Portable fish finders operate by using SONAR (SOund Navigation And Ranging). It emits sound waves into the water. These sound waves move at a constant speed. When they reflect back to the receiver the time between when the signal was sent and when it returns is measured to give the depth of the object the sound wave bounced off of. Any fish finder can also be used to test the depth of water and map the bottom. How this is graphically represented will depend on your specific model. Some will give a numbers readout, others will show the contours of the bottom and still others will only show “fish.”
It’s important to understand two things about fish finders. The first is that a “fish” is generally any object that floats above the bottom. It could be a piece of vegetation or debris. This doesn’t mean they aren’t a useful tool when trying to find a good spot, it only means that readings should be taken with a grain of salt.
The second thing is that, just as with any tool, using a fish finder takes practice. Stopping the boat when you see your first fish hit isn’t necessarily the best course of action. After a few hits a small, investigatory circle will give you a better idea of what the area looks like, how deep the water is and how deep the fish are.
Along with practice you’ll want to play with the settings on your finder to get it to work for you and the fish you are trying to find. Most finders can have the “window” adjusted to show an area between certain depths. The first ten feet or so of water rarely has fish, and many anglers set the window ten feet or so above the bottom. This can cut down the noise, or useless information, that the sonar returns.
Most fish finders have a fish identifier, or fish ID, setting that tries to identify certain types of input as a fish. This is good for beginners, but as you get more used to your finder turning this off will allow you to all of the information and interpret it yourself. This will allow you to see greater detail and interpret it yourself.
In the end a fish finder is not a magic wand to bring fish up from the unseen depths, but it does give you information that you’d have no other way to access. With a little practice you’ll learn to use it to more quickly find good fishing spots and have more productive outings.