Have you wondered how to use a Fishfinder or what does all the terms mean? Here is a short tutorial to help!

INTRODUCTION:
People have been fishing for thousands of years. Every person fishing has had the same problem - finding fish and getting them to bite. Although sonar can't make the fish bite, it can solve the problem of finding fish. You can't catch them if you're not fishing where they are - and the a quality sonar will prove it.

In the late 1950s, Carl Lowrance and his sons Arlen and Darrell began scuba diving to observe fish and their habits. This research, substantiated by local and federal government studies, found that about 90 percent of the fish congregated in 10 percent of the water on inland lakes. As environmental conditions changed, the fish would move to more favorable areas. Their dives confirmed that most species of fish are affected by underwater structure (such as trees, weeds, rocks, and drop-offs), temperature, current, sunlight and wind. These and other factors also influence the location of food (baitfish, algae and plankton). Together, these factors create conditions that cause frequent relocation of fish populations.

During this time, a few people were using large, cumbersome sonar units on fishing boats. Working at low frequencies, these units used vacuum tubes which required car batteries to keep them running. Although they would show a satisfactory bottom signal and large schools of fish, they couldn't show individual fish. Carl and his sons began to conceptualize a compact, battery operated sonar that could detect individual fish. After years of research, development, struggle and simple hard work, a sonar was produced that changed the fishing world forever.

Out of this simple beginning, a new industry was formed in 1957 with the sale of the first transistorized sportfishing sonar. In 1959, Lowrance introduced "The Little Green Box," which became the most popular sonar instrument in the world. All transistorized, it was the first successful sportfishing sonar unit. More than a million were made until 1984, when it was discontinued due to high production costs. We've come a long way since 1957.  From "little green boxes" to the latest in sonar and GPS technology.

HOW IT WORKS
The word "sonar" is an abbreviation for "SOund, NAvigation and Ranging." It was developed as a means of tracking enemy submarines during World War II.  A sonar consists of a transmitter, transducer, receiver and display.

In the simplest terms, an electrical impulse from a transmitter is converted into a sound wave by the transducer and sent into the water. When this wave strikes an object, it rebounds. This echo strikes the transducer, which converts it back into an electric signal, which is amplified by the receiver and sent to the display. Since the speed of sound in water is constant (approximately 4800 feet per second), the time lapse between the transmitted signal and the received echo can be measured and the distance to the object determined. This process repeats itself many times per second.

The frequencies most often used in sonar are 192 - 200 kHz (kilohertz); although some units use 50 kHz. Although these frequencies are in the sound spectrum, they're inaudible to both humans and fish. (You don't have to worry about the sonar unit spooking the fish - they can't hear it.)

As mentioned earlier, the sonar unit sends and receives signals, then "prints" the echo on the display. Since this happens many times per second, a continuous line is drawn across the display, showing the bottom signal. In addition, echoes returned from any object in the water between the surface and bottom are also displayed. By knowing the speed of sound through water (4800 feet per second) and the time it takes for the echo to be received, the unit can show the depth of the water and any fish in the water.

TOTAL SYSTEM PERFORMANCE There are four facets to a good sonar unit:

• High power transmitter.
• Efficient transducer.
• Sensitive receiver.
• High resolution/contrast display.

We call this our "Total System Performance" specification. All of the parts of this system must be designed to work together, under any weather condition and extreme temperatures.

High transmitter power increases the probability that you will get a return echo in deep water or poor water conditions. It also lets you see fine detail, such as bait fish and structure.

The transducer must not only be able to withstand the high power from the transmitter, but it also has to convert the electrical power into sound energy with little loss in signal strength. At the other extreme, it has to be able to detect the smallest of echoes returning from deep water or tiny bait fish.

The receiver also has an extremely wide range of signals it has to deal with. It must dampen the extremely high transmit signal and amplify the small signals returning from the transducer. It also has to separate targets that are close together into distinct, separate impulses for the display.

The display must have high resolution (vertical pixels) and good contrast to be able to show all of the detail crisply and clearly. This allows fish arches and fine detail to be shown.

FREQUENCY
Most Lowrance sonar units today operate at 192 or 200 kHz (kilohertz), with a few using 50 kHz.

There are advantages to each frequency, but for almost all freshwater applications and most saltwater applications, 192 or 200 kHz is the best choice. It gives the best detail, works best in shallow water and at speed, and typically shows less "noise" and undesired echoes.  Target definition is also better with these higher frequencies.  This is the ability to display two fish as two separate echoes instead of one "blob" on the screen.

