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I posted this over on Crappie main and got a lot of cute remarks but no answers. So, I will try it here.

We were fishing on a lake yesterday, water temperature was about 52 degrees, water clarity was about 6 feet. Sonar (fish finder) is a Lowrance 135c and the digital read out indicated 15 ft. water depth, same as the screen display. The screen display showed a level bottom (we were not moving) that looked like soft mud. When I dropped anchor it went down 49 feet!! Then I dropped a weighted line, same thing, 49 FEET! Yet the sonar was showing only 15 feet.

Anyone have an answer? ....thermocline?, zoo plankton?, Loch Ness?

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There were some honest answers there... check if you are set on Meters instead of Feet. Check to be sure you are not in demo mode. It could be grass sub-surfece. Try setting your sensitivity to a mid-level, if it's set high. I ha mine set high when I first bought the boat and one spot was showing some sort of soft structure almost to surface in 25ft of water.... I reset the sensitivity and bang... cleared up the problem and I started marking good solid bottom and bottom structure very distinctly.

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I'm with paddlefishing follow him. I take it the transducer is level etc not on land but when boat in the pond. You don't say if this happened in one place or all the time @ several locations. Go to a known area depth & make your adjustments(esp sensitivity) there, that way U will know when its reading right, then try another area @ same depth.

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I don't suffer from insanity; I enjoy every minute of it.

I'm having trouble with my 522c also. It shows a flat bottom and never changes depth. I'm thinking it is the transducer but I don't find any kinks, nicks, cuts, or any other damage to it. HELP!!!!!

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You are correct, there were two honest answers. Your suggestion was taken seriously and I intend to check it. Also the suggestion that my transducer could be covered with scum is valid and will be checked.

The unit worked fine all day except that one spot where it gave a false reading. That led me to think that something was in the water that caused the reading. Can the thermocline cause a false reading? How about a thick cloud of zoo plankton?

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could be leaves settling from the fall... could be theremal.... could be grass dieing back

Is there a current there?

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Did you change any settings?.....

If so ... reset to factory and see how it looks.... make your adjustments in known water depth and structure contours.... only adjust one setting at a time.... not all at once.....

It's like sighting in a bow.... adjusting the hight to center first and then the windage....

Soft/muddy bottoms can make for some weird readings too.

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Thanks for your replies. I have been told and believe that the "bottom" that the FF was "seeing" was a thick layer of zoo plankton. I have seen the zoo plankton before down 5 to 10 feet and it looks just like the bottom but I have never before noticed it on the fish finder. Live and learn!

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When anchored and night fishing, I've had 30 feet of water turn into 3 foot deep. Shad gathered so thick the sonar wouldnt read thru 'em. 'Course this kept bouncing from 3 to 30, whenever the shad got that thick under the sonar.

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Longlining, Spider Rigging, and Nightstalking...Did I Miss Anything?!?

I have a HD5 and this exact thing happened to me last night. I was sitting over a crappie condo for about 3 hours anchored and not moving. I saw several bait balls come and go. Then I noticed my depth went from 19 feet to 4 feet for no reason. Then it went back to 19. I white line came across my screen at 4 feet, like the sonar was seeing a new bottom. At first I thought it was a thermo reading of some sort, then noticed this happened on and off. I tried turning off the unit and even reset it back to factory specs. Once it came back on reading 4 feet and once it came back on reading 19 feet.
I thought "great" I've got me a piece of crap for a sonar. Then after stewing a few mins... I noticed the very large amount of shad around my lights. I fine tuned my settings (changed to 83 mhz) and saw the very large bait ball under my boat. When that bait ball was gone, I read the bottom.

Sometimes, you just gotta think about what's actually going on before condemning the unit. Mine actually did what would be expected. It was seeing a wall of bait fish and was reporting back to me exactly what I had asked it to do.

Best of luck with yours!