A Humminbird Side Imaging examination of commercial gill nets on the Mississippi River
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In waters around the US (and beyond!), sport anglers often share the water with commercial fishing operations. In many bodies of water, particularly in the upper midwest, commercial fishermen target rough fish (carp, buffalo, sheephead), catfish, and other species deemed "less desirable" by traditional sport anglers. Commercial fishing in the Great Lakes maintains an active harvest of whitefish, chubs, trout, salmon, smelt, and walleye. In other regions of the country, freshwater commercial operations that target walleye, salmon, paddlefish, or sturgeon remain quite active. One of the most common tools of commercial fishing is the gill net. In this article, I will illustrate how these gill nets appear with Humminbird's Side Imaging technology, and demonstrate ways in which you can tune your unit to enable you to see these pieces of commercial fishing gear. 
Dr. Jason Halfen 
Professional Walleye Angler
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Illustration of a bottom gill net
(Michigan Sea Grant ) 
​Gill nets that target fish in the lower part of the water column are anchored to the bottom with lead weights, and extend off the bottom with the help of small floats to lift the main body of the net. Set in this way, gill nets sit like an upright fence in the water. Gill nets can range in length from 100 to 400 feet long. The mesh size of the gill net can vary, allowing small fish to pass through but ensnaring larger fish within the mesh. The presence of a set gill net is indicated by two large floats, typically with flags attached, at either end of the net. Most of our regional commercial anglers tend their nets by hand, running their nets and removing their catch once every 24 hours. 
Sonar imaging of a gill net can be challenging. The mesh itself is typically monofilament or nylon line, not much thicker than heavy line used for traditional angling. As such, the mesh is essentially invisible. The small lead anchor weights sink into bottom sediments and there therefore also invisible. To image the net, we must be able to identify the nylon cord that forms the top and bottom borders of the net, or the small floats that are spaced along the top border of the net. While these represent fairly limited sonar targets, the power of Humminbird Side Imaging still allows us to identify these pieces of commercial fishing gear while they're in the water....and even see the fish they have caught! 
While fishing Lake Pepin, which forms a portion of the border between Minnesota and Wisconsin, I regularly encounter gill nets tended by commercial anglers targeting carp, buffalo, sheephead and catfish. These nets are often set perpendicular to shore, and can be found in water as shallow as 6 feet and as deep as 30 feet. While these nets do not present a particular hazard to boaters during the day, the net buoys can be challenging to see during low-light periods. Also, determining the direction in which a net is set can be difficult when fog, drizzle, or other weather conditions obscure the locations of the buoys that mark the two ends of the net. As a result, finding ways to identify the gill nets while they are in the water, using technology other than simple eyesight, would be valuable to boaters of all sorts. Luckily, users of Humminbird Side Imaging fishing systems already own the tools that they need to clearly image these nets....here's how I do so using my Humminbird 1198c. 
To identify gill nets using Humminbird Side Imaging, make use of the "SI Enhance" options found in your Side Imaging X-Press menu. There are three options in the SI Enhance menu: SI sensitivity, SI contrast, and sharpness. To collect the images below, I increased SI sensitivity to 17 (from my normal setting of 10-12), SI contrast to 15 (from my normal setting of 10), and kept sharpness set to off. I adjusted by SI range to keep the image in the middle of one of the two Side Images. These simply adjustments allowed the top and bottom cords of the gill net shown in the picture above to literally jump off the screen. These cords are clearly visible as two parallel lines in the left Side Image below. 
I collected this image by driving parallel to a line established by the two net buoys, at 5-6 mph. This net was set in an area of light current, and you can see that wind, wave, and current action have caused the middle portion of the net to bow downstream (more towards the center of the Side Image). Another thing to note in this Side Image is the presence of a couple of bright white sonar returns between the two lines that represent the top and bottom of the gill net. I assume that these are fish that have become entangled in the net, and are waiting to be retrieved by the owner of the net. 
Having the ability to identify a gill net when it is set underwater represents another powerful use for your Humminbird Side Imaging fishing system. Imagine this situation: you're trolling a spread of 4 crankbaits on leadcore line in deep water on a foggy morning. Atmospheric conditions prevent you from seeing the two gill net buoys that you have just passed between. If you can image the gill net beneath your boat with Humminbird Side Imaging, you'll have time to slow down and retrieve your baits before they snagged by, and are lost in, the commercial gear below. You just saved your 4 favorite baits, and the commercial fisherman who shares the water with you doesn't have to dodge treble hooks while harvesting the rough fish you're trying to avoid. 
Thanks Humminbird, for providing anglers with the power to find fish, structure, and now gill nets with Side Imaging!