There are some applications where a 50 kHz frequency is best.  Typically, a 50 kHz sonar (under the same conditions and power) can penetrate water to deeper depths than higher frequencies.  This is due to water's natural ability to absorb sound waves.  The rate of absorption is greater for higher frequency sound than it is for lower frequencies.  Therefore, you'll generally find 50 kHz used in deeper saltwater applications.  Also, 50 kHz transducers typically have wider coverage angles than 192 or 200 kHz transducers. This characteristic makes them useful in tracking multiple downriggers.  Thus, even when these downriggers are in relatively shallow depths, 50 kHz is preferred by many fishermen.

In summary, the differences between these frequencies are:

192 or 200 kHz	50 kHz
Shallower depths. Narrow cone angle. Better definition and target separation. Less noise susceptibility.	Deeper depths. Wide cone angle. Less definition and target separation. More noise susceptibility.

TRANSDUCERS:
The transducer is the sonar unit's "antenna." It converts electric energy from the transmitter to high frequency sound. The sound wave from the transducer travels through the water and bounces back from any object in the water. When the returning echo strikes the transducer, it converts the sound back into electrical energy which is sent to the sonar unit's receiver. The frequency of the transducer must match the sonar unit's frequency. In other words, you can't use a 50 kHz transducer or even a 200 kHz transducer on a sonar unit designed for 192 kHz! The transducer must be able to withstand high transmitter power impulses, converting as much of the impulse into sound energy as possible. At the same time, it must be sensitive enough to receive the smallest of echoes. All of this has to take place at the proper frequency and reject echoes at other frequencies. In other words, the transducer must be very efficient.

CRYSTAL:
The active element in a transducer is a man-made crystal (lead zirconate or barium titanate). To make these crystals the chemicals are mixed, then poured into molds. These molds are then placed in an oven which "fires" the chemicals into the hardened crystals. Once they've cooled, a conductive coating is applied to two sides of the crystal. Wires are soldered to these coatings so the crystal can be attached to the transducer cable. The shape of the crystal determines both its frequency and cone angle. For round crystals (used by most sonar units), the thickness determines its frequency and the diameter determines the cone angle or angle of coverage (see Cone Angles section).  For example at 192 kHz, a 20 degree cone angle crystal is approximately one inch in diameter, whereas an eight degree cone requires a crystal that is about two inches in diameter. That's right. The larger the crystal's diameter - the smaller the cone angle. This is the reason why a twenty degree cone transducer is much smaller than an eight degree one - at the same frequency.

HOUSING:
Transducers come in all shapes and sizes. Most transducers are made from plastic, but some thru-hull transducers are made from bronze. As shown in the previous section, frequency and cone angle determine the crystal's size. Therefore, the transducer's housing is determined by the size of the crystal inside.

SPEED AND THE TRANSDUCER:
Cavitation is a major obstacle to achieving high speed operation. If the flow of water around the transducer is smooth, then the transducer sends and receives signals normally. However, if the flow of water is interrupted by a rough surface or sharp edges, then the water flow becomes turbulent. So much so that air becomes separated from the water in the form of bubbles. This is called "cavitation." If these air bubbles pass over the face of the transducer (the part of the housing that holds the crystal), then "noise" is shown on the sonar unit's display. You see, a transducer is meant to work in water - not air. If air bubbles pass over the transducer's face, then the signal from the transducer is reflected by the air bubbles right back into it. Since the air is so close to the transducer, these reflections are very strong. They will interfere with the weaker bottom, structure, and fish signals, making them difficult or impossible to see.

The solution to this problem is to make a transducer housing that will allow the water to flow past it without causing turbulence. However, this is difficult due to the many constraints placed upon the modern transducer. It must be small, so that it doesn't interfere with the outboard motor or its water flow. It must be easy to install on the transom so that a minimum of holes need to be drilled. It must also "kick-up" without damage if struck by another object. Again, the patented design of the HS-WS transducer is Lowrance's latest improvement in high-speed transducer technology. It combines high speed operation with easy installation and will "kick-up" if struck by an object at high speed.

The cavitation problem is not limited to the shape of the transducer housing. Many boat hulls create air bubbles that pass over the face of a transom mounted transducer. Many aluminum boats have this problem due to the hundreds of rivet heads that protrude into the water. Each rivet streams a river of air bubbles behind it when the boat is moving, especially at high speed. To fix this problem,  mount the face of the transducer below the air bubbles streaming from the hull. This typically means you have to mount the transducer's bracket as far down as possible on the transom.

