January 25, 2016
January 25, 2016

Down & Side Imaging in Saltwater

Imaging sonar systems may have first been introduced as a way of catching more bass, but they’ve proved their worth in saltwater, too.

In this day and age if you don’t upgrade your phone and your computer on an annual basis, you’re operating in the dark ages. Whether we want to admit it or not, the same is true of the sonar systems on our boats.

There’s no substitute for on-the-water experience, so we rigged a boat with three units and tested them while looking for a set of pilings and a sea wall at different ranges.

Down and side imaging systems have revolutionized our ability to see beneath the water’s surface. Although they might have initially targeted sweetwater anglers, salty guys have figured out that imaging can give them a serious leg up on the competition, too.

Sonar vs Imager

There’s a lot of confusion as to exactly what constitutes an “imager” versus a regular old fish finder, so for starters, let’s take a look at what we’re talking about. Traditional down-looking sonar commonly uses a frequency in the 200-kHz range to ping through the water. Compared to imagers, this is a very low frequency. Generally speaking, imaging units send out their transmissions at 350, 450, or even as high as 800-kHz. 

Why difference does it make? A big one, once you understand how sonar waves work. Here’s an easy way to wrap your head around it: Picture a calm pond. Now throw a boulder into the water; it will create large waves that travel for long distances, and roll right over small obstructions or floating objects without being reflected back. This is similar to a low frequency sonar ping. 

Humminbird’s new “supersized” HELIX CHIRP imagers offer bigger screens and networking features.

Next, throw a pebble into the pond. It will make tiny waves which don’t go nearly as far—but are reflected by the smallest stick or leaf they strike. That’s how high frequency waves act. In practice, the low frequency sonar waves get you more range (often up to and even beyond 1,000 feet) but significantly less detail. 

The high frequency waves provide stellar detail, but at significantly less range (usually just a few hundred feet). When the sonar waves and their reflections are digitized and projected onto an LCD screen, the difference between the low and high frequency waves is akin to the difference between an X-ray and an MRI.

The latest and greatest imaging models take things a step farther by incorporating a “CHIRP” transmission, which pings through a spread in the frequency spectrum instead of pinging at a single or dual frequency. Just like the potent CHIRP units used on bluewater battlewagons, they send their signals out through an entire range, in quick succession.

Interestingly, the returns you see on-screen when an imager’s transmission bounces back from a fish is a bit different from the return provided by a traditional fishfinder. Schools usually show up as individual fish instead of being lumped together. Fish are distinctly separate from the structure they’re hiding near, even when they’re hovering between tree branches or right next to a wreck. And at these higher frequencies, fish tend to show up as dots or blobs more often than arches.

Fresh vs Salt

Whatever type of fishing you do, if you’ve seen the stellar on-screen images provided by a modern imager, be it down-looking or side-scanning, you already know that the detail levels are far and away better than a traditional fishfinder can provide. But awesome though they may be, there is a substantial difference between how they behave in freshwater versus the salty stuff.

The first thing you’re likely to notice is a significant reduction in range due to the common presence of suspended solids, water density differences (freshwater had a density of 1.0 grams per milliliter while salt is 1.025), the presence of more algae and/or plankton, and other variables.

As a general rule of thumb, fishfinders of any type can penetrate the depths better in freshwater than they can in saltwater. There are, of course, exceptions. In a cloudy lake churned up by strong winds, for example, range may be lower than it is in a clear saltwater bay.

So, just how much range can you expect? Among the different manufacturers currently producing side and down imaging units, the claimed maximum range (in ideal conditions, such as gin-clear freshwater) is between 500 and 750 feet. During a recent test in saltwater, using three different side imaging systems from three different manufacturers, I was able to consistently see large obstructions (a seawall and a cluster of pilings) out to 300 feet. At 400 feet, they were visible on about half the passes. Fish hanging around the pilings were identifiable out to about 75 feet, but beyond that, became very difficult to distinguish.

Why would the fish become difficult to detect so much sooner than solid structure? Size is part of the issue, but that doesn’t tell the whole story because of another difference between saltwater as opposed to freshwater. The fish—or more accurately, the sea life—found in the brine is quite different and more diverse.

In saltwater you’re likely to encounter more shrimp, jellyfish, and other assorted critters that will create a good deal of clutter on your fishfinder screen. As a result, it can be much more difficult to pick out individual fish. This problem is compounded when using a side-scanner, where the LCD screen is divided in half to look in two directions at the same time.

Your screen has no more pixels than it would if you were using it to look directly down, so all other things (such as range and zoom variables) being equal, a target will appear to be half the size on-screen when in side-finding mode. Now mix in a few jellyfish and a crab or two, and it’s easy to see how a redfish might be tough to pick out among all the specks and blobs. 

Finally, we need to consider the different ways saltwater anglers often use their electronics, most importantly, the way chartplotters are used to keep tabs on your exact position in open waters. Without a close-by shoreline and easy visible targets, many saltwater anglers need to have a chartplotter screen front and center as they fish, zoomed in tight to track drifts, keep trolling courses on track, or maintain station over structure.

Splitting the screen for side-finding cuts into your available LCD space, which in turn, makes it harder to spot details on the fish-finding section of your screen.

Concept vs Reality

Understanding these imagers and the differences that show up between fresh and saltwater should have an impact not only on how you use them, but also on how you choose them. First and perhaps most important—when you choose a new imaging system get the biggest screen possible, period.

It’s very difficult to take advantage of all the detail an imager has to offer if you’re squinting at a four- or five-inch diagonal screen. It’s sort of like watching a high-definition movie on your cell phone. Sure, it looks great, but you simply can’t pick out all the little details you’d see if you were watching it on a full-sized TV.

The problem is exacerbated if you’re using a side-finder, since you’re effectively cutting the viewing area of your LCD in half by looking out in two directions at once. It becomes even worse when you split the screen between chartplotter and fishfinder. 

Now add in all the blobs and specks that are no more than clutter. Remember that fish are likely to appear as dots and blobs rather than arches (especially on side-finders).

The bottom line? I’ve personally lived with a seven-inch unit, and found it frustrating. I currently have a nine-inch display and find it barely sufficient. While using a 12.1 inch display, I felt like a whole new world of fish-finding ability was opened up to me.

Out on the water, the most important thing to keep in mind is that you’ll have a much easier time finding structure than you will finding fish. In fact, most experienced saltwater anglers who depend on side-scanners depend on them more to find items and features that attract fish, than to find the individual fish themselves. Don’t be discouraged if you spot a submerged tree or a rockpile and don’t also see fish around it. Use that unit to mark the spot. Most modern scanners allow you to scroll into the unit’s history and make a waypoint where a piece of structure appears on the screen. Always take a few casts before moving on.

One final thought, which I hate to say as much as you hate to hear: plan on your unit going obsolete within a couple of years. Yes, I know it’s painful to think about replacing your fishfinder every other season, but these units are improving so quickly and so significantly that if you don’t constantly swap them out, you’re at a technological disadvantage.

Even though you may not be targeting bass, you can bet each and every one of us salty guys wants to turn that disadvantage into an advantage—and out-fish the competition.




—story by Lenny Rudow


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