NorthStar Navigation NorthStar 8000i - Manual (Page 113)

Northstar 8000i Operation Manual 113
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