FREQUENCY:
Most sonar units operate at 192 or 200 kHz (kilohertz). Some models use 50 kHz. A few are dual frequency capable, meaning they can use both 50 and 200 kHz transducers. Typically, high frequency (192 or 200 kHz) sonar units provide the best resolution and definition of structure and targets. They excel at showing minute details of the underwater world. 50 kHz units have much greater depth penetration capability, but show less definition. 50 kHz transducers also usually have a much wider cone angle than 192 or 200 kHz transducers.

You must match the transducer's frequency to the sonar unit. For example, a 192 kHz sonar unit requires a 192 kHz transducer.

CONE ANGLE:
A transducer's cone angle determines its coverage area of the underwater world. The wider the cone angle, the greater the area that's covered. They are offered in a variety of 192 kHz transducers with either a wide (20°) or narrow (8°) cone angle. They are also offered in a variety of 200 kHz transducers with either a wide (20°) or narrow (12°) cone angle. The 50 kHz transducers come with a 35° cone angle. And typically the dual frequency transducers come with both a narrow (12°) 200 kHz and a 50 kHz cone angles.

Generally, use a wide cone angle for fishing shallow to medium depths. The narrow cone penetrates to deeper depths, but shows less fish and structure due to its narrow beam.

The depth capability of your sonar units depends on its transmitter power, receiver, sensitivity, frequency, transducer and transducer installation. Other things that effect depth capability are: water conditions and type, (all sonars will show deeper depth readings in fresh water than salt) and bottom conditions.

TYPES OF TRANSDUCER INSTALLATION:
Most of our permanent-mount transducers are designed for high-speed operations. For the best results, the transducer should be placed where a smooth, undisturbed flow of water will pass across the face of the transducer at all boat speeds. Be sure to READ your transducer's owner's manual before installing the transducer!

TRANSOM MOUNT:

The transom-mount transducer is the most popular, and it's generally the easiest to install. This transducer design performs best when it is slightly below the boat's hull. A plastic transducer is recommended on aluminum or steel-hulled boats to avoid potential electrolysis problems.

Mounting the transducer on the transom is recommended for outboard and stern-drive (I/O) powered boats only. Transom mounting is ideal for high-speed operation and models with the "kick-up" feature will prevent damage if the transducer strikes an object.

Make certain that the chosen location doesn't interfere with the boat's trailer. DO NOT mount the transducer directly behind the ribs, or thru-hull fittings. Typically, on aluminum boats, mounting the transducer between two ribs works best. On all hulls, mount the transducer at least one foot away from the engine's lower unit. This helps to prevent air bubbles from the transducer interfering with the propeller.

Periodically wash the bottom of the transducer with soap and water to remove any oil film or growth that may collect. Oil and dirt reduce the transducer's sensitivity and can even prevent its operation.

SHOOT THRU THE HULL MOUNT:

In this installation, the transducer is bonded to the inside of the hull with epoxy. Ideally, the transducer is placed in the aft third of the hull close to the centerline. The signal "shoots through" the hull with some loss of signal strength. This installation must be made in an area of the hull that is made from solid fiberglass, with no air bubbles or separated layers. If the hull is of multi-layer or "sandwich" construction, you will have to remove the inner layer of fiberglass and the wood or foam core to expose the outer layer of the hull. This type of mount is recommended only with 192 or 200 kHz transducers.

BOLT THRU THE HULL MOUNT:

In this type of installation, a hole is cut in the hull and the transducer is mounted through the hull by means of a threaded shaft and nut. If the boat hull has a dead rise higher than 10 degrees, fairing blocks made from wood or plastic must be fabricated so that the transducer will mount in a completely vertical position.  Some fish-finder models do not require a fairing block.

On in-boards, the transducer must be installed ahead of the propeller, shaft(s), and engine water intake(s).

If the boat's hull is made of steel or aluminum, use a plastic transducer to prevent electrolysis problems.

TROLLING MOTOR MOUNT:

The "pod" or "puck" style transducer is designed for mounting on an electric trolling motor. It typically has two slots for a hose clamp (which usually must be purchased separately).

TRANSDUCER CONE ANGLES:

20 degree cone angle | 8 degree cone angle

The transducer concentrates the sound into a beam. When a pulse of sound is transmitted from the transducer, it covers a wider area the deeper it travels. If you were to plot this on a piece of graph paper, you would find that it creates a cone shaped pattern, hence the term "cone angle."  The sound is strongest along the center line or axis of the cone and gradually diminishes as you move away from the center.

