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 used by Eagle in our sonar is 200 kHz
(kilohertz); we also make some units that 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.
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.
Housings
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.
For more information on transducer types and their
applications see The
Transducer Selection Guide.
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 our 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.
Transducer Cone Angles
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.
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 obviously 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
transmitters 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 transmitters 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.
Eagle 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. 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 that 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.


Soft Bottom | Hard
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 Thermodlcines

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 thermocline.
The depth and thickness of the thermocline can vary with the
season or time of day. In deep lakes there may be two or more
thermoclines. This is important because many species of game fish
like to suspend in, just above, or just below the thermocline.
Many times bait fish will be above the thermocline 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 thermocline
shows on the screen.
Operation
Automatic

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-Symbol I.D.™

Every Eagle LCG offers the convenience of our Advanced
Fish-Symbol I.D.™. Activated by the press of a button,
Advanced Fish Symbol I.D.™ lets your unit do the work of
interpreting return sonar signals. Advanced Fish Symbol
I.D.™ 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.
Advanced 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.
ASP™ (Advanced Signal
Processing)
Advanced Signal Processing (ASP™)
is another innovation that uses sophisticated programming and
advanced digital electronics to continually monitor the effects
of boat speed, water conditions and other interference sources -
and automatically adjusts the sonar settings to provide the
clearest picture possible.
ASP™ sets the sensitivity as high as possible while keeping
the screen free of "noise." It automatically balances sensitivity
and noise rejection. The feature can be turned off and on
and will work whether the sonar is in automatic or manual
mode. With ASP™ operating behind the scenes you'll
spend less time making routine sonar adjustments and more time
spotting fish.
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 GRAYLINE® 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. With ASP™ 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.
Grayline
GRAYLINE® lets you distinguish
between strong and weak echoes. It "paints" gray on targets
that are stronger than a preset value. This allows you to
tell the difference between a hard and soft bottom. For
example, a soft, muddy or weedy bottom returns a weaker symbol
which is shown with a narrow or no gray line. A hard bottom
returns a strong signal which causes a wide gray line.
If you have two signals of equal size, one with gray and the
other without, then the target with gray is the stronger
signal. This helps distinguish weeds from trees on the
bottom or fish from structure.
GRAYLINE® is adjustable. Since GRAYLINE® shows the
difference between strong and weak signals, adjusting the
sensitivity may also require a different GRAYLINE® level.
Zoom

You may see fish arches while trolling with the unit in a 0 -
60 foot scale, however it it 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.
It also helps to see the Why Fish Arch section below. This
explains how arches are created on your sonar's screen.
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.
PIXEL HEIGHT |
|
PIXEL HEIGHT |
100 VERTICAL PIXEL
SCREEN |
|
240 VERTICAL PIXEL
SCREEN |
RANGE |
PIXEL HEIGHT |
|
RANGE |
PIXEL HEIGHT |
0-10
feet |
1.2
inches |
|
0-10
feet |
0.5
inches |
0-20
feet |
2.4
inches |
|
0-20
feet |
1.0
inches |
0-30
feet |
3.6
inches |
|
0-30
feet |
1.5
inches |
0-40
feet |
4.8
inches |
|
0-40
feet |
2.0
inches |
0-50
feet |
6.0
inches |
|
0-50
feet |
2.5
inches |
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.
 |
|
 |
100 pixels |
|
240 pixels |
On the right above is a section of a 240 vertical pixel
screen. On the left is a simulated version of the same screen
with only 100 vertical pixels. As you can see, the screen on the
right has much better definition than does the one on the left.
You can see fish arches much easier on the 240 pixel
screen.
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™) 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.
Actual On-The-Water Chart Recordings
Sample 1

This shows a split-screen view of the water beneath the boat.
The range on the right side of the screen is 0 - 60 feet. On the
left, the screen has a 30-foot "zoom" rangeof 9 to 39 feet. Since
the unit is in the automatic mode, (shown by the word "auto" at
the top center of the screen) it picked the ranges to keep the
bottom signal on the screen at all times. The water depth is 35.9
feet.
The unit was used with an HS-WSBK "Skimmer®" transducer
mounted on the transom. The sensitivity level was adjusted to 93%
or higher. Chart speed was one step below maximum.
A. Surface Clutter
The markings at the top
of the screen can extend many feet below the surface. This is
called Surface Clutter. It's caused by many things, including air
bubbles created by wind and wave action or boat wakes, bait fish,
plankton and algae. Many times larger fish will be seen feeding
on the bait fish and other food near the surface.
B. GRAYLINE®
GRAYLINE® is used to
outline the bottom contour which might otherwise be hidden
beneath trees and brush. It can also give clues to the
composition of the bottom. A hard bottom returns a very strong
signal, causing a wide gray line. A soft, muddy or weedy bottom
returns a weaker signal which is shown with a narrow gray line.
The bottom on this screen is hard, composed mainly of rock.
C. Structure
Generally, the term
"structure" is used to identify trees, brush, and other objects
rising from the bottom that aren't part of the actual bottom. On
this screen, "C" is probably a tree rising from the bottom. This
record was taken from a man-made lake. Trees were left standing
in several areas when the lake was built, creating natural
habitats for many game fish.
D. Fish Arches
The X-85 has a significant
advantage over many competitive units in that it can show
individual fish with the characteristic arched mark on the
screen. (See Why Fish Arch for more information.) On this screen,
there are several large fish holding just off the bottom at "D,"
while smaller fish are hanging in the middle of the screen and
near the structure.
E. Other Elements
The large, partial arch
shown at "E" is not a fish. We were trolling near the entrance to
a cove that had hundreds of tires banded together with wire
cables. Other cables anchored the tires to the bottom. The large
arch at "E" was created when we passed over one of the large
cables that anchored the tires.
X-85 Sample 2

This shows a full-screen zoom view of the water beneath the
boat. The range is 8 - 38 feet, which gives a 30-foot zoom. Since
the unit is in the automatic mode, (shown by the word "auto" at
the top center of the screen) it picked the ranges to keep the
bottom signal on the screen at all times. The water depth is 34.7
feet.
The unit was used with an HS-WSBK "Skimmer®" transducer
mounted on the transom. The sensitivity level was adjusted to 93%
or higher. Chart speed was one step below maximum.
A and B. Fish Arches
The X-85 has a
significant advantage over many competitive units in that it can
show individual fish with the characteristic arched mark on the
screen. (See Why Fish Arch for more information.) On this screen,
there are several large fish holding just off the bottom at "B",
while an even larger fish "A" is hanging directly above
them.
C. Structure
Generally, the term
"structure" is used to identify trees, brush, and other objects
rising from the bottom that aren't part of the actual bottom. On
this screen, "C" is probably a large tree or trees rising from
the bottom. This record was taken from a man-made lake. Trees
were left standing in several areas when the lake was built,
creating natural habitats for many game fish.
D. Surface Clutter
Surface Clutter "D" at
the top of the screen extends below 12 feet in places. Small fish
can be seen beneath the surface clutter. They are probably
feeding.