Anglers who have never used a fishfinder before may find themselves quickly baffled by what they see on the screen. Rather than fish, or at the very least pictures of fish, the screen will have small icons or, more likely, small arcs floating in the middle of the water. Neither representation is truly accurate, and even the most advanced units can’t give fishermen the sort of “picture” they might be expecting. However, with a basic understanding of how a fishfinder works and what information it is reporting, a fisherman can quickly learn what is in the water beneath him and how to best take advantage of it.
A fishfinder works by sending out sonar “pings” into the water, and then drawing an image based on the echo of such pings when they return to the transponder. Because of the shape of a fish, and the way it tapers in both directions, the “ping” is reflected back in an arc, with less signal as it swipes across the face and tail of the fish, but more in the center. This produces the “arc” seen on the screen. The size of the arc represents the approximate size of the fish, and with time, practice and guides, it is possible even to make a reasonable guess as to the species of fish and whether or not it is above the legal limit for capture.
There are many objects which the fishfinder can detect which are in fact not fish, and without a careful understanding, fishermen can easily be fooled. Even more advanced modern units are limited by physics, and so it will likely never be possible to get a visual and complete “map” of the water below. Instead, the device can at best provide fishermen with a rough outline of what’s below, with clues for interpretation to determine the truth. As previously mentioned, fish tend to appear as “arcs” due to their unique shape, and the way that sound waves bounce off of them. This enables fishermen to separate them from “lines,” which are generally submerged or semi-submerged objects like logs, garbage, and other obstructions. Likewise, the “arc” of a single large fish can be separated from the multiple arcs of small fish, which quickly turn into clouds as a large number of pings are returned.
Not all clouds are small fish, and seeing a cloud on the fishfinder may, in fact, be a good thing. A cloud is simply the sonar image of a large number of small objects too small to be properly resolved. It may indicate a school of small fish, or it may indicate a field of underwater plants. Both can obscure a large fish who is feeding or resting within the cloud, although many modern fishfinders can resolve the fish even if he is obscured. As such, it may be worth investigating clouds, or passing a lure through them to see what comes up.
Larger, more solid objects that are not arc-shaped are generally hazards. The most common shape will, of course, be the lake bed, but fallen trees, rocks, and man-made structures may be visible, indicating places where it may be inadvisable to run a line for fear of having it get caught or damaged. Most units will only be able to provide vague shapes, but some more advanced units may be able to show exactly what is down there, and how far away it is. This is very useful when fishing around artificial reefs, as it enables the fisherman to know where to cast his line and how deep to set his lure to approach the reef without getting his line caught.