Interpreting sonar

[smallmouth89]

I am enjoying my new Piranah Max 20 but being my first ever sonar unit I have a lot of questions about what I see on the screen. Unfortunately the Humminbird manual is rather sketchy on intrepreting what is on the screen. This unit has a dual cone 20/60 degree coverage. I think I've got it figured that solid fish icons are close to the tube and the hollow outlines are fish that are more in the periphery. Do any of you have units that work this way?

When trolling a breakline I'm kicking back to the shallows, finding the deeper dropoff point and then turning parallel to that and following the deeper water. However at this point I'm only getting a reading for the depth I'm covering and I can't see that shallower water anymore as I am parallel to it and in deeper water. If the water had dropped off from 4 to 8 feet and I'm maintaining a path in that 8 feet of water I really can't tell how far that 4 foot level is away from shore now. I can tell when the 8 foot level dips to 10 or 12 feet but in my present direction of travel I'm not certain where that 4 foot edge is. Does this make sense or is it just my spatially/ mathematically challenged brain confusing the issue?

By the way my unit helped me catch a few trout yesterday. With the surface water temperature a constant 45 degrees the fish were spread out all over the ponds and I didn't mark a lot of fish. However I came over one area where the fish alarm was constantly sounding so I stayed in that area and eventually caught 4 nice rainbows. Without the sonar I would have just kept trolling through. But with the unit suggesting there were fish in the area my stopping to explore the area with a series of fan casts paid off. Your thoughts, gentlemen?
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[TubeDude]

[cool][#0000ff]Sorry, no magic words for ya. Not all sonars are created equal, and it is usually an individual matter to learn to read what your sonar is telling you. [/#0000ff]
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[#0000ff]One thing I think I have learned is that fast drops and breaklines do not always show up as such on units that are not high wattage and "real time" display. Even then, some interpretation is necessary. Fast changes can temporarily confuse the sonar...and the fisherman.[/#0000ff]
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[#0000ff]If you want to work a break line, buy some marker buoys, like the bass boys do. Use your sonar to find the dropoff points and throw a buoy several feet back, to avoid tangles. Go along the line and put out several of those buoys and you will have a visual reference.[/#0000ff]
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[#0000ff]I think your interpretation of the strength of the fish display is probably on target. My Eagle displays all "hits" solid, but it could be useful to know that the fish are out at the edge of the "cone". However, in shallow water, that is not a very big difference.[/#0000ff]
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[#0000ff]One of the keys to catching fish is to first find them. Second is to fish with confidence. Sonar helps you do both by letting you know that you are in the ZONE.[/#0000ff]
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[#0000ff]Sounds like you are educating yourself right well, and having some fun in the process.[/#0000ff]
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[Fishhound]

Hey smallmouth89,

You are understanding your Humminbird's functions correctly. Mine is the same way - basically.

Think of the beams from your fishfinder like the beam of a flashlight. The 20 degree is a narrow intense beam, like a spot light. The 60 degree is a broad more diffused and less intense beam, like area lighting.

The solid fish figures are the ones directly under you - those are the fish in your 20 degree beam(that's the higher frequency beam). The 20 degree beam is the one that goes down the deepest.

The hollow figures are the ones in your 60 degree beam(that's the lower frequency beam). This is the beam that covers the most area.

The coverage on each beam is as follows(if you'll remember some of your geometry from school):

The 20 degree beam is measured at the widest part of the beam - that will equal little over 1/3 of the depth you read(actually it's about .35 - or a circle of 10feet(depth) X .35). In other words, if the depth is 10 feet, the cone will cover a circumference under you that is a little over 3 feet in diameter. At a depth of 4 feet that would be a cone on the bottom of about 1and 1/3 foot diameter on the bottom.

The 60 degree beam puts a circle on the bottom that is roughly about(just a little bit over) the same diameter as the depth(actually the number would be 10 feet X 1.15). Or, if the depth is 10 feet, the cone on the bottom is about 10 feet in diameter. At a depth of 4 feet the cone is a little over 4 feet in diameter on the bottom.

To follow a contour of a specific depth - for example, a breakline(like a submerged roadbed), a drop off, or a deep weed edge, you can use a GPS and mark spots along it(this is difficult to do in a tube while fishing though). Or, use bouy markers that you can buy them fairly cheaply - Or, make your own very easily and even more cheaply.

