Understanding How to Read Your Fishfinder & Sonar Readings
Lowrance has put together quite possibly one of the best guides to understand your fish finder/sonar readings and how to make sense of it all. What do the arches really display and how to understand feeding/moving fish versus still fish.
The full article will be posted - but the first few pieces will be an overview.
What are the sonar images really showing?
A: This is a prime example of what is referred to as “streaking”. This effect occurs when you begin to mark a fish arch, however reach a point where the fish notices the boat shadow, motor, or other stimulus and move rapidly away from the transducer. Remember, although the streaks go down, it doesn’t mean the fish moved deep, only that they moved away from the transducer. They may have stayed at the same depth and simply showed their tail fins to the boat.
B: On this chart we can see the beginnings of a faint thermocline which has formed at approx. 22 feet. Even if faint it is important to note thermoclines and how the fish and bait are relating to these temperature differences.
A: Indicates the beginning of a track made by an active and moving fish. The “arch”, if we can call it that, shows that although the fish moved a little farther away from the boat it did for a while actually parallel the boat. It may have been inspecting our trolled live bait.
B: Interspersed around the active fish are less active fish which would rather sit still than move around. This results in relatively clean rounded arches. This flat rounding of the arch may also indicate the fish is closer to straight under the transducer. Without going into the mathematics of bisecting cone shapes for face area, a good rule of thumb is a fish arch becomes sharper or pointed the farther from center the echo is. So sharp pointed arches indicate fish on the edges of the coverage areas and rounded flatter arches are more directly under the transducer.
C: Smaller schools of bait fish can also produce or be displayed in an arching pattern. But as displayed the peaks of these arches show color indicating good return signal strength while also being dispersed. In some cases it can become possible to actually see arches from individual baitfish.
D: Is showing fish on the bottom but without solid sonar returns they may not be directly under the transducer.
A & B: This chart is a good representation of the difference between a fish under the transducer and one on the fringes of the coverage area. A-arch shows color which indicates return signal strength and it is also well rounded with an even length of arch closer and arch away. B-arch shows a slightly more pointed appearance to the peak and this echo is weaker showing only the faintest of echo returns. This indicates this arch was generated be a fish closer to the edge of the coverage area and A was generated by a fish closer to directly under the transducer.
C: indicates balls of baitfish. Both are relatively the same size schools of bait and we would assume the same size baitfish. The first shows more color in the center indicating more echo signal return strength and the next one is weaker. The second ball is also depicted a little farther away or lower on the range scale. This may indicate a ball of bait which is not as close to center under the transducer as the first bait ball.
A: Depicts the thermocline. In this portion of the chart it is easily seen, in others it is not. You may have begun to notice that most of the activity is very close to this thermocline. This chart also shows active fish (B closer to the surface which streak away when we reach a distance of approx. 10 feet from them. Other deeper inactive fish (C are not disturbed by our trolling approach.
Thanks to Lowrance and Luke Morris for the preparation and publishing of this article/information - Lowrance.com
Last edited by Striper; 03-02-2010 at 03:53 PM.
Bump because fish finders arent a magic bullet unless you know how to use them...