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settle an argument or ake it bigger??


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when using your sonar does a 20" fish look the same on the screen when it's 40ft. down and when it's 80ft. down. this has been an ongoing discussion between a freind and myself. i say as long as the auto depth doesn't change they will be the same size.. he says the deeper the "20 inch " fish is the bigger it shows up .......opinions please... GOD bless rollie

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Based on alot of hours watching my screen, a mature King 100 ft down will appear as a smaller arch than a mature King 40 ft down. I run a Furuno 582 with thru hull transducer in manual mode. If memory serves me I had the same condition on my old Lowrance. Can't comment with any other brand. Also I beleive the location of the fish in the cone angle might have something to do with the arch size and shape.

IE fish directly under the boat compared to a fish on the outside edge of the cone angle.

Glen

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I think the biggest varible in REEL time would be what the fish was doing. If it was 40' or a 100' and NOT moving they should look the same but if one was on the move and going the same direction as the boat the arc would be bigger on that one.................but what I know....... :lol: .........this will be fun............... :clap:

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Here is a link to a study trying to judge salmon size from sonar. They actually tethered some fish for readings. It was pretty good down to about 38 feet. (12 meters) Deeper than that not so good. Just pretend any time you see any arches they are 35 lb salmon and scream hysterically to friends/family/clients when they show up on the sonar. Unless of course you go by 10 or 20 such arches without catching anything then declare they are bait.

http://www.sf.adfg.state.ak.us/FedAidPDFs/fds07-44.pdf

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I would drop a 12# cannon ball in the lake, and watch its decent.

Seriously, on a moving boat, the deeper fish would be in the cone angle longer, because the cone angle would be of a larger diameter at 80' than it is at 40'. That being a fact of trig, the deeper target would appear larger mostly because of the "tails" of the arch. That is the result of your sonar bouncing off the target more times.

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I would think that you would get a "fainter" echo back the deeper the fish is so the same size fish would appear smaller but it's been bloody hard for me to catch the same size fish 40' apart in the same troll to verify this.

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Can't explain it, don't know how or why.

All I can tell you is what I've seen after marking and catching many hundreds of them with the same fishfinder. Many of the largest deep water Salmon I've caught appeared to be nothing more than a small more roundish than arch shape with a red spot in the center.

At 40 or 50 feet a stationary mature fish would be a larger arc with a red spot in the center.

And I'm not complaining one bit.

On the 200 side of the screen these fish usually appear smaller than on the 50 side.

Again don't have a clue as to why just the way it is.

Glen

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Aren't we arguing apples and oranges? There's several variables at play here: first, the width of the mark itself (think of it as being drawn with a fat crayon versus a thin crayon) and second, the length of the arc. The second variable depends upon both the depth of the mark and it's position relative to direction the boat is traveling; a fish on the outside of the spread will mark as a shallower arc than a fish directly beneath the boat because its in the cone for a shorter period of time.

Also, I don't know how the algorhythms used by the graph alter its output depending on signal strength...the bottom is infinitely big, but it marks as a discrete structure because, like a sponge, it absorbs the signal...the actual bottom appears darkest, then beneath that becomes increasing light until the signal fades out...good way to tell mud from rock. It could very well be that the graph software generates a bigger mark as the fish get deeper to compensate for the fact that the fish is absorbing so much less of the total signal output. Therefore, it could be that both you and your friend are right. The fish at 40 and 80 look the same on the graph, but that's because the graph is compensating in the case of the deeper fish by amplifying the signal (ie "making it bigger"). How's that for compromise?

For practical purposes, without knowing exactly where the fish is and how fast it's moving, I'd say that experience is the best teacher. In my limited experience, you can't eat marks...but it sure beats a blank screen. Fish on!!!

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Rule #1, Don't troll any where around Vince when there are big marks on the screen. The marks have already been caught and released and they wont bite or they are scared sh-tless and are a waste of time as Vince is merely upgrading his King Of The Lake box !!

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Dear FF & Other Experts,

A high quality video screen type fish finder 50, 108 or 200 hz. is actually measuring the difference in DENSITY of a target. A large fish with a full stomach will mark differently, then a small fish with an empty stomach. A 300 LB. Atlantic Tuna will leave a very small mark on a good quality unit! (but have a dense red core) NO UNIT can detect the THERMOCLINE. The actual static line seen are small organisms in the water column drifting about because of water current etc. Every good fisherman knows to look for bait pods in the thermocline and look for activity (streakers) around the bait pods. This technique usually brings excellent results. And now enter the WHIZ kids with Computer enhanced graphics built into "Attractively priced Fish Finders" and small schools of minnows now become fish shaped!! I wouldn't buy any mfg's units until I could see it in operation. If you can't mark your downrigger weights 100 ft. down.(at slow speed) your missing a lot of other important data like small bait pods!! Active streaking small marks (thru your riggers)also tell you great info!! Commercial fishermen go to special schools put on by the equipment manufacturers in order to learn how to operate the units and READ THEM. The commercial guys can detect species, weight, and how full the net drag bag is several hundred feet from the boat (several $$$$$ cost!). If you can properly set your unit and find bait and fish, you will dramatically increase your fishing fun!

Respectfully Submitted, Jet Boat Bill (on the way back to Webster, NY)

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Well Put knowing all your equipment that well and how to use it is really important. I was always told the sonar signal bounces off the air bladder and deeper fish wll have reduced sized air bladers, and if it is moving tru your cone stationary or following, depends how long it stays in the cone will change size. Most commercial boats also use forward scanning sonar to detect there prey, or obsrtruction for the boat.

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So, who's going to volunteer to put on a seminar at Gander on how to interpret sonar readings?

Come on, JBB, it's your density...I mean destiny :D Couldn't resist...BTTF is a great movie.

Gator

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