Using Humminbird Down Imaging to find the fish, including ones that traditional sonar misses
Professional Walleye Angler
As I pulled away from the docks at Everts Resort on Pool 4 of the Mississippi River to start today's Humminbird Down Imaging test drive, I immediately noted how well Down Imaging performed at spotting bottom-relating fish. Walleye and sauger are my normal targets of choice, particularly at this time of year, and these fish can be easily found (and identified by sonar) in slow, deep water as well as in shallow water areas near current breaks. The two pictures below show how well Down Imaging performs in relation to traditional 2D sonar in both shallow and deep water. Fish are identified in both pictures, and are noted by red arrows.
These results really came as no surprise to me. What did surprise me, however, were the number of occasions with Down Imaging revealed the presence of fish that were completely absent in the traditional 2D sonar returns. A couple of notable examples are included below. Note that in each case, a fish is identified in the Down Imaging view (noted with red arrows), but a corresponding return from traditional 2D sonar is absent.
As you start to use Down Imaging, you'll find more and more examples of fish that traditional 2D sonar misses, but Down Imaging (and Side Imaging) find. This will help put more fish in your boat, and make your time on the water more productive and enjoyable.
It's important to point out that these are two separate fish, identified in two different locations with two different bottom compositions (tip of a wingdam in the left image, deep sand flat in the right image). That fact alone convinced me that I was not observing some sort of sonar or instrument artifact caused by something in the water or on the bottom. Moreover, on inspection of the corresponding Side Images, I could see that the fish were also identified by Side Imaging. The combination of Down and Side Imaging was showing me fish that traditional 2D sonar was missing. The full screen captures corresponding to the two pictures above are shown below. The fish missing in the 2D sonar returns are again noted with red arrows; one fish's prominent sonar shadow is also noted with a blue arrow.
What these Down Imaging-identified fish share in common is that they are both relatively high in the water column, both about 10 feet down over 15-20 feet of water. As I began to think about this common thread and the physics of sonar transmission from the transducer, the appearance of these fish in Down Imaging but not in traditional 2D sonar began to make sense.
Traditional 2D sonar is recorded using a frequency of 200 kHz, and the cone angle of this beam is 20 degrees. This relatively narrow beam receives returns from a small portion of the water column and the bottom beneath the transducer. For example, in water that is 20 feet deep, a 20 degree soanar beam will receive returns from a circular portion of the bottom that is ~ 7 feet in diameter. This circular area that is interrogated by the 200 kHz sonar becomes smaller in higher portions of the water column. For example, in water that is 20 feet deep, traditional sonar only interrogates a ~ 3.5 foot diamter circle of water 10 feet below the boat. For a shallow fish to appear in your 2D sonar, it is constrained to be within a very small piece of water that is directly below the transducer. If it is outside of this circle, it will not appear on your 2D sonar screen.
Humminbird's Down Imaging and Side Imaging make use of a pair of 455 kHz sonar beams, each of which has a sonar cone angle of 86 degrees, providing for nearly 180 degree coverage of the bottom and water column above it. If you review the diagram at left, you'll see that the 455 kHz beams cover the water column well outside of the 200 kHz beam that is used to produce the traditional 2D sonar image. The fish that are identified by Down Imaging and Side Imaging in the screen captures above, yet absent in the traditional 2D sonar views, are outside of the 200 kHz beam but well within the cones of the 455 kHz sonar. The combination of Down Imaging and Side Imaging allow the angler to find these fish that traditional 2D sonar misses. Using Down Imaging will help you find these fish, and give you more confidence to spend your time targeting water that is holding shallow, active fish....water that appears fish-free in traditional 2D sonar.