Inflorescencia /yema masculina .1 Longitud del pedúnculo [cm]

With practice and experience it's possible to interpret the echoes on the sounder screens quite accurately. You can learn to recognize different fish, the type of bottom, and shadows.

The water depth, the scroll speed and the echo strength can affect the quality of the display. The range and gain settings can also affect the amount and quality of echoes returned.

17.1 What are fish arches?

In good conditions, a fish swimming through the sonar cone will usually be shown on the Sounder screen as a fish arch.

A fish arch starts when the fish first enters the edge of the sonar cone, generating only a weak echo. This appears as the thin leading edge of the fish arch on your Sounder screen.

As the fish swims towards your vessel and the center of the sonar cone, the echoes are returned more strongly and more frequently. This means that the fish arch increases in thickness and is shown at progressively shallower depths. This produces the first half of the fish arch, which peaks as the fish passes through the center of the sonar cone.

As the fish moves away from your vessel towards the edge of the sonar cone, the reverse happens, producing the downturned side of the fish arch on your Sounder screen.

17.2 Problems with fish arches?

There are several reasons why fish may not always appear as fish arches on your Sounder screen. For example:

• when you're at anchor, the fish appear as horizontal lines on the Sounder screen as they swim in and out the sonar cone. A slow boat speed is more likely to produce fish arches, especially in deep water

• when you're in shallow water, the sonar cone is very narrow because it doesn't have time to spread out, so the fish don't stay within it long enough to generate a good fish arch on the display screen. Instead, the fish tend to appear as randomly stacked blocks of pixels

• when the water is choppy, the wave motion can distort the fish arches

• the transducer may not installed in the best position

• the range and range shift may need adjusting to show a smaller vertical extent of water. Also try using the split zoom screen to increase the resolution. It can be easier to see fish arches when the Sounder screen is concentrated on a smaller area of water, rather than displaying everything from the surface to the bottom.

17.3 How to recognize different types of fish

The swim bladders inside fish vary in their size and shape, depending on the type of fish. The air inside the swim bladder reflects the sonar pulse so the size of the swim bladder can affect the strength of the echo. With experience, it's possible to recognize the type of fish from the strength of the echoes that they're producing.

If you want to practice recognizing different types of fish, the A-Scope screen is useful.

When you're fishing amongst a school of fish and catching them, note the type of fish and the strength of the echo that they're returning on the A-scope screen.

Then, when you see that particular echo again it's likely to be the same type of fish.

17.4 How to recognize and avoid shadows

Shadows can hide fish from view. Shadows are likely to be created around areas such as hollows in the bottom or next to individual rocks and rocky ledges. The weak echoes returned from any fish in these areas are often obscured by the stronger echoes that are returned from the whole area.

A rocky ledge may produce a double bottom trace because the echoes are being returned from two different depths.

If you're fishing in areas with hollows, rocks and ledges, the 200 KHz frequency is the best choice because the high frequency, narrow sonar cone tends to reduce the effect of shadows. Shadows are more likely to occur with the 50 KHz frequency.

17.5 How to recognize the type of bottom

Soft bottoms such as mud, weeds, or sand tend to scatter the sonar pulses resulting in a weak echo. These tend to appear as a narrow band on your Sounder screen.

Hard bottoms such as coral or rock usually reflect the sonar pulses strongly. These tend to appear as a wide band on your Sounder screen.

A rocky ledge reflects echoes from two different depths and is likely to produce a double trace on your Sounder screen.

If you're at anchor, all the echoes are returned from the same area of bottom. This produces a flat, uniform trace on your Sounder screen.

17.6 How the water depth and scroll speed affect the history

The Sounder screen doesn't show a fixed distance traveled by your vessel; instead, it shows what's passed below the keel over a period of time. This is called the history.

The history is affected by two variables:

• the depth of water. In shallow water, the sonar signals have only a short distance to travel, so the Sounder screen shows a history that covers only a brief period of time and moves quickly. In deep water, the sonar signals have a

much longer distance to travel so the Sounder screen shows a history that covers a longer period of time and moves more slowly.

• the scroll speed. You can change this setting so that the Sounder screen shows a longer history with less fish detail, or a shorter history with more fish detail (see "Change the scroll speed" page 103).

17.7 Factors that affect the echo strength

The colors on the Sounder screen indicate differences in the strength of the echoes, which can be useful in interpreting the Sounder screen. However, when you're trying to interpret the strength of an echo, remember that it can be affected by many factors such as:

• the size of the object (bigger objects return more echoes).

• the depth of the object and the sonar frequency you've selected. The 50 kHz frequency penetrates to a greater depth than 200 kHz.

• the location of the object relative to the position of your vessel. The area of water covered by the sonar pulse is roughly cone-shaped and spreads out and down from your vessel so echoes are returned most strongly from the central area under the transducer.

• the clarity of the water. Particles or air bubbles in the water can reduce the strength of echoes. In particular, planing hulls moving at speed can produce air bubbles and turbulent water that may bombard the transducer, producing ultrasonic noise that may obscure the real echoes. Using the 200 kHz

frequency can reduce the amount of noise.

• the composition and density of the object, such as a bed of seaweed or a rocky bottom. Hard surfaces return stronger echoes.