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Fish air bladders .

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It just come to my attention that transducer signals dont bounce off the fish but rather the air bladder . I'm not disputing this but rather questioning it . A depth finder is sensitive enough to pick up a thermocline but not a fish ? 

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CHIRP technology can show a fish that doesn't have a swim bladder, like tuna.  Usually at the lower frequencies.  Though mostly at the high frequencies we use, it's the air bladder coming back.

 

The Vexilar site explains it well.

http://vexilar.com/blog/2014/08/28/how-sonar-works

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Thanks for posting that, John.  The section on "How the Sonar Shows Fish" is excellent.  

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We all know that bass use their air bladder to maintain nuetral weightlessness at a specific depth like a astronaut in space. What we tend to overlook as bass anglers is the airbladder is also used as an organ to hear with. The airbladder acts like a drumb that has nerves connected to the inner ear. Bass definately hear your sonars pulses when the pulses contact it's body. How the sonar pulses affect bass is debatable and appears to have no affect most of the time and can alarm some bass.

Scanning sonar units are relatively new to recreational bass anglers and have been around decades for off shore salt water anglers and may have a greater impact on bass.

I have observed bass leave when the sonar is turned on and have no affect when turned on. It's been my belief when quietly hunting giant bass to avoid using my sonar until after fishing an area if I suspect big bass are at the spot. Why take a chance? On the other side of the coin I survey areas using my sonar to locate bass and bait. 

Tom

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2 hours ago, scaleface said:

It just come to my attention that transducer signals dont bounce off the fish but rather the air bladder . I'm not disputing this but rather questioning it . A depth finder is sensitive enough to pick up a thermocline but not a fish ? 

I think I get where you're coming from on this, so I'll add a bit more to the discussion.

 

I've seen some incredibly passionate/heated arguments over the subject, but I'll throw this out there anyway. I don't think we can completely rule out that the fishes body doesn't come into play to some degree, especially with more sophisticated units now on the market, just that it's the air bladder that is the primary thing responsible for us seeing fish on our units. With that said then, it's not the actual thermocline (temperature gradient) we're picking up in most cases, but more often the things living and suspended in and around the thermocline instead (plankton, detritus, silt, micros, etc.). Let me explain...

 

Water at its most dense (4 C) has a density of 1000 g/cm3. At 20 C, water is only slightly less dense at 0.9982 gm/cm3, about a .18% difference. But, 4 C to 20 C would make for a pretty significant thermocline. However, the average fish body has a density of 1080 g/cm3 (range 1040-1090 gm/cm3 @ 20 C), which is a significantly greater difference in density than the temperature difference mentioned above. As such, if it's not the fishes body we're picking up mostly on sonar returns, then it's also likely not the temperature/density gradient we're picking up in thermoclines either. The change in density (whether talking fish or temperature) changes the reflectivity of the water (the speed of sound increasing or decreasing with density differences, as well as the slight bending of the signal through a different density medium), and sensitive units might be able to detect some of this occurring, but in most cases with typical lower powered units, it's more likely the "stuff" in the thermocline we're picking up, "giving away" the presence of the actual thermocline itself.

 

Hopefully that makes some sense.

 

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While I agree with most of this a thermocline can be a dark thin line in lieu of a slow fuzzy transition depending on how quickly the temperature changes. Cold water being dense slows down the sonar return and we see that as background noise or a color change depending on the sonar ulit.

To understand suspended particulates affect on sonar is fish the same lake at night and your clear background becomes all clutter at the same settings.

Good discussion and the illustration posted should clarify the airbladder affect on sonar.

Tom

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17 minutes ago, WRB said:

 

To understand suspended particulates affect on sonar is fish the same lake at night and your clear background becomes all clutter at the same settings.

 

Tom

Done this many times, and shared the results with several  water biologists. Haven't confirmed completely, but all generally agreed diel movement of zooplankton as what we were seeing.

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Agree it's plankton because it happen at sea at night hundreds of miles off shore.

Tom

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On 3/7/2018 at 8:52 AM, scaleface said:

It just come to my attention that transducer signals dont bounce off the fish but rather the air bladder . I'm not disputing this but rather questioning it . A depth finder is sensitive enough to pick up a thermocline but not a fish ? 

Fish flesh, like people flesh, is mostly water.

 

Being cold blooded, fish flesh tends to be be pretty much the same temperature as the surrounding water...

 

The air bladder is, OTOH, a completely different  element in the water surrounding it...thus it stands out like...a big ol' bubble...underwater.

 

THat's how I think of it.  Hope that helps...

 

Note that with DI and SI, that's not what you see...

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Makes sense . After all depth finders pickup air bubbles rising to the surface . Now  thats a signal that confused me for awhile .

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1 hour ago, scaleface said:

Makes sense . After all depth finders pickup air bubbles rising to the surface . Now  thats a signal that confused me for awhile .

Try upside down, suspended leaves with little air pockets under them.  Took me two years to figure that one out...looked like eight billion panfish...just hanging there...

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