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Author Topic:   SONAR Interpretation
jimh posted 12-31-2006 01:01 PM ET (US)   Profile for jimh   Send Email to jimh  
Please interpret this rasterized SONAR display for me. What do the several targets near the bottom represent? Sorry, I don't know what the real answer, as I did not investigate, and, yes, I am serious. I would be very interested to hear what experienced users of SONAR think this might be.

The device shown is a LOWRANCE X87, we're in fresh water (Ottawa River), using the standard HS transom mounted transducer. The water depth is about 28-feet, and we are in a little bight that is out of the main river current.

Photo: SONAR display

Chuck Tribolet posted 12-31-2006 02:04 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Fish, or something else in the water column, close to the
bottom. They are arched because they are a little further
away as you approach, get closest when you are right over,
then get further away after you pass over.


bigjohn1 posted 12-31-2006 07:20 PM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
That would be my bet also. Looks like a small school of bottom-dwelling fish.
jmorgan40 posted 12-31-2006 08:43 PM ET (US)     Profile for jmorgan40  Send Email to jmorgan40     
It would help knowing what your settings are since Lowrance units are very sensitive. Without knowing your chart speed and sensitivity settings they look like fish hugging the bottom with another about 5 feet off the bottom.
timing posted 12-31-2006 10:45 PM ET (US)     Profile for timing    

Highly probable those are fish suspended just above the bottom.
I would guess they are hanging out in hopes of something edible moving down that slight slope.
That slope could be a slow drop off, or perhaps a lump.
If a lump, I would guess the currents are flowing right to left
as those fish wait for a meal to sweep over the lump.

In terms of arch graphics on the sonar unit – this site offers a good briefing:
[ Replaced link with the Lowrance original web page at:]

Hopefully a follow up post after you revisit that spot and a pic of you holding the catch.

And most importantly, this would be a good time of the year to express sincere thanks for all your efforts in providing this awesome site! Happy New Year!


jimh posted 01-01-2007 09:43 AM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for reminding me of the LOWRANCE SONAR tutorial. It has a great explanation for the arch effect seen in my screen shot: asp#Why%20Fish%20Arch

I suppose there is no way to guess at the size of the fish. I don't recall the sensitivity setting. I think I probably adjusted it to give the best depth readings when running at speed so the unit would hold the bottom.

We had our nephew aboard, who is a very avid fisherman, and he was impressed with the resolution of our SONAR.

cwolf posted 01-01-2007 12:17 PM ET (US)     Profile for cwolf  Send Email to cwolf     
Image coupled with your comment about being out main current indicates a school of fish. Fish like being out of main current when at rest so they're not expending too much energy. You'll also see them in depressions/holes or around structure of any kind. BTW - my guess is that they are catfish.
jimh posted 01-01-2007 01:48 PM ET (US)     Profile for jimh  Send Email to jimh     
If anyone has an interesting screen shot of a SONAR display--and even better if they happen to know what the target producing the echo actually is--it would be interesting to see it. Send me an email with the image and I can host it for you, and we can include it in the discussion.

Now back to the image shown above, about how big would these fish be? Any guesses?

jmorgan40 posted 01-01-2007 07:24 PM ET (US)     Profile for jmorgan40  Send Email to jmorgan40     
Based on my knowledge with my Lowrance...I would say they are a few pounds based on the depth of the water. When I mark bigger stripers, my arches are normally a little larger.
Chuck Tribolet posted 01-01-2007 09:25 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I've seen lots of arches like that, and if I dive the site
they are schools of blue and yellow tail rockfish.

The higher the arch, the bigger the fish, because the SONAR
will start picking it up farther away.


jimh posted 01-01-2007 11:32 PM ET (US)     Profile for jimh  Send Email to jimh     
Chuck--That is a very good point: a correlation between target size and amount of arch. Thanks.
jimh posted 01-01-2007 11:33 PM ET (US)     Profile for jimh  Send Email to jimh     
By the way, how many fish are in that echo cluster?
JMARTIN posted 01-02-2007 12:44 PM ET (US)     Profile for JMARTIN  Send Email to JMARTIN     
OK, I will guess five on the bottom, maybe a little one up to the left. Big fish will show some color, like the bottom does, some yellow then red. Get those volts and water temperature off the screen. Get some color in the background. Put on the fish symbol, then the Lowrance will put a "fish" picture where it thinks it sees a fish. Bigger picture, bigger fish. The problem is, the Lowrance thinks everything suspended in the water is a fish.
If you are fishing, learn to read the arches. John

David Pendleton posted 01-08-2007 12:51 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Here's the extra credit question (and yes, I know the answer).

How does a Fishfinder know a fish is a fish?

Tom W Clark posted 01-08-2007 01:02 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
How does a Fishfinder know a fish is a fish?

It doesn't. Oh, it might have software to try to discern whether the echo is from a air bladder or not, but it is far from perfectly consistent.

Lots of experience fishing a given area with a fish finder will given operator a good sense of what they are looking at.

Distance of fish, size of fish, type of fish, water movement,salinity, haloclines thermoclines, subsurface currents, etc, all contribute to confusion of a fishfinder, but for the most part, when you see arches like those five pictured in Jim's shot above, think: Fish.

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