If possible, scout out the area to fish in advance - get a map of the area before hand(this is the easiest way). Or, you can kick back and forth(in a serpentine or figure "S" fashion) across the contour and throw markers as you intersect the different depth changes.
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[smallmouth89]

Thanks for the very informative reply. I've found the deepest water in the ponds I fish to be about 15 feet. Without the sonar I would have assumed many spots to be 20 feet deep or even more. I'm often marking trout (Solid icons) cruising 3 feet down over deeper water. Does that mean they are right under the tube? If so I could almost kick them with my fins. I believe I read somewhere that the sonar detects fish from the air in their air bladder. Is that correct?

If the fish symbols do not appear again when maintaining a relatively stationary position I'm assuming that indicates the fish were on the move. If you were anchored and the symbols were still there after the screen scrolled by and refreshed itself that should indicate the fish are holding in that position and depth. Would that be correct?

What type of sensitivity setting is best for water under 15 feet deep?
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[Fishhound]

Hey smallmouth89,

OK leaving the mathmatics of it out, let's look at your situation as you posted it.

Of course I don't know how you have your transducer mounted nor what kind of tube you have(next time describe your set up to me so I can tell more about what is happening). A number of things could be at play here. For example, if you are in a belly boat, you could be seeing your fins on the fish finder or your jig could show up as a fish if it is directly under the transducer(given the size of the jig or the sensitivity setting you are using). Also, I don't remember off hand if your Max 20 has Real Time Sonar(RTS). In any case, remember that when you use the Fish ID option(the fish icons) the readings are actually produced from an algorithym of the sonar returns from both frequencies and displayed as what is interpreted by that software as a possible fish sonar return.

You see a fish icon register at 3 feet while you are in 15 feet of water. The 3 feet depth is measured from the bottom of your transducer - not the surface of the water(unless the Max 20 has an offset adjustment like mine). When the fish registered at 3 feet(assuming a solid icon), that means it passed through a beam directly under the transducer that was about 1 foot in diameter. If the icon scrolled off the screen, that means the fish moved out of the cone(on your fishfinder, while using the fish icons, I don't think it indicates the direction the fish moved - mine does). If the icon was hollow, that means it passed through a cone(the lower frequency) of about 3 feet in diameter under the transducer, but just off to one side(could be in front or behind as well) of the 1 foot cone that is produced by the higher frequency.

You can tell the direction of the fish if you are just reading the raw sonar data and not the fish icons(it's the shape of the arch that tells you the information about the encounter - the thickness of the arch tells you something about the size of the fish). If you are moving and the fish is moving with you, you will see a straight line on your screen. The same thing will happen if you are stationary and the fish is holding directly under you. Your jig will also do the same thing - show up as a straight line across your display - moving your jig up and down will show up as an up and down zig zag line.

Sensitivity - If you increase the sensitivity, you will read more weaker sonar returns(smaller fish). If you decrease the sensitivity you will read fewer sonar returns(only the larger fish will appear). Check your manual to see if the adjustments for sensitivity can be done for each frequency individually.

Again, I dont remember if your finder has a surface clutter adjustment or noise filter, but if it does, decreasing those adjustments will remove some of the algea or interference from aeration, turbulence or even the thermocline readings - increasing the adjustment will do the opposite - allowing you to identify more baitfish and or debris in the water.

I don't know if I explained that very well - let me know.
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[smallmouth89]

Fishhound,

I think you are explaing things quite well especially, since you are talking to a mathematically challenged individual. My talents lie within the creative realm an math concepts always come harder.

I would suspect that at this time of the year especially in shallow water there would be no thermocline, Does the thermocline only occur in deeper waters? How does the thermocline register on the sonar unit?

My setup has the transducer mounted to a floating block of foam suspended off the point of my Fat Cat. It is only a couple of inches under the surface. I didn't think my PiranahMax20 was powerful enough to pick up my jig.
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[Fishhound]

Hey smallmouth89,

Yes, that is correct, shallower waters do not typically experience a thermocline. The thermocline usually occurs in the summer between about 20-40 of water in bodies that are quite a bit deeper than that. On your fish finder(given the sensitivity setting) it will show up as a fairly or relatively thick or thin(depending on the actuall depth of the body of water) band across the entire screen at those depths.

Also, if your Max20 does not have RTS(Real Time Sonar), it will probably not register your jig.
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