In order to measure the transducer's cone angle, the power is first measured at the center or axis of the cone and then compared to the power as you move away from the center.  When the power drops to half (or -3db[decibels] in electronic terms), the angle from that center axis is measured. The total angle from the -3db point on one side of the axis to the -3db point on the other side of the axis is called the cone angle.

This half power point (-3db) is a standard for the electronics industry and most manufacturers measure cone angle in this way, but a few use the -10db point where the power is 1/10 of the center axis power. This gives a greater angle, as you are measuring a point further away from the center axis. Nothing is different in transducer performance; only the system of measurement has changed. For example, a transducer that has an 8 degree cone angle at -3db would have a 16 degree cone angle at -10db.

Wide cone angles will show you more of the underwater world, at the expense of depth capability, since it spreads the transmitter's power out. Narrow cone angle transducers won't show you as much of what's around you, but will penetrate deeper than the wide cone. The narrow cone transducer concentrates the transmitter's power into a smaller area. A bottom signal on the sonar unit's display will be wider on a wide cone angle transducer than on a narrow one because you are seeing more of the bottom. The wide cone's area is much larger than the narrow cone.

High frequency (192 - 200 kHz) transducers come in either a narrow or wide cone angle.  The wide cone angle should be used for most freshwater applications and the narrow cone angle should be used for all saltwater applications.  Low frequency (50 kHz) sonar transducers are typically in the 30 to 45 degree range. Although a transducer is most sensitive inside its specified cone angle, you can also see echoes outside this cone; they just aren't as strong. The effective cone angle is the area within the specified cone where you can see echoes on the display. If a fish is suspended inside the transducer's cone, but the sensitivity is not turned up high enough to see it, then you have a narrow effective cone angle. You can vary the effective cone angle of the transducer by varying the receiver's sensitivity. With low sensitivity settings, the effective cone angle is narrow, showing only targets immediately beneath the transducer and a shallow bottom. Turning the sensitivity control up increases the effective cone angle, letting you see targets farther out to the sides.

WATER AND BOTTOM CONDITIONS:
The type of water you're using the sonar in affects its operation to a large degree. Sound waves travel easily in a clear freshwater environment, such as most inland lakes.

In salt water however, sound is absorbed and reflected by suspended material in the water. Higher frequencies are most susceptible to this scattering of sound waves and can't penetrate salt water nearly as well as lower frequencies. Part of the problem with salt water is that it's a very dynamic environment - the oceans of the world. Wind and currents constantly mix the water. Wave action creates and mixes air bubbles into the water near the surface, which scatters the sonar signal. Micro-organisms, such as algae and plankton, scatter and absorb the sonar signal. Minerals and salts suspended in the water do the same thing. Fresh water also has wind, currents and micro-organisms living in it that affect the sonar's signal - but not as severely as salt water.

Mud, sand and vegetation on the bottom absorb and scatter the sonar signal, reducing the strength of the return echo. Rock, shale, coral and other hard objects reflect the sonar signal easily. You can see the difference on your sonar's screen. A soft bottom, such as mud, shows as a thin line across the screen. A hard bottom, such as rock, shows as a wide line on the sonar's screen.

HARD BOTTOM	SOFT BOTTOM

You can compare sonar to using a flashlight in a dark room. Moving the light around the room, it's easily reflected from white walls and bright, hard objects. Moving the light onto a darkly carpeted floor returns less light because the dark color of the carpet absorbs the light, and the rough texture scatters it, returning less light to your eyes. Adding smoke to the room (children, don't try this at home!), you'll see even less. The smoke is equivalent to salt water's effect on the sonar signal.

WATER TEMPERATURE AND THERMO CLINES

Water temperature has an important influence upon the activities of all fish. Fish are cold-blooded and their bodies are always the temperature of the surrounding water. During the winter, colder water slows down their metabolism. At this time, they need about a fourth as much food as they consume in the summer.

Most fish don't spawn unless the water temperature is within rather narrow limits. The surface water temperature gauge built into many of our sonar units helps identify the desired surface water spawning temperatures for various species. For example, trout can't survive in streams that get too warm. Bass and other fish eventually die out when stocked in lakes that remain too cold during the summer. While some fish have a wider temperature tolerance than others, each has a certain range within which it tries to stay. Schooling fish suspended over deep water lie at the level that provides this temperature. We assume they are the most comfortable here.

