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Catching up with fish
What can we learn from this Fish Finder screen?
This screen shot was captured while moving at a speed of 0.2 knots by using the engine to hold the position. The frequency is 200 kHz, and the Bottom Discrimination mode is displayed in graphic mode.
Although we do not see any fish echoes here, this Fish Finder image was shot at the point where we caught tilefish.
Let's take a look at the characteristics of this fishing spot with the information obtained from this screen image.
Since the seabed is on a gradual downhill slope as the boat moves forward (to the right on the Fish Finder), we can determine that the boat has been advancing towards the deeper water. The left edge of the screen is 50 m deep and 72.7 m at the right. The water depth difference goes up almost 23 meters.
To think about the distance the boat has traveled, you can use the "minute mark" at the top of the screen. The minute mark is displayed with red and white lines that alternate every 30 seconds. In this screen there are four sets of red and white marks, so it indicates us that it took 4 minutes for the boat to move from the left side, to the latest information displayed in the vertical row at the right side of the screen.
Ship speed 0.2 knot is equivalent to about 6.2 meters/ m, so in 4 minutes the boat traveled a distance of about 25 m. Meanwhile, since the water depth has changed by about 23 m, it can be inferred that the inclination angle of the seabed is less than 45 degrees from the moving distance of the boat and the water depth change. (If it is derived by a trigonometric function, 43 degrees is obtained).
Although the method of discriminating the bottom composition by the length of the “tail” extending downward from the seabed line is easy when the tilt angle of the seabed is small, it becomes more difficult to distinguish when the inclination angle becomes larger. As the tilt angle becomes large, the trail gets longer and the variation becomes larger, so that the difference of the tail, due to the difference in bottom composition tends to be difficult to judge just by looking at the screen.
When it comes to defining seabed sediment, the Bottom Discrimination mode works perfectly. The model (FCV-627) used this time converts the strong analog signal of the echo reflection from the seafloor to a digital signal, process the information, and provides highly accurate bottom composition discrimination.
The swimming layer of tilefish is near the seabed and the echo of tilefish itself is hardly caught by a Fish Finder, because most of the time tilefish stay in their nest hole. Therefore, the key in searching for tile fish is to look in the range of 40 to 120 meters of water depth, and a seabed made of sand, which is easy to make a nest.
In actual fishing, water depth inevitably changes when exploring your fishing grounds. It is one of the most valuable functions for boat anglers that the Fish Finder qualifies bottom composition even in the seabed slopes and displays the discrimination results with high accuracy.
DAIWA field tester, Writer at Boat Club, a monthly Japanese boating publication
Nobuaki is a hobby angler who travels around Japan together with his beloved car-top boat Tomoe-maru. With a keen interest in fishing as well as scuba diving, he has gained a substantial amount of knowledge in how to utilize fish finders to the fullest. Nobuaki regularly holds well-attended training courses in the use of fish finders, as well as being a proponent for proper sea manners and safety at sea. Sharing his knowledge is a passion for Nobuaki, and he manages his own homepage as well as being a sought after writer in Japanese boating publications.