Author Topic: Fishfinder Advice  (Read 206 times)

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Offline FlyingAnchor

  • Posts: 109
Fishfinder Advice
« on: March 11, 2018, 11:47:56 PM »
I’ve seen quite a few questions about fish finders and their utility, and I thought I might offer some tips on things I’ve learned over the years messing around with them. They are certainly not required with most of the skinny water fishing we do around here (in fact, unless you’re in 6 ft or deeper you’ll likely never see a fish on one), but they can be immensely helpful tools.

Fish finders work on a relatively simple principle, but use some very complex technologies to maximize their capability. Fundamentally, a sound wave is transmitted out of the transducer, and then the time that signal takes to return after bouncing off of something is processed by a computer and compared to other returns, to build a 'picture' of what is under the boat. By modulating the frequency, the unit can optimize between search area (beam width), depth (lower frequency waves travel farther), and fidelity (higher frequency waves provide greater resolution and detail). For most of the fishing we do around here, high frequencies are just fine. CHIRP sonar uses a longer transmission (pulse) with varying wavelength, and then compares the returns of the different wavelengths to get a very high resolution 'picture.' But that is all...BORING! What we care about is how to use it to find fish, so here are some tips I've learned.

It's important to understand what you are seeing, and what you are not. The return (or ‘mark’) you see on the screen is the result of an object of different density than water reflecting the wave from the transducer. With fish, this is generally the swim bladder (since it is filled with air). So…a large return (or ‘mark’) doesn’t necessarily mean a large fish. Additionally, different species tend to have different sized swim bladders. Striped bass have larger swim bladders than bluefish, so stripers tend to look like larger more defined marks, and bluefish as smaller splotches. When I used to fish the lakes out west, very large mackinaw trout holding close to the bottom would barely show up, while their smaller brethren holding in the middle of the water column would be very well defined (their swim bladders being more inflated).

It doesn’t take a large object to draw a large return. For proof, tie a one ounce weight, or jig, to your line and drop it directly under your boat in limited current so it stays within the sounders beam width. If you jig it up and down you should see the return move in a wave along the screen. A typical one ounce lead weight will draw a fairly sizable return, even though it’s physical size is smaller than the average human thumb.

So what causes ‘fish arches?’ The wave transmitted by the transducer propagates in a cone, but the only measurement analyzed by the fish finder is time. As you move through the water, the fish will first be on the outer edge of the cone. Using some simple trigonometry, this distance is slightly further than when the fish is directly under the boat. Since the wave has to travel slightly further (longer time) the unit interprets that return to be slightly deeper. As you move closer and subsequently over the fish, that time is shortened (slightly) and the return is shown as being shallower. Repeat the process as the fish exits your beam, and that is the construction of the arch, an object that appears to rise slightly and then subsequently descend over a short time. The same principles would apply for fish that are swimming under a stationary boat (and subsequently in and out of the beam). A large fish arch doesn't necessarily mean a large fish, it could just be a slow moving fish that remains in the beam for a long time, which brings us to...

What about ‘length’ of returns? Most fish finders use a scrolling display, and the line of pixels and the right side (the most recently created one) is what is under the boat at that moment. Every line of pixels you move to the left is a snapshot in time of what was. So, as an example, if you were anchored and a bubble was released from the bottom directly under you, you would first see a return adjacent to the bottom, as it rises, each return would appear higher on the screen. As time passes, you'd see a diagonal line from the lower left of the screen to the upper right, essentially a depiction of the bubble's depth with respect to time. If you were moving, you'd only pass that bubble at one point in time, so you'd only see one return which would march left across the screen until it was off the display. If you want a good example of this, go to Whitehurst and play around over the deep borrow pits adjacent to the runway. When peddling around, it looks like there a tons of fish all over the water column. However, if you stop and remain stationary, many of those returns will slowly creep up the display from left to right as time passes. They’re all just bubbles rising from the bottom (Disclaimer: I don’t know what causes the bubbles, but you can see them pop on the surface. And there ARE fish in that portion of the lake that you will mark, but they’re generally all hugging the bottom). Now, let’s go back to being anchored. If a fish were slowly swimming around your boat (remaining within the beam width the entire time but never changing depth) you would see one constant return at a constant depth. After a while, it would look like a straight line from left to right somewhere above the bottom on the display. The other return that can look like a straight horizontal line is the thermocline (since different water temperatures have different densities), but it's generally less defined than a hard return.

So where exactly are the fish under the boat? Since the transducers beam is cone shaped, any return within that cone will be displayed. Sooooo, because most displays (side imaging units operate a bit differently) are simply a function of depth of return over time, there is no way to discern where exactly in relation to the boat that return actually is. If at 20 feet the beam was 8 feet wide, anything within a four foot radius at 20 feet deep would show up as a return. This is why ‘video game fishing’ is tough, but it’s usually close enough to get you in the ballpark.

Where to mount a transducer? Three things can severely degrade transducer performance: Material in between the transducer casing and the water, turbulent water flow around the transducer, and mounting the transducer at an improper angle. To mitigate the first, direct contact with water is generally best (think through a scupper or an over the rail mount). If not that, most kayak hulls are thin enough as to not noticeably degrade performance if mounted ‘thru-hull’, provided that the amount of epoxy, glue or other method to hold it to the hull is minimized. I have my transducer mounted to the Lowrance mount on a Hobie and it works just fine. To minimize the second, keep it somewhere where the water won’t be turbulent, away from paddle strokes or peddle systems. For the third, ensure it is parallel to the water line (or according to manufacturers’ instructions). This is often the most common mistake. A transducer mounted at an angle will transmit the beam outward instead of straight down, and the returns will be inaccurate.

A word about settings: I’ve found the best way to get what you want with most units is to bounce a weight or jig under the boat and mess with the display until that return is very sharp and clear, without a lot of clutter. Settings for water clarity and clutter rejection will be helpful here. The fish ID functionality on most units is absolute garbage, and you’re far better off interpreting the raw display. Bottom line? Read the instruction manual (each unit functions a bit differently) and then go try it out, a lot! In my humble opinion, fish finders are best for determining water temperature, bottom contour, composition, and structure, and balls of bait. If you catch a fish after seeing or chasing a bunch of marks, try to establish a pattern of what the mark looked like. Eventually, you'll get a sense for what different underwater environments look like on your particular display. Above all else, don't be a slave to them! I can't count how many times I was staring at the screen of a sounder and missed fish crashing bait on the surface...
« Last Edit: March 12, 2018, 07:56:57 PM by FlyingAnchor »


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