The temperature in a lake is seldom the same from the surface to the bottom. Usually there is a warm layer of water and a cooler layer. Where these layers meet is called a thermo cline. The depth and thickness of the thermo cline can vary with the season or time of day. In deep lakes there may be two or more thermo clines. This is important because many species of game fish like to suspend in, just above, or just below the thermo cline. Many times bait fish will be above the thermo cline while larger game fish will suspend in or just below it. Fortunately, this difference in temperatures can be seen on the sonar screen. The greater the temperature differential, the denser the thermo cline shows on the screen.

OPERATION:

AUTOMATIC MODE:

After starting your boat, go to a protected cove and stop. Leave the engine on. You may want to take a partner along to operate the boat while you learn how to use the sonar. Press the sonar unit's ON key and idle slowly around the cove. You'll probably see a screen similar to the one to the left. The dashed line at the top of the screen represents the surface. The bottom shows in the lower part of the screen. The current water depth (33.9 feet) shows in the upper left corner of the screen. The depth range in this example is 0 to 40 feet. Since the unit is in the automatic mode, it continually adjusts the range, keeping the bottom signal on the display.

FISH ID:

Many units offers the convenience of the Fish-Symbol I.D. Activated by the press of a button, Fish Symbol I.D. lets your unit do the work of interpreting return sonar signals. Fish Symbol I.D. normally works in automatic mode only. If you turn it on while in manual mode, it will switch to automatic mode. Fish and other suspended targets are clearly displayed as fish-shaped symbols in four different sizes.

Fish Symbol I.D. is designed to give a simplified, easy to interpret display of suspended targets that are assumed to be fish. After gaining experience with your sonar, you will probably turn it off much of the time so you can see all of the detailed information on fish movement, thermoclines, schools of baitfish, weed beds, bottom structure, etc. NOTE: Many types of interference signals will also generate the Fish Symbol I.D., which may create confusion as to what is a fish and what is interference.

SENSITIVITY:
The sensitivity controls the ability of the unit to pick up echoes. A low sensitivity level excludes much of the bottom information, fish signals, and other target information. High sensitivity levels enable you to see this detail, but it can also clutter the screen with many undesirable signals. Typically, the best sensitivity level shows a good solid bottom signal with Gray line and some surface clutter. When in the automatic mode, the sensitivity is automatically adjusted to keep a solid bottom signal displayed, plus a little more. This gives the unit the capability to show fish and other detail. In automatic mode, the unit also adjusts sensitivity automatically for water conditions, depth, etc.  When you adjust the sensitivity up or down, you are biasing up or down the normal setting the unit's automatic control would choose. Many units have an Automatic Sensitivity function, when enabled, the automatic mode picks the proper sensitivity level for 95% of all situations, so it is recommend to always use this normal mode first. But, for those unusual situations where it is warranted you can bias it up or down. You can also turn off the automatic sensitivity control for special uses.

To properly adjust the sensitivity while the unit is in the manual mode, first change the range to double its current setting. For example, if the range is 0 - 40 feet, change it to 0 - 80 or 0 - 100 feet. Now increase the sensitivity until a second bottom echo appears at twice the depth of the actual bottom signal. This "second echo" is caused by the echo returning from the bottom reflecting off the surface of the water, making a second trip to the bottom and returning. Since it takes twice as long for this echo to make two trips to the bottom and back, it shows at twice the depth of the actual bottom. Now change the range back to the original scale. You should see more echoes on the screen. If there is too much noise on the screen, back the sensitivity level down a step or two.

ZOOM:
You may see fish arches while trolling with the unit in a 0 - 60 foot scale, however it is much easier to see the arches when using the zoom feature. This enlarges all echoes on the screen. Turning the zoom feature on gives you a screen similar to the one at left. The range is 8 - 38 feet, a 30-foot zoom. As you can see, all targets have been enlarged, including the bottom signal. Fish arches (A & B) are much easier to detect, and important structure (C) near the bottom is magnified. This also shows small fish hanging just beneath the surface clutter (D). The above steps are all that's required to manually adjust your sonar unit for optimum fish finding capability. After you've become more familiar with your unit, you'll be able to adjust the sensitivity properly without having to look for a second echo.

FISH ARCHES:
One of the most common questions that we receive is "How do I get fish arches to show on my screen?" It's really pretty simple to do, but it does require attention to detail, not only in the way you make the adjustments to the unit, but to the whole sonar installation.

