Help with fishfinder

[Marie Maddie]

I am new to this game and wanted some advice on understanding the fishfinder. How can you tell the difference between species just by looking at the screen? And how do you find the thermocline? I have a Furuno Navnet 2 VX series 10" screen. Which is better at finding fish low freq. or high freq. I have tried different settings and now i am totally lost. Thanks, Mark

[gottaflylee]

Ok, lets start with this. High frequency is for shallow depths up to about 300 to 400 hundred feet and low frequency is from there deeper. If you are not marking on high frequency (200hz) you may not have the gain up high enough. When you switch to low frequency you will need to turn down the gain.
To find a the/ a thermo-cline, turn the gain up to where you have clutter on most of the screen and then turn it down to where it is mostly gone, if there is a thermo-cline there is should show up as clutter at what ever depth it is at. Hope this helps.

[fish4bills]

An fish-finder electrical signal is sent to the transducer. The electrical signal to energize the transducer crystal is generated by an amplifier. The energized crystal reverberates at a particular frequency, to convert the electrical signal into mechanical acoustic or sound energy. The acoustic energy or sound waves causes oscillation of the water molecules through which the sound travels. The sound is pulsed out in a defined beam. These do not travel out in a straight line but in a wave pattern. The acoustic pulse travels through the water at a rate of approximately 4800 ft (1500m) per second in saltwater and 4920 ft/sec in fresh water. When the energy strikes an object within that beam such as fish, the sea bottom, or a structure then some of the energy is reflected, or echoed back to the transducer. The transducer collects this return, passes it back to the unit and this processes it into display data. As the processor is programmed with the rate of sound transmission in the water it calculates the time difference between the transmission and reception of the returned signal to give a range or depth figure. This result is then displayed as a number or as an image on a screen.

The depth determines the fishing technique, and what type of lure or bait. The finder lets you see who may be at home down there, but it will not guarantee a catch. That part is up to your skills. In most fishing you will use your unit to look for underwater structure, such as ledges and significant bottom changes. You may also be looking for drop-offs which are gullies and channels or break-lines, which is a sharp bottom drop off point. You may just be looking for fish activity. If you locate a tight ball of baitfish then they are probably being chased by a predator. A loose school indicates no activity. A finder is not necessarily going to tell you what the fish is. Image interpretation involves understanding both the underwater features and fish behaviour to distinguish between bait fish and the target fish. .


The acoustic transmission frequency affects both the water depth range and the cone angle. The lower frequencies are used in deeper waters and have lower power losses. They also tend to have wider beam angles and cover wider viewing areas. There are many frequencies used in fish-finder sonar systems. Typical are 38, 40, 50, 75, 107, 120, 150, 192, 200, 400, and 455 kHz. Shallow waters less than 300 feet give the best results with high frequency transducers of 200kHz and wide cone angles up to 20 degrees. In depths greater than 300 feet low frequency transducers of 50kHz with small cone angles of 8 degrees are the best option.