Fish detection and display, Fish symbols

4-3 Fish detection and display

Where to find fish

Underwater features like reefs, wrecks and rocky outcrops attract fish. Use the 50 kHz or 50/200 kHz frequency display to find these features, then look for fish by passing over the feature slowly several timesusingtheZoomdisplay(seesection5-2Sonar Zoom display). If there is a current, the fish will often be found downstream of the feature.

WhenfishingwiththeFISH4500/4600withtheFish symbols Off, a weak fuzzy band may appear between the bottom trace and surface. This might indicate a thermocline - a rapid change in water temperature, such as the edge of a warm or cold current. The temperature difference can form a barrier which the fish may not swim through. In fresh water, fish often collect around a thermocline.

Fish symbols

The fish symbol can be customized or switched off altogether so that the echoes are not con- verted to fish symbols on the display. See section 3-2 Setup > Sonar. The differences between Fish symbol On and Off are:

Fish symbols On

Using Navman’s SBN-IIsonar technology the fishfinder analyses all echoes and eliminates most false signals and clutter so that remaining targets are most likely fish. Depending on the strength of the remaining echoes, they are displayed as either small, medium or large fish symbols - with or without depth. While the SBN-IIprocessing is very sophisticated it is not foolproof - there will be times when the fishfinder will not be able to differenti- ate between

large air bub- bles, rubbish containing air, fishing floats etc. and gen- uine fish.

The follow- ing picture shows the Sonar display with the Fish symbol: On + depth:

Fish symbols Off

For experienced users this always provides the best information as every echo is displayed, wheth- er it is surface clutter, a thermocline or a fish.

The picture in section 4-1 Interpreting the display, shows the Sonar display with the Fish symbols Off. The fish appear as arches.

Fish arches

In good conditions and with Fish symbols Off, a fish passing through the cone-shaped ultrasonic pulse is displayed as a fish arch. The 50 kHz frequency uses a wider cone than the 200 kHz frequency. This makes the fish arches easier to see.

A fish arch occurs when a fish enters the weak edge of the sonar cone, generating a weak echo that is displayed as the first pixel of the fish arch. As the fish moves closer to the middle of the cone, the distance between the transducer and the fish reduces and the echo is displayed at progres- sively shallower depths, producing the start of an arch. When the fish passes directly beneath the middle of the cone, the echo becomes strongest and thickest. As the fish passes out of the middle of the cone the reverse happens with a progres- sively weaker and deeper echo.

There are many reasons why fish arches may not be seen. For example:

•Poor transducer installation (see Transom Transducers Installation Guide).

•If the boat is anchored then fish will tend to show on the display as horizontal lines as they swim into and out of the transducer sonar beam. Slow speeds in deeper water give the best fish arch returns.

•Range is important. It will be much easier to see fish arches when using zoom mode to concentrate on a particular section of water, rather than just displaying everything from the surface to the bottom. Zooming in- creases screen resolution and is necessary for good fish arches.

•It is difficult to get fish arches in shallow water as the transducer sonar beam is very narrow near the surface and fish do not stay within the beam long enough to display an arch. Several fish in shallow water tend to display as randomly stacked blocks of pixels.

•Wave motion may result in distorted fish arches.

FISH 4500/4600 Installation and Operation Manual

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