Short course (very) on Acoustic Image
        interpretation.

While there are no "absolutes" in Acoustic Imaging, you can generally count on the following -    The denser an object is, the better it will be displayed in an Acoustic Image.  This is also true for objects with a smooth surface.
	In most lakes and rivers, objects on the bottom attract fish. The most commonly found objects are standing trees, stumps, logs on their sides, rocks, a few small boats and sometimes man-made structures.  Submerged logs and fallen trees are usually obvious because of their horizontal orientation to the bottom (logs) and the multiple "acoustic shadows" caused by branches of any trees. Trees are often still standing vertically and cast very long "acoustic shadows". They can be a hazard to navigation but, most acoustic imaging equipments are not  capable of making that determination alone (divers and / or other equipments are needed to evaluate the level of hazard that a detected object presents). Logs and trees laying on the bottom will usually have a short acoustic shadow.         A man-made structure of steel or concrete will usually produce a very good image (like the tunnel entrances in the image to the left).
FISH are often displayed as white lines or just white specs on the acoustic images.  A long white line, could very well be a fish or a school of fish that are swimming at the side of the boat and moving in the same direction.  Small white specs are usually fish moving in a direction different than that of the boat or, not moving at all. Another indicator of fish is the lack of acoustic shadow on the bottom of the lake (telling us that the object is suspended in the water between the surface and the bottom). The reason that the fish don't generate the "arch" of a conventional fishfinding sonar is because the transducer is pinging them from the side instead of from above.