.

Big screens and live sonar change the sum. MEGA Live 2 is rated at 930 mA nominal, and LiveScope 2 draws up to about 2.5 A, on top of the display. Wired2Fish’s advice for a boat with that load is a dedicated lithium battery and a heavy-gauge wiring harness for the electronics, which also helps prevent screen interference and voltage drops.

Live sonar, and why the leagues limit it

Garmin’s Panoptix LiveScope launched on June 11, 2018, at $1,499.99, as “the first and only live scanning sonar for recreational fishing.” Lowrance ActiveTarget followed at the end of 2020 and Humminbird MEGA Live in 2021. MEGA Live 2 was announced with the XPLORE heads in December 2024 and shipped in January 2025. Garmin put LiveScope 2 on sale on July 7, 2026, with no black box and a range of up to 250 ft (76 m).

A live transducer alone costs $1,399.99 to $2,199.99 at Tackle Warehouse, before the display. The exception is Lowrance’s Eagle Eye 9 Live: a 9 in screen, one transducer with live forward and down views, and charts, for $999.

Is it cheating? Most pro circuits ration it, and one bans it:

• B.A.S.S. has limited Elite Series boats to one live-sonar transducer and 55 in of total screen since 2025. In 2026 it allows the technology in at most five of nine regular-season events, chosen by coin flip.
• Major League Fishing caps every boat at two forward-facing or 360 transducers. The Bass Pro Tour allows live sonar in one of three periods a day; from 2026 the Pro Circuit, Toyota Series and BFL allow one continuous 3-hour block.
• The NPFL banned live forward-facing sonar in practice and competition from 2025.

For fun fishing no rule applies, so the question is only whether it suits your water. Live sonar finds suspended fish that 2D sonar misses; an angler who fishes shallow cover gets more for the money from 2D and side imaging.

A bass boat on the run. Photo: songbird839 / Depositphotos.

Which fish finder for a kayak, the bank or the ice?

The smaller the craft, the smaller the answer.

• Kayak. A 4–7 in unit with GPS and a small draw, on the gear track in front of the seat, with the transducer in a scupper or on an arm. The Striker 4 and Striker Vivid 4cv are the standard kayak fish finder picks, drawing so little that a 10 Ah battery runs either for 25 hours or more.
• The bank. A castable sonar, a floating ball tied to the line that streams to a phone. Deeper’s PRO+ 2 reads to 330 ft (100 m); Garmin’s Striker Cast GPS reads 150 ft (46 m) down and records contours as you reel. For a rental boat or a canoe, the other portable fish finder is a carry kit: unit, transducer and battery in one case.
• The ice. For ice fishing, a flasher shows the bottom, your jig and any fish under the hole as colored bands on a dial, live. Vexilar’s FLX-28 Pro Pack II comes with an ice transducer, a 12 V 9 Ah lithium battery and a charger for $619.99. A screen unit can do the same in an ice bundle. Garmin’s Striker Vivid 5cv Ice-Fishing Bundle has a built-in flasher; Outdoor Life found it for $300 at Bass Pro in October 2025, against $450 at FishUSA.

Garmin, Humminbird or Lowrance?

Each of the big three makers sells a ladder from a pond unit to a tournament console, and the rungs line up closely.

The trap in every ladder is compatibility. A Garmin Striker or ECHOMAP cv cannot run LiveScope, and a Humminbird HELIX cannot run MEGA Live 2 or network with an XPLORE. Buy the display for the sonar you want in two years, not the one you want today.

Which brand is best depends on what you want most. For live sonar, Outdoor Life’s Pete Robbins, comparing the first MEGA Live with LiveScope Plus in 2024, called it “close to a tie, with Garmin getting a slight edge”. For the finest imaging, Humminbird MEGA or Lowrance Active Imaging HD. For the cheapest complete live sonar, the Eagle Eye 9 Live.

The trolling motor decides it for some boats. Humminbird and Minn Kota are both brands of Johnson Outdoors, so a Humminbird display controls a Minn Kota trolling motor and anchor over one network, and MEGA Live 2 mounts are made for Minn Kota shafts. Garmin’s LiveScope 2 integrates with its own Force Pro and Force Kraken motors, and Lowrance builds its own Ghost X.

How do you read the screen?

The six marks a 2D screen shows. Drawing: fishing.info.

On 2D sonar a fish is an arch because it moves through the cone. It enters at the edge, is closest in the middle and leaves at the far edge, and the echo time draws a curve. A full arch means the fish crossed the whole cone; bigger fish draw longer, thicker arches. On down imaging the same fish is a dot, because the beam is too thin to draw a curve.

A hard bottom returns a thick, strong band, a soft mud bottom a thin, faint one; color palettes paint a hard bottom in bright yellow or red. Baitfish appear as a cloud of dots and weed as a fuzzy carpet above the bottom. In summer the thermocline shows as a faint horizontal band where the water temperature drops.

Two settings make the biggest difference. Turn the fish symbols off and read the arches, because the symbols turn every echo into a fish, your own jig included. And set the gain, the sensitivity, high enough that bait and the thermocline show, then back it off until the open water clears. The how to read a fish finder page has every mark and setting, and the bass and catfish hubs say what the sonar is looking for, season by season.

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