Fishfinder 101: How To Get The Most Out Of New Technology

For those just getting into fishing or who have bought new fishfinders that use the latest technology, knowing how how to dial in the settings to get the best view and how to interpret what’s on the screen can help anglers find and catch more fish. That’s why getting the most out of your new fishfinder is vital.

Raymarine Axiom Pro9 CHIRP

Above: A Raymarine Axiom Pro9 CHIRP. Photo by Raymarine.

Advanced In Fish Finding Technology On Boats

In the last decade, incredible advances have been made in fish finding technology and the price of these state-of-the-art devices has dropped drastically. CHIRP is one of the breakthroughs and was formerly a military-only technology that migrated to high-end fishfinders and is now available on even the least expensive models. CHIRP bombards the bottom with 10-1,000 times the energy of traditional short-duration, single-pulse sonar technology and uses alternating, long-duration pulses to paint a far more detailed view of what’s down under.

Those looking at a fishfinder screen for the first time will likely be puzzled by what they’re seeing. Interpreting the identity of big, multicolored blobs and things that look like Nike swooshes can be confusing. Some might assume the big blobs are big fish and the curved images are proportionally-sized to indicate how large the fish are. Actually, the big blobs — especially if they are smooth on top — are likely big schools of small fish. If the blob is close to the bottom and is ragged on top, with no separation from the surrounding bottomscape, it’s probably structure jutting up from the bottom. And the size of the curved images anglers call boomerangs don’t necessarily relate to the actual size of the fish. Easy, right?

What Does Sonar Actually Mark?

Think of a sonar pulse as a golf ball. If you drop it on a cart path, it rebounds high into the air. If you drop it in the grass, it doesn’t bounce so well. A fish’s flesh is mostly water so it doesn’t give much traction for a beam to bounce off of; it blends into the waters surrounding it. In contrast, the skeleton and air bladder of a fish provide the structure needed for a strong sonar return and the size of the image is dependent on a few things. If it’s in the center of a beam’s cone, it will appear larger and, conversely, if it’s on the outer edge, it will appear smaller. The velocity of the boat or a fish’s speed through the water will affect its size and shape. The quality of an image is in direct proportion to how many pings of sonar the target is hit by.

Picture the sonar’s cone as a flashlight beam. The closer to the transducer, the narrower the beam, the farther down it goes, it spreads out and becomes more dispersed. The curved shape of a return means either the boat or the fish is moving. A fish that lays suspended beneath a stopped boat will project a long, straight return. A fast-moving fish will show up as a streak and if the boat is moving fast, fish may just appear as small dots since they may have only received a single ping each. So what’s the key to determining the actual size of a fish? Color has a lot to do with it.

Dial-In Your Fishfinder Settings For The Best Image

Although the auto settings on most modern fishfinders do a respectable job of producing a decent representation of what’s beneath the boat, learning to fine-tune the settings makes a huge difference. The first thing to do is change the range of the display to make it two to three times the depth of the water. Then boost the unit’s sensitivity until a second bottom appears beneath the actual bottom but not too much to add clutter to the screen. Not only will this give you the clearest view of the fish and bottom, but it will also tell you the relative hardness of the floor. The closer the two bottom depictions, the harder the composition, which is where most fish reside.

The next move is to adjust the color palette. Look at the seafloor color while adjusting this and tweak it until the bottom is a solid, bright color like red or yellow. The color you designate to represent the bottom will also be the color of the biggest fish since it indicates the strongest return. Say you’ve chosen red as the primary color to identify the densest items. Now try to find a fish that also has the most red in it and that will usually mean it’s a relatively large one. Keep tweaking the color until red isn’t the only color of a fish but represents its majority. When looking at schools of bait, if they are a solid color the same hue as the bottom, chances are they are small baitfish packed tightly together. If you see large predators around schools of baitfish, they are likely feeding and are usually easier to catch.

Where Are the Fish?

