This little ditty here is about answering that enigmatic question, "What the I heck am I looking at on this here fishfinder?" "Oh", says someone, "sand bass are yellow, and calicos are more green." Then someone else says, "My fish finder shows little fish on the screen, with numbers." Then someone else says, "I can tell when the bottom gets harder because it gets thicker." What the heck is the deal?
Modern fishfinders are full of features, often so complicated to adjust and change, most simply turn them on and hope for the best. Some draw little pictures of fishies, with little numbers by each. Some automatically adjust the depth range and other features.
While most private boaters see the Lowrance meter as one of the top offerings, with brands like Garmin and Hummingbird being tough competitors, a quick trip to the wheelhouse of any commercial or party boat reveals the pros seldom if ever run these machines.
Instead it's all Furuno all the way. Sometimes you'll see an old Sitex machine, or Ratheon. You may spot the side-scan sonar instead, usually a Wesmar displaying on a computer monitor. On scientific research vessels, you might see a Simrad or two. In commercial applications, Furuno has a reputation for big power, robust performance, and until recently, a lot fewer seldom used, software derived bells and whistles.
In the past it was all about transmit power, gain control, range and zoom. All the rest was fluff. Tuning the meter was a tech's job, done once for each transducer.
Commercial grade transducers weigh in pounds, not ounces, and are always mounted through the hull, for best performance.
The basics are these, and I'm staying away from CHIRP technology and multi-beam, phase-shift analysis. The transducer smacks the heck out of the water, clicking off an audible sound. Standard frequencies are 50,000 (50 kHz) and 200,000 hertz (Hz, or 200 kHz), or cycles per second, and it's acoustic, SOUND.
For comparison, we hear sounds from about 30 Hz to about 20,000 Hz. The sounder's audible click is a result of the quickness and power behind the transmission. It's too quick and too high frequency to hear as a tone.
As any sound, it spreads out in water, traveling out in all directions, but more powerfully in one. The transducer also listens for echos, (echo sounder), and can further limit the directionality by limiting the listening direction.
Nevertheless, this is not perfect, and tech types talk about the width of the "cone." The fish finder finds stuff, not just directly under the boat, but also off to the sides some. But it plots it all on the display along a straight down line, even though a lot of echos are heard from off to the side.
This is where physics plays a roll. Lower frequencies tend to spread more easily. They also have a harder time reflecting off small stuff, unless it's packed together in mass. Lower frequencies remain strong over longer distances too. Sound also travels much faster and better though water than air.
So, that 50 kHz setting is better for deeper water. At the same time, you will get a sharper image from 200kHz. and it will still work at higher hull speeds. More transmit power is always better and costs a lot more.
Here's where it gets tricky. That cone of broadcast/reception... -though manufacturers give hard numbers in degrees, as if the cone had a hard edge, it does not. Instead, that spec. typically only refers to the point at which a given target, or "fish's" echo would be at half of loudness of what it would be a the same distance straight under the ’ducer.
Imagine a single fish out in front of the boat a little, you would first begin to see it a little deeper than it really is, because it's farther away. As you come over it, it get's closer and the echo is stronger, then passes under, becoming weaker and farther again. You end up seeing one of those graceful little fish arches!
Some fish never pass straight under, and you get a weaker, less arched fish. Sometimes fish are all jammed together and you get an amorphous mass, with half-arches on the edges.
It's really hard to tell species from color, takes some practice with a specific meter and gain setting. If the gain is self-adjusting all the time, you're hosed. You need the thing set on manual gain. Manual depth helps too, so the screen doesn't jump ranges as you come over big kelp, or bait schools.
Different types of fish do reflect sound differently, and they behave differently too. A key trick here is to use the higher frequency if you are looking for them in a cluttered water column. Where you might do fine looking for 100-pound tuna in open water with 50 kHz, looking for sandbass inshore, you'll need to use the 200 to see individuals.
Fish-wise, two things are really reflective, air and heavy bones –basically any big density change. So, air filled anchovies boom back echos, and so do big air bladdered seabass and barracuda. Sharks have those "dentigenous scales" and show well. Bat rays are big and flat. Salmon are thick but fine boned and have small gas bladders. Individual salmon show thick but faintly, and are often mixed in bait schools, very tough to spot.
You are really looking for the heaviest marks in the school, to identify how reflective your targets really are. Barracuda tend to be high in the water, while seabass tend low. Both tend to stay at a given depth. Tuna and yellows zoom up and down pretty easily, but tuna tend to dart up under the boat to feed while the main sleepy herd holds down near the first hard thermocline. It's hard to see anything except bottom super shallow because that cone tip is so small there.
A faint "schmootz layer" of gellies and other plankton frequently forms at the uppermost thermocline. Then there is squid, highly mobile, big blobs to small slashes, not as reflective as sardines and anchovies, sometimes forming a thin "carpet" on the bottom during the day on nests.
Lastly, there's the bottom. This meters out to the side too, and it's always out there. That's why the bottom has thickness on the meter. You're not really seeing "below the bottom" much. But, if there's a rock or wreck off to the side/in front, you'll begin to see it below the bottom first, as it echos back at distance. As you get close, it will show more and more, the bottom will thicken. Finally as you come over it the depth will begin to shallow.
Booming power, just the basics, a clutter free screen and practice seeing marks on the meter and seeing what you catch are what you need to learn what you are looking at. Letting software developers guess what fish look like is like letting non English speakers develop voice recognition software. So just turn those little fishy icons off, if you can figure out how.
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Merit McCrea is saltwater editor for Western Outdoor News. A veteran Southern California partyboat captain, he also works as a marine research scientist with the Love Lab at the University of California at Santa Barbara’s Marine Science Institute. He can be reached at: firstname.lastname@example.org.