Fish Finder Features Explained – How to Read a Fishfinder

Fishing is one of the oldest methods for gathering food used by humanity. These days, the art of fishing is more accessible thanks to technology. Where people once relied on their eyes to find fish in the open ocean, now we have gadgets like fish finders.

Fish Finders are a type of instrument that uses sonar to detect the presence of objects in the water below. The sound waves bounce off an object, and then the Fish Finder uses that to find things. In the last few decades, they have become standard tools for commercial fishing. They may seem very complicated to those unfamiliar with them, but they do have some common traits. In this guide, we will best explain some standard features of fish finders.

Fish Finder Features Explained

Standard Sonar

Sonar is standard on all types of Fish Finders. Back in the 80s and 90s, it was common to use a 200-kHz sonar to locate things in the water. Anything it bounced off of would show up on an LCD screen. Those screens were later upgraded to show color. When the sonar picks up something big and solid, it will show up as something close to red.

The 200-kHz sonar is the standard to which all fish finders are set to. The reason is that it is versatile and able to tell the difference between fish and other objects. That kind of versatility can work on any significant body of water.


Using a pair of very high-frequency sonars, usually in the range of 455/800 kHz, they appeared a few years back. They can create high-quality images on par with MRI’s, capturing details down to a branch on a tree.

They do have a few downsides, though. At that high frequency, they tend to distort after a few hundred feet. They are also less adept at finding fish than lower frequency. They’ll appear as smudges on the screen. Instead, you will have to look for areas where fish will gather.

On the plus side, this is perfect for people who go fishing in shallow waters, usually a hundred feet or less.

Side-Finders and Down-Imagers

There is some confusion about the difference between these two types of fish finders. The key is the direction they emit sonar. Down-imagers emit sonar right below your boat, while side-finders or side-imagers send them to the sides.

Due to trademarks by different companies, side-finders and down-imagers can be hard to differentiate. Side imagers and Down imagers are trademarked by Humminbird. Others companies use terms like “DownScan”, “SideScan”, and “DownVu”.

Side imagers were the predecessors to imagers and scanners. They can pick up images of things that are not underneath a boat so that they can scan over a wide range. Since they can cover both sides of the boat, they cover twice as much ground. Due to their range, anglers like to use them in shallow waters. This lets them cover as much ground as possible.

Using a side imager can get confusing because you have to factor in the range and palette used. You will need to look in several directions at once, meaning a bigger screen is necessary. These are also expensive, so think of that first.

Down imagers, meanwhile, are best for the deep waters. Since the depth is greater, it makes more sense to look for fish underneath the boat. Down imagers can produce high-end images, even when sonar works at high speeds.

The disadvantage to down imagers is that they provide less detail, especially in the horizontal. This means that they can’t tell what side of the boat a fish may be located, limiting their effectiveness. Consideration should be given to all accounts when purchasing a down or side imager.


This stands for “Compressed High-Intensity Radar Pulse.” It does have some of the characteristics of radar, but it operates underwater. By sending pulses at different frequencies at once, it can get a high-end, detailed map under the sea. It can measure up to several miles and tell which fish is which.

These are the most expensive of the different kinds of fish finders out there. They can cost thousands of dollars, but the results can be worth the price. A CHIRP is for the anglers who fish in the deep ocean beyond the continental shelf.

Single/Dual Frequency Transponder

Both of these are types of fish finders, but they differ on the levels of kHz they can use. A single frequency transponder can only handle 200 kHz. They’re used in freshwater locations like lakes and rivers. Any body of water that’s less than 200 feet deep would work for these transponders.

A dual transponder can use both the 50 and 200 kHz. Unlike the single transponders, these work best in waters with a depth of more than 200 feet. This makes them suited for deep water and saltwater bodies.

Keep in mind that where you fish will determine the frequency needed for a fish finder. Someone who wants to catch in water of all depths will want a fish finder with a dual frequency transponder.

Temperature Gauge

The water’s temperature plays a vital role in where fish go. Large bodies of water will also have different levels of warm and cold water. The closer to the surface, the warmer the water will be. Go down beyond the sunlight, and it will usually be colder. The spot between them is the thermocline.

Smaller fish like to hang out in spots away from the thermoclines. The bigger fish will stay below or between that sweet spot. A temperature gauge can be useful because it will show where those soft spots are on the screen. This will help someone narrow down where to look for the fish they are after.


GPS has become a staple to navigation in recent years, and fishing is no exception. Depending on where a person wants to fish, a GPS can help, especially if they are going off into uncharted spots. The GPS on a fish finder can create waypoints that will help someone track their way back to shore, even after dark.

As a bonus, a GPS can save the location of a fishing spot so that you can find your way back there on your next trip. That can be a tremendous help to hardcore anglers.

Display Screen

This may not seem as important as the others, but a proper screen can determine how useful a fish finder is. A bigger screen means a better view of the area around the boat. It is also helpful to consider whether you should get it in black and white or in color. Color is preferable, as it will help distinguish between objects and fish with the sonar.

Some other features to consider are symbols for fish, the number and types of fish, and if it lights up or not. Having sensors that can detect the subtle changes in the water temperature is a good idea, along with alarms to show them.

Fixed Mount or Portable

Portable fish finders are smaller, come with their containers, and will attach to a wall with suction cups. A fixed mount version will be able to hook up to a boat’s transom via its transducer. In some cases, it can even get installed on the inside part of the hull.


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