SCREEN RESOLUTION: The number of vertical pixels that the screen is capable of showing is called Screen Resolution. The more vertical pixels on a sonar's screen, the easier it will be for it to show fish arches. This plays an important role in a sonar unit's capability to show fish arches. The chart below lists the pixel sizes and area they represent down to 50 feet for two different screens.

As you can see, one pixel represents a larger volume of water with the unit in the 0 - 100 foot range than it does with the unit in the 0 - 10 foot range. For example, if a sonar has 100 pixels vertically, with a range of 0 - 100 feet, each pixel is equal to a depth of 12 inches. A fish would have to be pretty large to show up as an arch at this range. However, if you zoom the range to a 30-foot zoom (for example from 80 to 110 feet), each pixel is now equal to 3.6 inches. Now the same fish will probably be seen as an arch on the screen due to the zoom effect. The size of the arch depends on the size of the fish - a small fish will show as a small arch, a larger fish will make a larger arch, and so on. Using a sonar unit with a small number of vertical pixels in very shallow water, a fish directly off the bottom will appear as a straight line separate from the bottom. This is because of the limited number of dots at that depth. If you are in deep water (where the fish signal is displayed over a larger distance of boat travel), zooming the display into a 20 or 30 foot window around the bottom shows fish arches near the bottom or structure. This is because you have reduced the pixel size in a larger cone.

CHART SPEED:
The scrolling or chart speed can also affect the type of arch displayed on the screen. The faster the chart speed, the more pixels are turned on as the fish passes through the cone. This will help display a better fish arch. (However, the chart speed can be turned up too high. This stretches the arch out. Experiment with the chart speed until you find the setting that works best for you.)

TRANSDUCER INSTALLATION:
If you still don't get good fish arches on the screen, it could be the transducer's mounting is incorrect. If the transducer is mounted on the transom, adjust it until its face is pointing straight down when the boat is in the water. If it is angled, the arch won't appear on the screen properly. If the arch slopes up but not down, then the front of the transducer is too high and needs to be lowered. If only the back half of the arch is printed, the nose of the transducer is angled too low and needs to be raised.

FISH ARCH REVIEW:

1. Sensitivity
   Automatic operation with Advanced Signal Processing (ASP^TM) turned on should give you the proper sensitivity settings but, if necessary, the sensitivity may be increased.
2. Target Depth
   The depth of the fish can determine if the fish will arch on the screen. If the fish is in shallow water, the fish is not in the cone angle very long, making it difficult to show an arch. Typically, the deeper the fish, the easier it is to show an arch.
3. Boat Speed 
   The boat's engine should be in gear at an idle or just above. Experiment with your boat to find the best throttle location for good arches. Usually, a slow trolling speed works best.
4. Chart Speed
   Use at least 3/4 chart speed or higher.
5. Zoom Size 
   If you see markings that are possible fish, but they do not arch, zoom in on them. Using the zoom function lets you effectively increase the screen's resolution.

FINAL NOTES ON FISH ARCHES:
Very small fish probably will not arch at all. Because of water conditions such as heavy surface clutter or thermoclines, the sensitivity sometimes cannot be turned up enough to get fish arches. For the best results, turn the sensitivity up as high as possible without getting too much noise on the screen. In medium to deep water, this method should work to display fish arches.

A school of fish will appear as many different formations or shapes, depending on how much of the school is within the transducer's cone. In shallow water, several fish close together appear like blocks that have been stacked in no apparent order. In deep water, each fish will arch according to its size.

WHY FISH ARCH:
The reason fish show as an arch is because of the relationship between the fish and the cone angle of the transducer as the boat passes over the fish. As the leading edge of the cone strikes the fish, a display pixel is turned on. As the boat passes over the fish, the distance to the fish decreases. This turns each pixel on at a shallower depth on the display. When the center of the cone is directly over the fish, the first half of the arch is formed. This is also the shortest distance to the fish. Since the fish is closer to the boat, the signal is stronger and the arch is thicker. As the boat moves away from the fish, the distance increases and the pixels appear at progressively deeper depths until the cone passes the fish.

If the fish doesn't pass directly through the center of the cone, the arch won't be as well defined. Since the fish isn't in the cone very long, there aren't as many echoes to display, and the ones that do show are weaker. This is one of the reasons it's difficult to show fish arches in shallow water. The cone angle is too narrow for the signal to arch.

Remember, there must be movement between the boat and the fish to develop an arch. Usually, this means trolling at a slow speed with the main engine. If you are anchored or stopped, fish signals won't arch. Instead, they'll show as horizontal lines as they swim in and out of the cone.