Much of what is seen on the screen is the history of where fish were a few moments before. The newest information is located at the right of the screen and should be the primary area of focus. But the history can be valuable as well. Most modern touchscreen multi-function displays (MFD) allow the operator to tap on the screen where a large group of fish was represented and this will automatically create a waypoint on the GPS navigation page that allows the driver to go back to that spot later. This is most valuable if significant bottom structure is located nearby that could be attracting and holding the fish in that spot. If an operator would rather focus on the latest sonar returns, increase the scroll speed and most of the targets shown on the screen will be new ones. New fishfinders have what’s called an A-scope, which shows what’s beneath your boat at that exact moment. It’s a gray bar that just shows horizontal streaks of color and if you are targeting large fish, look for a streak of red (or whatever color you’ve chosen to represent the bottom).

How To Use A Fishfinder

Above: Knowing how to use an onboatd fishfinder to find fish is in the water around you is vital when casting from a boat. Photo by William McAllister from Pexels.

Side And Down Imaging

Probably one of the best tools available for finding fish is side imaging and down imaging, which can display a near-photographic image of the bottom but is also the most confusing for new users to interpret. The most common complaint is while they can see the bottom in great detail, users can’t see fish like when using CHIRP sonar. Plus, the side imaging display is flipped 90 degrees on the screen. All major fishfinder companies have this technology under different names: SideScan (Lowrance/Simrad), SideVü (Garmin), SideVision (Raymarine), and Side Imaging (Humminbird).

There are several key principles to understand before the images start to make sense. First, the black area at the center is the water column. This representation often creates confusion about where structure is actually located in relation to the boat’s position. Say the operator sees a rock pile off to the right that when looking at the scale on the top of the screen indicates it’s at the 50-foot mark. If a boat is floating in 20 feet of water, that rock pile is actually only 30 feet to the right since you have to subtract the depth of the water. And how does an angler know it’s a rocky bottom? Hard images show up brighter than mud or sand bottoms and the near-photographic image lets the operator see exactly what any structure is, unlike traditional sonar.

Identifying The fish

The trickiest part of side imaging is identifying the fish. Unlike CHIRP sonar that shows each fish as a large, colored image, a fish on side imaging is usually just a small bright dot. And it’s often hard to see a fish next to hard structure because both with be bright; there’s no contrast. But, often a fish that’s pinged with side imaging sonar will create a distinct black shadow on the bright structure behind or below it that often will have the outline of the fish itself. Conversely, when you see large areas of black, these are areas that are shadows cast by structure or drop-offs. If you see light-colored images that aren’t connected to anything, these are fish that would have likely been invisible were it not for the dark background.

The easiest way to prospect for fish in an area where there are lots of docks or on a shoreline is to idle slowly. Or, if you are in open water side imaging can help find structure. Side imaging allows the operator to change frequencies to match a given situation and this will help quickly find fish or locate structure like rock piles, wrecks or submerged trees in areas you’ve never fished before. The lowest frequencies are best for covering large areas. Most side image transducers use 455 kHz and 800 kHz. With this, you can expand the range on your screen to cover as much as several hundred feet on boat sides of the boat. At this range, details will be less vivid and you won’t see fish but it allows the user to see larger structures. Once a likely spot is found, change the frequency to 800 kHz and move closer to the structure or when prospecting docks or shorelines. Companies like Humminbird have taken it a step further with Mega Imaging, which is a 1200 kHz frequency. With these higher frequencies, if you are close enough and idle slowly, you should be able to easily see if fish are present.

While modern technology does an incredible job of finding fish, learning how to tweak the settings and interpret what you see will raise your fishing game to a new level.

Written by: Alan Jones

Alan Jones is a marine journalist who has tested more than a thousand boats and written more than 500 boat reviews and video walkthroughs online, including fishing boats, cruisers, PWC, pontoons and ski boats. If it floats, he's tested it. One of his passions is testing new products that make boating easier and safer. Currently the president of Boating Writers International, Alan is a gearhead who's tested almost every engine that's come out in the last 20 years. He also loves fishing of all kinds.


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