Trying to get my head around sounders.

[GreasePit]

Hey raiders,
Just want to get some opinions on what the heck i'm actually seeing on my sounder.
It's sonar imaging (Lowrance Elite 5), with the sensitivity sitting on 75% at a stand still position on 200khz. (I now know i should probably be running 83khz when anchored up).

For a bit of context, this is what the sounder for the most part usually looks like.

As well as that, the two flat lines around 25ft being my bait jig, with two small pieces of bait around the same size as a finger nail on some tiny size 8 hooks.

Now, every now and again, this would pop up on the sounder, sometimes being even thicker than as pictured.

I dropped baits to that depth, small and big, different varieties to see what it could be but no takers. Now given the colouration of the bait jig being solid green, and the baits not being touched indicates that these are not actually fish. I initially thought they may have been big schools of mullet rolling through but i think they would have come up as more of a solid colour given the appearance of the relatively small bait on the sounder.

Can anyone shed any light on this?

On top of this, i know the sounder sends a sonar pulse straight down, but how wide does it send that pulse to the port and starboard, as well as how long? Or in other words, what is the physical area that the sonar projects onto the screen?

Thanks in advance.

Edited July 20, 2016 by Laborgatory

[amkr]

Let me see if I can help.

The transducer will have an angle rating on it, so the area it covers will vary depending on the depth - I think the lowrance ones are 18 degrees off the top of my head.

It also shows more than what is currently under the boat, for example the items on the far left of the screen have probably passed you by

The whole screen isn't dynamic, it draws the signal on the left hand side of the screen and scrolls it across. So you could have a fish swim past your transom and swim off but the sounder will display it as if it hasn't moved.

Basically the beam width isn't all that important.

You're right in saying that more dense objects show up a darker colour.

What I think you're seeing on the sounder there is a school of very small fish, like sweep for example. They usually show up without any great density and will easily cover the entire sounder screen. I have had similar images appear on the sounder and downsized to sabiki rigs to find out what was down there. It's always been plagued of sweep for me.

I think the sounder gets tricked by the size of the 'school' and identifies it as a larger less dense object.

Abither trick I've started using recently is to bump my sensitivity up on the sounder so there's just a little bit of noise in the clean water - it gives me a better target seperation.

I Also split my screen into 3 views. 83khz, 200khz and downscan/chirp. I find this lets me view the same area with different parameters and helps me decide what is actually there and what is noise or crap fish. Sometimes things show up on one frequency and barely register on another.

Using this set up we trolled over a school of tailor, only showed up on the 83khz, didn't even register on the 200 or chirp screens - I actually said to Welst at the time "we've just driven over a school at the right depth, you should hookup" and sure enough, just after the images moved off the left of the screen the rod bent over.

My 2c

[locodave]

Boat show coming up, take your pics and speak to the reps - just an idea.

[GreasePit]

Let me see if I can help.

The transducer will have an angle rating on it, so the area it covers will vary depending on the depth - I think the lowrance ones are 18 degrees off the top of my head.

It also shows more than what is currently under the boat, for example the items on the far left of the screen have probably passed you by

The whole screen isn't dynamic, it draws the signal on the left hand side of the screen and scrolls it across. So you could have a fish swim past your transom and swim off but the sounder will display it as if it hasn't moved.

Basically the beam width isn't all that important.

You're right in saying that more dense objects show up a darker colour.

What I think you're seeing on the sounder there is a school of very small fish, like sweep for example. They usually show up without any great density and will easily cover the entire sounder screen. I have had similar images appear on the sounder and downsized to sabiki rigs to find out what was down there. It's always been plagued of sweep for me.

I think the sounder gets tricked by the size of the 'school' and identifies it as a larger less dense object.

Abither trick I've started using recently is to bump my sensitivity up on the sounder so there's just a little bit of noise in the clean water - it gives me a better target seperation.

I Also split my screen into 3 views. 83khz, 200khz and downscan/chirp. I find this lets me view the same area with different parameters and helps me decide what is actually there and what is noise or crap fish. Sometimes things show up on one frequency and barely register on another.

Using this set up we trolled over a school of tailor, only showed up on the 83khz, didn't even register on the 200 or chirp screens - I actually said to Welst at the time "we've just driven over a school at the right depth, you should hookup" and sure enough, just after the images moved off the left of the screen the rod bent over.

My 2c

Thanks for your reply mate, great info in there.

No sweep in my system, but could definitely be some really tiny baitfish.

Ill have to try the split screen when i hit the water next, hopefully be able to sound up some bigger bait schools and get into some quality fish with a little help from modern technology.

Cheers again