Raymarine e7 User Reference page 193

The depth of individual targets can be displayed by switching on the
Target Depth ID in the fishfinder menu Menu > Presentation. The
number of target depths displayed is influenced by the fish alarm
sensitivity level.
Water depth
As sea depth increases signal strength decreases, resulting in a
lighter on-screen image of the bottom.
Size of the target
The larger the target, the larger the return on the fishfinder display.
The size of a fish target is also dependent upon the size of the fish's
swim bladder rather than its overall size. The swim bladder varies in
size between different breeds of fish.
Transducer frequency
The same target will appear differently when the transducer
frequency is changed. The lower the frequency the broader the
mark.
Using the fishfinder
Clutter / Background noise
The fishfinder picture may be impaired by echoes received from
floating or submerged debris, air bubbles or even the vessel's
movement. This is known as 'background noise' or 'clutter' and is
controlled by the gain modes. The system will automatically control
the gain settings according to the depth and water conditions. You
can however adjust the gain settings manually if you prefer.
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